US 4,774,422 AGrant

High Speed Low Pin Count Bus Interface

Issue Date:1988-09-27

•17 Claims

•8 Drawing Sheets

Abstract

Bus interface apparatus is provided to drive a high speed bus with two nonoverlapping clock signals. The apparatus takes advantage of the inherent bus capacitance which will temporarily hold data signals placed on the bus by using bus interface circuitry having high input and output impedances. That circuitry can thus be activated by coincident signals.

Metadata

Assignee

Digital Equipment Corporation

Inventors

Darrel D. Donaldson
Richard B. Gillett, Jr.
Douglas D. Williams

Application Information

Application Number:US 0447803

Filing Date:1987-05-01

Priority Date:1987-05-01

Art Unit:254

Classifications

IPC:

H03K 1716H03K 19017H03K 19092

Field of Search:

307473;475;443;243;448

Patent Drawings (8 sheets)

Description

BACKGROUND OF THE INVENTION

This invention is related to U.S. patent application Ser. No. 044,467, filed on May 1, 1987, by Darrel D. Donaldson, Richard B. Gillett, Jr., and Douglas D. Williams entitled "High Performance Low Pin Count Bus Interface."

The present invention relates to the field of computer data buses generally and more specifically to high speed buses which allow bidirectional communication. In most buses, especially buses that contain multiple lines to transfer data in a parallel format, there are windows of time during which data on the bus is valid. In synchronous buses, periodically repeating cycles form the basis for timing of such buses and in each of those cycles windows are usually defined by a separate signal which becomes active once per bus cycle. During the time between windows, the data on the bus is allowed to change to new levels and is considered to be invalid.

In general, data communication on a bus involves a bus driver to place data onto the bus and a storage device to retrieve and store data from the bus. The bus driver, can generally be one of two types. One type can only drive a bus line to one level, e.g., to a ground potential, and thus requires a pull-up or precharging device to establish the other bus level. The other type actively drives the bus line to both levels and does not require precharging or pull-up devices. When a drive signal controlling the bus driver is in one state, it enables the bus driver and causes the bus driver to drive the bus to a level (either the same or inverted) corresponding to input data at the input terminal of the bus driver. When the drive signal is in the other state, it disables the bus driver causing the bus driver to cease driving the bus. Immediately following the enabling of the bus driver, while the data on the bus is still changing, the data is also considered to be invalid. For each particular driver technology and interface characteristics, this data invalid time is relatively fixed, as it depends on conditions like bus length, bus driver propagation delay, etc. As bus frequencies increase, the cycle times of the bus decreases, and the bus driver enable time decreases correspondingly. Thus, the amount of the bus driver enable time during which data is valid also decreases.

The storage device responds to the actuation of a latch signal by storing the data currently on the bus. The latch signal must be timed so that the storage device stores data while it is still valid on the bus, and conventional practice is to time the latch signal so that the storage device stores data while the bus driver is still enabled. Thus the conventional drive signal continues to drive the bus for some "hold time" after the latch signal is actuated. Conventional practice for driving buses between integrated circuit chips is to actuate the latch prior to disabling the bus driver because the voltage levels on the bus may be indeterminate when the bus is not being driven, and during such times the storage device may not actually store the data intended to be transferred over the bus from the bus driver.

Generation of the bus driver and latch signals to obtain the hold time requires two separate clock signals: one for the latch signal and one for the drive signal which extends beyond the latch signal. Those two clock signals, however, are only sufficient for unidirectional communication on the bus lines. Full bidirectional communication on the same bus lines requires two storage device/bus driver pairs, as well as four clock signals (two signals for each direction).

Furthermore, the drive signal used for communication in one direction on the bus must not overlap the drive signal used for communication in the other direction so that the bus drivers used for communication in each direction are not driving the bus simultaneously. If they do drive the bus simultaneously, even for only a small amount of time because of clock skew, for example, then the drivers and the bus lines will experience current spikes, and the availability of the data transferred by the second drive signal will be delayed.

The generation of the multiple clocks for bidirectional communication is further complicated if that communication must be synchronized to an overall system clock. For example, if there were a system bus with its own bus timing coupled to one of the elements of the bus, then the four clock signals necessary for bidirectional communication would have to be synchronized with the timing of this system bus. Such synchronization may be difficult for several reasons. First, the cycle time of the clock used for the sytem bus may be so small as to make it impractical to further subdivide the clock cycle time to obtain four different clock signals conforming to the requirements for bidirectional communication. Furthermore, even if it were possible to obtain those clock signals, their pulse widths may be so narrow that logic circuitry in the elements could not reliably respond to them.

One design approach would be to eliminate the need for the four separate clock signals by using two unidirectional buses, one for communication in each direction. The addition of another set of lines for the other unidirectional bus, however, doubles both the number of bus lines and the area which must be dedicated for those lines. In addition, the use of such buses doubles the number of pins on the components which couple to that bus. Thus, for example, to transfer 64 bits of data in parallel using two unidirectional buses would require an additional 64 pins per interface as compard to a single bidirectional bus.

Of all these disadvantages, the increased pin count may be the most severe. If the number of pins required exceeds the number that can be supported in a single integrated chip, then multiple chips must be used for the circuitry. This can be disadvantageous because circuitry generally operates more slowly when divided between multiple chips. Thus, the design of high speed circuitry frequently involves the development of techniques that minimize the required number of pins to avoid dividing functions across chip boundaries. Pin counts are also a limiting factor in circuit design because of limited space for those pins on printed circuit boards.

Accordingly, it is an object of the present invention to minimize the number of clock signals needed for high speed bus transfer.

Another object of the present invention is to provide high speed bidirectional bus transfer without driver overlap.

A still further object is to minimize the number of pins needed to interface to a system bus.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

The present invention overcomes the problems and disadvantages of the prior art by using high impedance interface circuitry coupled to a bus so that the inherent capacitance of the bus can be used to maintain the data levels on the bus after bus drivers have ceased driving the bus. This allows bus drivers to be driven by the same signal as storage elements on that bus to allow high speed bidirectional communication with only two nonoverlapping clocks.

To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, interface apparatus is provided for coupling across integrated circuit boundaries by way of a multiple line, parallel signal transfer bus having an inherent electrical capacitance on each line of the bus, the bus sequentially propagating data by maintaining on the bus lines for a predetermined period of time voltage levels representing the data. The interface apparatus of this invention comprises: a plurality of bus drivers each corresponding to a different line of the bus and being capable of driving the corresponding bus lines to either one or two different voltage levels, a plurality of latches each corresponding to a different line of the bus, and signal generating means. Each of the bus drivers includes an input terminal for holding input data to be transmitted to the corresponding line of the bus, a bus interface terminal coupled to the corresponding line of the bus, and an enable terminal for receiving a drive signal which, in a first state, causes the bus driver to drive the corresponding bus line to one of the levels according to the input data and, in a second state, causes the bus driver to cease driving the corresponding bus line and present at its bus interface terminal a high electrical impedance. Each of the latches includes an input terminal coupled to the corresponding line of the bus, the latch presenting at its input terminal a high electrical impedance and a control terminal for receiving a control signal that causes the latch to store data representing the voltage level on the corresponding bus line when the control signal is actuated. The signal generating means is coupled to the plurality of latches and to the plurality of bus drivers and generates the drive and control signals to transfer the input data at the input terminals of the bus drivers, through the bus, to the latches. The signal generating means includes first means for switching the drive signals from the first stae into the second state at substantially the same time as the control signals are actuated. As a result of this invention, the high electrical impedances presented by the bus drivers and the latches in combination with the inherent capacitance of the bus lines maintain the bus lines at voltage levels corresponding to the input data for a sufficiently long period of time after the bus drivers have ceased driving the bus to ensure that the latches correctly store the input levels even if the control signals are actuated after the bus drivers cease driving the bus lines.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and together with the description serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a data processing system including a system bus using the present invention;

FIG. 2 is a block diagram of a node in the data processing system of FIG. 1;

FIG. 3 is a timing diagram showing timing signals used in the data processing system of FIG. 1;

FIG. 4 is a block diagram of the data interface in the node of FIG. 2;

FIG. 5 is a block diagram of the arbiter in the data processing system of FIG. 1;

FIG. 6 is a detailed block diagram of the data interface in FIG. 2 and of interface circuitry for the node bus in FIG. 2;

FIG. 7 is a block diagram of portions of clock decoder 63 shown in FIG. 2;

FIG. 8 shows a CMOS output circuit;

FIG. 9 shows a CMOS input circuit; and

FIG. 10 shows timing signals for the node bus 67 shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings.

A. System Overview

FIG. 1 shows an example of a data processing system 20 which embodies the present invention. The heart of system 20 is a system bus 25 which is a synchronous bus that allows communication between several processors, memory subsystems, and I/O systems. Communications over system bus 25 occur synchronously using periodic bus cycles. A typical bus cycle time for system bus 25 is 64 nsec.

In FIG. 1, system bus 25 is coupled to two processors 31 and 35, a memory 39, one I/O interface 41 and one I/O unit 51. I/O unit 53, is coupled to system bus 25 by way of I/O bus 45 and I/O unit interface 41.

A central arbiter 28 is also connected to system bus 25 in the preferred embodiment of data processing system 20. Arbiter 28 provides certain timing and bus arbitration signals directly to the other devices on system bus 25 and shares some signals with those devices.

The implementation shown in FIG. 1 is one which is presently preferred and should not necessarily be interpreted as limiting the present invention. For example, I/O unit 53 could be coupled directly to system bus 25, and arbiter 28 need not operate in the manner described for the present invention.

In the nomenclature used to describe the present invention, processors 31 and 33, memory 39, and I/O interface 41, and I/O device 51 are all called nodes. A "node" is defined as a hardware device which connects to system bus 25.

According to the nomenclature used to describe the present invention, the terms "signals" or "lines" are used interchangeably to refer to the names of the physical wires. The terms "data" or "levels" are used to refer to the values which the signals or lines can assume.

Nodes perform transfers with other nodes over system bus 25. A "transfer" is one or more contiguous cycles that share a common transmitter and common arbitration. For example, a read operation initiated by one node to obtain information from another node on system bus 25 requires a command transfer from the first to the second node followed by one or more return data transfers from the second node to the first node at some later time.

A "transaction" is defined as the complete logical task being performed on system bus 25 and can include more than one transfer. For example, a read operation consisting of a command transfer followed later by one or more return data transfers is one transaction. In the preferred embodiment of system bus 25, the permissible transactions support the transfer of different data lengths and include read, write (masked), interlock read, unlock write, and interrupt operations. The difference between an interlock read and a regular or noninterlock read is that an interlock read to a specific location retrieves information stored at that location and restricts access to the stored information by subsequent interlock read commands. Access restriction is performed by setting a lock mechanism. A subsequent unlock write command stores information in the specified location and restores access to the stored information by resetting the lock mechanism at that location. Thus, the interlock read/unlock write operations are a form of read-modify-write operation.

Since system bus 25 is a "pended" bus, it fosters efficient use of bus resources by allowing other nodes to use bus cycles which otherwise would have been wasted waiting for responses. In a pended bus, after one node initiates a transaction, other nodes can have access to the bus before that transaction is complete. Thus, the node initiating that transaction does not tie up the bus for the entire transaction time. This contrasts with a non-pended bus in which the bus is tied up for an entire transaction. For example in system bus 25, after a node initiates a read transaction and makes a command transfer, the node to which that command transfer is directed may not be able to return the requested data immediately. Cycles on bus 25 would then be available between the command transfer and the return data transfer of the read transaction. System bus 25 allows other nodes to use those cycles.

In using system bus 25, each of the nodes can assume different roles in order to effect the transfer of information. One of those roles is a "commander" which is defined as a node which has initiated a transaction currently in progress. For example, in a write or read operation, the commander is the node that requested the write or read operation; it is not necessarily the node that sends or receives the data. In the preferred protocol for system bus 25, a node remains as the commander throughout an entire transaction even though another node may take ownership of the system bus 25 during certain cycles of the transaction. For example, although one node has control of system bus 25 during the transfer of data in response to the command transfer of a read transaction, that one node does not become the commander of the bus. Instead, this node is called a "responder."

A responder responds to the commander. For example, if a commander initiated a write operation to write data from node A to node B, node B would be the responder. In addition, in data processing system 20 a node can simultaneously be a commander and a responder.

Transmitters and receivers are roles which the nodes assume in an individual transfer. A "transmitter" is defined as a node which is the source of information placed on system bus 25 during a transfer. A "receiver" is the complement of the transmitter and is defined as the node which receives the information placed on system bus 25 during a transfer. During a read transaction, for example, a commander can first be a transmitter during the command transfer and then a receiver during the return data transfer.

When a node connected to system bus 25 desires to become a transmitter on system bus 25, that node asserts one of two request lines, CMD REQ (commander request) and RES REQ (responder request), which are connected between central arbiter 28 and that particular node. In general, a node uses its CMD REQ line to request to become commander and initiate transactions on system bus 25, and a node uses its RES REQ line to become a responder to return data or message to a commander. Generally, central arbiter 28 detects which nodes desire access to the bus (i.e., which request lines are asserted). The arbiter then responds to one of the asserted request lines to grant the corresponding node access to bus 25 according to a priority algorithm. In the preferred embodiment, arbiter 28 maintains two independent, circular queues: one for the commander requests and one for the responder requests. Preferably, the responder requests have a higher priority than the commander requests and are handled before the commander requests.

The commander request lines and responder request lines are considered to be arbitration signals. As illustrated in FIG. 1, arbitration signals also include point-to-point conditional grant signals from central arbiter 28 to each node, system bus extend signals to implement multi-bus cycle transfers, and system bus suppression signals to control the initiation of new bus transactions when, for example, a node such as memory is momentarily unable to keep up with traffic on the system bus.

Other types of signals which can constitute system bus 25 include information transfer signals, respond signals, control signals, console/front panel signals, and a few miscellaneous signals. Information transfer signals include data signals, function signals which represent the function being performed on the system bus during a current cycle, identifier signals identifying the commander, and parity signals. The respond signals generally include acknowledge or confirmation signals from a receiver to notify the transmitter of the status of the data transfer.

Control signals include clock signals, warning signals, such as those identifying low line voltages or low DC voltages, reset signals used during initialization, node failure signals, default signals used during idle bus cycles, and error default signals. The console/front panel signals include signals to transmit and receive serial data to a system console, boot signals to control the behavior of a boot processor during power-up, signals to enable modification of the erasable PROM of processors on system bus 25, a signal to control a RUN LIGHT on the front panel, and signals providing battery power to clock logic on certain nodes. The miscellaneous signals, in addition to spare signals, include identification signals which allow each node to define its identification code.

FIG. 2 shows an example of a node 60 connected to system bus 25. Node 60 could be a processor, a memory, an I/O unit, or an I/O interface. In the example shown in FIG. 2, node 60 includes node specific logic 65, a node bus 67, and a system bus interface 64 containing a data interface 61 and a clock decoder 63. Preferably, data interface 61, clock decoder 63, and node bus 67 are standard elements for nodes connected to system bus 25. The node specific logic 65, which uses different integrated circuits from system bus interface 64, preferably includes, in addition to the circuitry designed by a user to carry out the specific function of a node, standard circuitry to interface with the node bus 67. In general, data interface 61 is the primary logical and electrical interface between node 60 and system bus 25, clock decoder 63 provides timing signals to node 60 based on centrally generated clock signals, and node bus 67 provides a high speed interface between data interface 61 and node specific logic 65.

In the preferred embodiment of node 60 and system bus interface 64 shown in FIG. 2, clock decoder 63 contains control circuitry for forming signals to be placed on system bus 25 and processes clock signals received from central arbiter 28 to obtain timing signals for node specific logic 65 and data interface 61. Since the timing signals obtained by clock decoder 63 use the centrally generated clock signals, node 60 will operate synchronously with system bus 25.

FIG. 3 is a timing diagram showing one bus cycle, the clock signals received by clock decoder 63, and certain of the timing signals generated by clock decoder 63. The clock signals received by clock decoder 63 include a Time H signal, a Time L signal, and a Phase signal as shown in FIG. 3. Time H and Time L are inverses of the fundamental clock signals and the Phase signal is obtained by dividing the fundamental clock signal by three. The timing signals generated by clock decoder 63 include C12, C23, C34, C45, C56 and C61, all of which are shown in FIG. 3. Those timing signals required by data interface 61, which occur once per bus cycle, are provided to data interface 61, and a complete set of timing signals, including equivalent ones of the timing signals provided to data interface 61, is buffered and provided to the node specific logic 65. The purpose of buffering is to insure that node specific logic 65 cannot adversely effect the operation of the system bus interface 64 by improperly loading the timing signals. Clock 63 uses the clock signals to create six subcycles for each bus cycle and then uses the subcycles to create the six timing signals CXY, where X and Y represent two adjacent subcycles which are combined to form one timing signal.

Each node in the sytem bus has its own corresponding set of timing signals generated by its clock decoder 63. While nominally the corresponding signals occur at exactly the same time in every node throughout the system, variations between clock decoder 63 and other circuitry in multiple nodes introduce timing variations between corresponding signals. These timing variations are commonly known as "clock skew."

FIG. 4 shows a preferred embodiment of data interface 61. Data interface 61 contains both temporary storage circuitry and bus driver circuitry to provide a bidirectional and high speed interface between each of the lines of node bus 67 and each of the lines of system bus 25. As shown in FIG. 4, data interface 61 preferably includes storage elements 70 and 72 and system bus driver 74 to provide a communication path from node bus 67 to system bus 25. Data interface 61 also includes storage element 80 and node bus driver 82 to provide communication path from system bus 25 to node bus 67. As used in the description of data interface 61, the term "storage element" refers generally to bistable storage devices such as a transparent latch or a mater-slave storage element, and not to a specific implementation. Persons of ordinary skill will recognize which types of storage elements are appropriate.

As shown in FIG. 4, storage element 70 has an input connected to receive data from node bus 67 and an output connected to the input of storage element 72. The output of storage element 72 is connected to an input of system bus driver 74 whose output is connected to system bus 25. Storage elements 70 and 72 are controlled by node bus control signals 76 and 78, respectively, which are derived from the timing signals generated by clock decoder 63. Storeage elements 70 and 72 provide a tow-stage temporary storage for pipelining data from node bus 67 to system bus 25. Different numbers of storage stages can also be used.

System bus driver 74 is controlled by system bus driver enable 79. According to the state of the system bus driver enable 79, the input of system bus driver 74 either is coupled to its output, thereby transferring the data at the output of storage element 72 to system bus 25, or decoupled from that output. When system bus drive enable 79 decouples the input and output of the system bus driver 74, system bus driver 74 presents a high impedance to system bus 25. The system bus drive enable 79 is also generated by clock decoder 63 in accordance with clock signals received from system bus 25 and control signals received from the node specific logic 65.

Storage element 80 has an input terminal connected to system bus 25 and an output terminal connected to an input of node bus driver 82. The output of node bus driver 82 is connected back to node bus 67. Storage element 80, preferably a transparent latch, is controlled by a system bus control signal 85 which is derived from the timing signals generated by clock decoder 63. A node bus drive signal 87 controls node bus driver 82 similar to the manner in which system bus drive signal 79 controls system bus driver 74. Thus, in response to node bus driver signal 87, node bus driver 82 either couples its input to its output or decouples its input from its output and provides a high impedance to node bus 67.

In order to explain how data is transferred over system bus 25, it is important to understand the relationship between system bus drive enable control signal 85. In the present embodiment, this relationship is shown in FIG. 3. System bus drive enable 79 is nominally driven from the beginning to the end of a bus cycle. The new data become available for receipt from system bus 25 at some time later in the bus cycle after driver propagation and bus settling time has occurred. In the present embodiment, storage element 30 is a transparent latch. Control signal 35 is logically equivalent to clock C45. The bus timing assures that system bus 25 data is available for receipt sometime prior to the deassertion of control signal 85. Storage element 80 stores bus data that is stable at least a set up time prior to the deassertion of control signal and remains stable a hold time after the deassertion of control signal 85.

Node bus 67 is preferably a very high speed data bus which allows bidirectional data transfer between the node specific logic 65 and system bus 25 by way of data interface 61. In the preferred embodiment of node 60 shown in FIG. 2, node bus 67 is an interconnect system consisting of point-to-point connections between the system bus interface 64 and the node specific logic 65. In accordance with the present invention, however, there is no requirement for such point-to-point interconnection.

FIG. 5 shows a preferred embodiment of the central arbiter 28 which is also connected to system bus 25. Central arbiter 28 provides the clock signals for system bus 25 and grants ownership of the bus to the nodes on system bus 25 ownership of that bus. Central arbiter 28 preferably includes an arbitration circuit 90, a clock circuit 95, and a oscillator 97. Oscillator 97 generates the fundamental clock signals. Clock 95 provides timing signals for arbitration circuit 71 and the basic Time H, Time L, and Phase clock signals for timing on system bus 25. Arbitration circuit 71 receives the commander and responder request signals, arbitrates conflicts between nodes desiring access to system bus 25, and maintains the queues referred to above for the commander and responder requests. Arbitration circuit 71 also provides certain control signals to clock 95.

B. Bus Interface Circuitry

In node 60, as shown in FIGS. 2 and 4, each data interface 61 is coupled to a corresponding line of system bus 25 by a single pin terminal. The connection can be direct but is preferably through a resistance for reasons not relevant to an understanding of the present invention. For the reasons explained in the Background of Invention, it is desirable that each data interface 61 also be coupled by a single pin terminal to a corresdonding line of node bus 67 for those lines of node bus 67 which correspond to the lines of system bus 25. By using single pin terminals, node bus 67 can be a single bidirectional bus instead of two parallel buses, and the number of pins or terminals needed is thus minimized. In addition, the circuitry connected to node bus 67 need not be divided among several integrated circuit chips only to satisfy increased pin count needed to interface to two parallel buses.

In addition, for the reasons explained in the Background of the Invention, the design of circuitry to accomplish a single pin terminal to single pin terminal, bidirectional transfer complex is complicated by the desire to place on node bus 67 a copy of all the data or messages from system bus 25, including the data on system bus 25 provided by node 60. Doing so allows the node specific logic 65 to view all the data or messages on system bus 25 which in turn helps to manage system bus 25. By making available to node specific logic 65 all the data or messages on system bus 25, node 60 can use the knowledge of which resources of data processing system 20 are being used and by whom those processes are being used to implement certain self-management techniques for use of system bus 25.

In addition, the provision of bidirectional communications between data interface 61 and system bus 25 through a single pin terminal for each line of system bus 25 allows node specific logic 65 to monitor and receive its own messages to system bus 25. Such connection also allows node specific logic 65 to send a message to itself by way of system bus 25 so other nodes can monitor those transactions.

Another benefit of cycle by cycle visibility of system bus 25 to user bus 67 is to maintain cache coherency. With regard to this use, if a node modifies the contents at memory locations accessible through system bus 25, the other nodes can monitor such accesses and determine whether their own caches include copies of these memory locations.

To provide single pin terminal connection as well as bidirectional transfer capability during each cycle of system bus 25, node bus 67 must operate at twice the speed of system bus 25, as explained in the Background of Invention. Node bus 67 must therefore have shorter transfer times than system bus 25.

In the preferred embodiment of the invention, with a 64 nanosecond cycle time for system bus 25, only six subhcycles are available due to the requirements of circuitry in, for example, system bus interface 64 which is composed of CMOS circuitry. Six timing signals, C12, C23, C34, C45, C56, and C61, each covering two subcycles, are formed for timing of node bus 67.

In the conventional method of bidirectional bus transfer requiring two pairs of clock signals, the timing necessary to form those signals is difficult to implement using the timing signals in FIG. 3 or any other signals based on the six subcycles. In fact, the implementation would only be possible by creating special timing signals for drivers which were longer than the timing signals shown in FIG. 3 (e.g., a three-subcycle long signal such as C123) or by creating latch control signals shorter than those timing signals (e.g., a single subcycle signal such as C1 or C2). In the first case, a high probability exists that two different drivers would drive the bus at the same time since the longer clock signals (e.g., C123 and C456) would need to be used to control the drivers and clock skew may cause those signals to overlap and drive the bus at the same time. In the second case, the shorter signals may not be long enough to be used effectively by the logic circuitry or to permit the data to propagate from the driver to the latch prior to actuation of the latch. Thus, a conventional bus driving method requires timing signals which may be obtained from a minimum of eight subcycles. This would allow two three-subcycle driver signals, separated by one subcycle to prevent overlap, and the latch control signals could lasts for the first two of the three-subcycle drive signals.

The present invention avoids problems of bidirectional data transfer when there is a limited number of subcycles and other circuit constraints by using a phenomenon of bus lines which has not been previously exploited for the problem of data transfer across integrated chip boundaries. Each of the lines of node bus 67 has an inherent capacitance of typically 5-10 pf. The inventors have found that this inherent capacitance can be used to maintain the proper voltage levels on the bus lines even after the bus is no longer being actively driven to a particular level by a bus driver. To store charge in the bus line inherent capacitance, the impedance of the discharge paths of that capacitance must be high. In general, the discharge paths for a bus are through the driver outputs and storage element inputs connected to that bus. The inventors have found that by using devices with high input and output impedances, specifically CMOS devices which also add several picofarads of capacitance to the bus lines, the inherent capacitance of the bus lines can be used to extend the time during which the data placed in the bus is valid even after the drive signals are removed from the bus drivers.

The interface apparatus of the present invention comprises a plurality of bus drivers each corresponding to a different line of the bus. Each of the drivers is capable of driving the corresponding bus line to either of two voltage levels. FIG. 6 shows an example of one line of bus 67 connected to circuitry in node specific logic 65 and in data interface 61. Bus driver 82, which has already been described, is shown as being coupled to that line of bus 67. Bus driver 82 includes an input terminal 90 for holding input data to be transmitted to the line of node bus 67 from storage element 80, and a bus interface terminal 92 which is actually coupled to the line of node bus 67. Bus driver 82 also has an enable terminal 94 which receives a two-state drive signal, called DRIVE 87 in FIG. 4 and shown specifically as C61 in FIG. 6.

FIG. 3 shows the timing of signal C61. When C61 is high (the designation of levels is a convention and not a requirement), the output terminal 92 drives the corresponding line of node bus 67 to one of the levels according to the input data. When signal C61 is low, then the bus driver 82 ceases driving the bus line and presents to the line of node bus 67 a high impedance.

The interface apparatus of the present invention also includes a plurality of latches corresponding to a different line of the bus. As FIG. 6 shows, a storage element 110 is coupled to the same line of node bus 67 as is driver 82. Storage element 110 has an input terminal 112 coupled to that line of node bus 67 and presents a high impedance to that line. Storage element 110 also has a control terminal 114 for receiving a control signal which, as shown in FIG. 6, is also C61. The control signal at terminal 114 causes storage element 110 to store the level on the corresponding line of node bus 67 when that control signal is actuated (i.e., when C61 deasserts). In general, a control signal is actuated during a single transition between states, such as either a rising or a falling edge, but not both.

In accordance with the present invention, the interface apparatus also includes signal generating means coupled to the plurality of latches to generate drive and control signals to transfer the input data at the input terminals of the bus drivers through the bus to the latches. FIG. 7 shows an example of clock decoder 63 including circuitry to derive signal C61. In FIG. 7, the Time L signal is received through buffer 130 and coupled to a clock input of a three bit shift register 132. The PHASE signal, after passing through buffer 134, is provided to the data input terminal of shift register 132. The Q.sub.1, Q.sub.2, and Q.sub.3 outputs of that register are passed through buffers 140, 138 and 136, respectively, to form the C12, C34, and C56 signals, respectively. The Q.sub.1 output of shift register 132 provides the data input to a three-bit shift register 142 whose clock input is coupled, through buffer 144, to the TIME H signal. The Q.sub.1, Q.sub.2, and Q.sub.3 outputs of register 142 are passed through buffers 150, 148, and 146 to form the C23, C45, and C61 signals, respectively. As FIG. 3 shows, the active portion of clock signal C61 in the preferred embodiment lasts for about one-third of the cycle of system bus 25.

Further in accordance with the present invention, the signal generating means includes first means for switching the drive signals from the first state to the second state at substantially the same time as the corresponding control signals are actuated. In other words, the drive signal need not remain an additional subcycle after the latch signal. In the preferred embodiment of the invention, clock decoder 63 generates timing signal C61 for both the driver and control signals. As explained previously, it is preferable to isolate the timing signal C61 which is sent to data interface 61 from the timing signal C61 which is sent to node specific logic 65 and thus storage element 110. Both timing signals, however, are substantially the same. This signal isolation prevents characteristics of the node specific logic 65 from altering the timing signals presented to data interface 61, for example due to improper loading, and adversely affecting the servicing of data interface 61 by the timing signals in clock decoder 63.

Contrary to conventional approaches, with the present invention there is no separate driver hold time for the bus lines. The absence of a separate driver hold time is possible with the present invention because the inherent capacitance of the bus lines is used to maintain the levels on those lines even when the node bus 67 is no longer being actively driven. Since the discharge paths for the line of node bus 67 are through the high input impedance of storage element 110 and the high output impedance of bus driver 82 when that driver is enabled, the voltage levels on the lines of node bus 67 remain relatively fixed for a predetermined period of time. That time can be calculated from the inherent bus capacitance, and the capacitances and impedances of driver 82 and storage element 110.

Preferably, the output circuitry for bus driver 82 is a CMOS driver as shown in FIG. 8. That circuitry includes a series-connected p-channel pull-up transistor 200 and n-channel pull-down transistor 210. The p-channel transistor 200 has a current path coupled between the supply voltage Vcc and output terminal 92. The n-channel transistor 210 has a current path coupled between the output terminal 92 and a reference terminal.

A prebuffer 220 sends out a GATE P signal to control the gate of the p-channel transistor 200 and a GATE N signals to control the gate of the n-channel transistor 210. When bus driver 82 is enabled, the GATE P and GATE N signals control transistors 200 and 210, respectively, to drive node bus 67 to either high or low levels, corresponding to "1" or "0" data. In particular, when node bus 67 is to be driven to a low level, the GATE P and GATE N signals are placed at a high level (approaching VCC); when node bus 67 is to be driven to a high level, GATE P and GATE N signal are placed at a low level (approaching ground). When bus driver 82 is disabled terminal 90 is to be decoupled from the output terminal 92, then prebuffer 220 sets the GATE P signal at a high level and sets the GATE N at a low level. This disables both transistors 200 and 210 and presents to node bus 67 a high impedance, typically several megohms.

Preferably, the input circuitry for latch 110 is also standard CMOS circuitry. An example of this shown in FIG. 9 by inverter circuit 230 comprising p-channel transistor 235 and n-channel transistor 237. The typical input impedance of the circuit in FIG. 9 is also of the order of several megohms.

By using driver 82 with the CMOS driver circuitry shown in FIG. 8, and by using storage element 110 with the CMOS input circuitry shown in FIG. 9, data is maintained on node bus 67 for what is effectively a "hold time" so at the falling edge of the control signal (C61) storage element 110 stores the level even though bus drive 82 has been disabled by its drive signal (also C61). In this manner, the "hold time" for the present invention overlaps with what would normally be considered to be the "driver non-overlap time" in conventional bus driving circuits.

Preferably, the timing of the present invention insures that the control signal is actuated during the "window" when the data is valid on the bus. As indicated, however, the present invention allows an extension of that window past the time that bus driver 82 ceases actively driving node bus 67.

For bidirectional communication over that same line of node bus 67, another driver/storage element pair is needed for transmitting data from the node specific logic 65 to the system bus 25. As shown in FIG. 6, node specific logic 65 includes a bus driver 120 having an input terminal 121, an output terminal 122, and an enable terminal 124. Preferably, bus driver 120 is structurally similar to bus driver 82. The input terminal 121 of bus driver 120 holds a second input level, shown in FIG. 6 as received from storage element 130, to be coupled to the same line of node bus 67 to which the output terminal 122 of bus driver 120 is coupled. The enable terminal 124 in the embodiment of the invention shown in FIG. 6 is connected to timing signal C34 which is also provided from clock decoder 63.

Data interface 61 preferably includes storage element 70 as the complement to bus driver 120. Storage element 70 has an input terminal 71 and a control terminal 75. In the embodiment of the invention shown in FIG. 6, the control signal at control terminal 75 is also signal C34.

The signal generating means for the interface apparatus of the present invention for providing bidirectional communication generates the different driver control signals for storage elements 70 and 110 and bus drivers 82 and 120 to transfer the data at the respective inputs of bus drivers 82 and 120 to node bus 67. That signal generating means includes means (1) for switching the drive signals for one of the bus drivers between a first and second state, thereby at substantially the same time as the corresponding control signals are actuated, (2) for switching the drive signals between the first and second state for the other of the bus drivers at substantially the same time as the control signals for the corresponding latches are actuated, and (3) for ensuring that the first and second drive signals are not simultaneously in the first state.

C1ock decoder 63, as shown in FIG. 7, generates timing signals C61 and C34 which do not overlap as can be seen from FIG. 3. Indeed, there are time periods between timing signals C34 and C61 corresponding to subcycles 2 and 5 and those time periods ensure that the node bus 67 will not be concurrently driven by bus drivers 82 and 120. Thus, the present invention does not have the bus driver overlap related problems discussed in the Background of the Invention.

The entire bidirectional data transfer operation, as well as its relationship to the other transfers through data interface 61, may be understood from the timing diagram of FIG. 10. In FIG. 10, timing signal C12 through C61 are reproduced as well as the present and previous system bus driver enables, the periods of valid data on system bus 25, and control signal 85. When timing signal C45 deasserts at the end of subcycle 5, the data on system bus 25 is valid, and transparent latch 80 captures that valid data. That data is then transmitted to node bus 67 while timing signal C61 is active because that timing signal, also shown as Drive 87, enables bus driver 82. At the end of subcycle 1, i.e., when timing signal C61 is deasserted, storage element 110 captures the data from node bus 67. In this way, the data from system bus 25 is transferred to storage element 110 once each cycle of system bus 25.

During subcycles 3 and 4 of that same system bus cycle, while timing signal C61 was not active, bus driver 120 transferred the data in storage element 130 to node bus 67. As FIG. 6 shows, when C34 is asserted, it enables bus driver 120. At the end of subcycle 4, when C34 is deasserted, the data which had been transferred to node bus 67 by bus driver 120 is captured by storage element 70 because of control signal 75 shown in FIG. 10. Later, at the end of subcycle 2, the data in storage element 70 would be captured by storage element 72 if the node had access to the bus such that GC12 was asserted.

C. Interface System

The interface system provides bidirectional communication between a node and a system bus. The system bus, such as system bus 25, propagates data during repetitive bus cycles and the node, such as node 60, has a user portion, such as node specific logic 65, for processing the data. The interface system comprises a node bus, such as node bus 67, which is coupled to node specific logic 65 and transfers data in parallel.

Transceiver means are coupled between the node bus and the system bus for providing bidirectional communication between the system bus and the node bus and for providing to the node bus a copy of all the data progagated on the system bus. In the preferred embodiment, data interface 61 provides such bidirectional communication.

Transceiver means includes first and second unidirectional communications means. The first unidirectional communication means has input terminals coupled to the node bus and output terminals coupled to the system bus and receives data from the node bus to be transferred to the system bus during selected cycles of the system bus. As shown in FIGS. 4 and 6, the first unidirectional communications means in the preferred embodiment includes storage elements 70 and 72 and bus driver 74. Storage element 70 has an input terminal 71 coupled to the node bus. The output of storage element 70 is coupled to storage element 72. Bus driver 74 is coupled to the output of storage element 72 and has an output terminal 77 coupled to system bus 25.

As described above, data is received from node bus 67 into storage element 70 during the actuating portion (e.g., fall time) of clock C34. That data is then transferred to storage element 72 when it is actuated by signal GC12 as shown in FIG. 10. Signal GC12 is the logical AND of two signals, C12 and GC12EN. The C12 signal shown in FIGS. 3 and 10 is one of the timing signals generated by clock decoder 63, and GC12EN is a signal which, when active, indicates that node 60 will be a transmitter on system bus 25 and that data can the be transferred out of data interface 61 onto system bus 25. The GC12 signal is generated by clock decoder 63 using signals from arbiter 28 and node specific logic 65.

As shown in FIGS. 4 and 6, driver 74 has as its input a drive signal called System Bus Drive enable 79 which is also received from clock decoder 63 and which causes the data in storage element 72 to be transferred onto system bus 25. The System Bus Drive enable 79 signal is based upon requests from node specific logic 65 as well as on certain grant signals received from the data processing system 20. Thus, data is transferred onto system bus 5 only during the cycles of system bus 25 when node 60 is a transmitter.

The second unidirectional communication means has input terminals coupled to the system bus and output terminals coupled to the node bus. The input and output terminals of the second unidirectional communication means are each respectively coupled to the corresponding output and input terminals of the first unidirectional communication means. The second unidirectional communication means transfers data progagated on the system bus to the node bus once every cycle of the system bus.

As shown in FIGS. 4 and 6, storage element 80 has an input terminal coupled to system bus 25 and is enabled by clock signal C45 received from clock decoder 63. Bus driver 82 has an output terminal 92 coupled to node by 67 and to the input terminal 71 of corresponding storage element 70. The enable terminal 94 of bus driver 82 is coupled to the C61 signal.

The interface system also includes timing means, coupled to the first and second unidirectional communication means, for controlling those means. The timing means controls the first unidirectional communication means to receive the data from the node bus during selected active portions of a first clock signal and controls the second undirectional communication means to transfer the data from the system bus to the node bus during active portions of a second clock signal. The first means also generates the first and second clock signals so that the active portions of those clock signals occur once each cycle and do not overlap.

As explained previously, clock decoder 63 generates clock signals C34 and C61. Storage element 70 receives data at the end of clock C34 and transfers that data to system bus 25 during selected ones of the cycles of system bus 25. Furthermore, storage element 80 receives data from system bus 25 during timing signal C45 and driver 82 transfers that data to node bus 67 during the C61 clock cycle. Thus, data transferred during each cycle of system bus 25 are also transferred to node bus 67 to allow node bus 67, and therefore node specific logic 65 obtains a picture of all messages transmitted on system bus 25. Since clock cycles C45 and C61 occur once per system bus cycle, the transfer from system bus 25 to node bus 67 also occurs once per cycle of system bus 25.

When used together, the bus interface circuitry and interface system of this invention provide several advantages over prior art systems. Those advantages include high speed bus transfer using a minimum number of clock signals and provides high speed data transfer between buses so that one bus, e.g., a node bus, can obtain a copy of all the messages transmitted via another bus, such as the system bus. By providing such a copy and using a single pin connection to system bus 25, a node coupled to the node bus can monitor its own messages and send messages back to itself by way of the system bus so other nodes can monitor those messages.

It will be apparent to those skilled in the art that various modifications and variations can be made in the bus interface circuitry and interface of this invention without departing from the spirit or scope of hhe present invention. The present invention covers such modifications and variations which are within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. Interface apparatus for coupling across integrated circuit chip boundaries by way of a multiple line, parallel signal transfer bus having an inherent electrical capacitance on each line of said bus, said bus sequentially propagating data by maintaining on said bus lines for a predetermined period of time voltage levels representing the data and said interface apparatus comprising: a plurality of bus drivers each corresponding to a different line of said bus and being capable of driving the corresponding bus line to either one of two different voltage levels, and each of said bus drivers including an input terminal for holding input data to be transmitted to the corresponding line of said bus, a bus interface terminal coupled to the corresponding line of said bus, and an enable terminal for receiving a drive signal which, in a first state, causes the bus driver to drive the corresponding bus line to one of said levels according to the input data and, in a second state, causes the bus driver to cease driving the corresponding bus line and to present a high electrical impedance at its bus interface terminal; a plurality of latches each corresponding to a different line of said bus and each including an input terminal coupled to the corresponding line of said bus, the latch presenting a high electrical impedance at its input terminal, and a latch control terminal for receiving a control signal that causes the latch to store data corresponding to the voltage level on the corresponding bus line when the control signal is actuated; and signal generating means, coupled to said plurality of latches and to said plurality of bus drivers, for generating the drive and control signals to transfer the input data at the input terminals of said bus drivers, through said bus, to said latches, said signal generating means including first means for switching the drive signals from the first state into the second state at substantially the same time as the control signals are actuated; whereby the high electrical impedances presented by said bus drivers and said latches in combination with the inherent capacitance of said bus lines maintain said bus lines at voltage levels corresponding to the input data for a sufficiently long period of time after said bus drivers have ceased driving the bus lines to ensure that said latches correctly store the input data even if the control signals are actuated after said bus drivers cease driving the bus lines.

2. The interface apparatus of claim 1 wherein said first means of said signal generating means includes means for setting said drive signals into said second state during the predetermined period of time that the voltage levels on said bus lines are maintained.

3. The interface apparatus of claim 1 wherein each of said plurality of bus drivers is contained in a different bus transceiver.

4. The interface apparatus of claim 1 wherein each of said plurality of bus drivers is constructed of CMOS components.

5. The interface apparatus of claim 1 wherein each of said plurality of latches is constructed from CMOS components.

6. The interface apparatus of claim 5 wherein each of said latches includes a CMOS inverter.

7. The interface apparatus of claim 1 wherein the control signals have a first and a second state and wherein the control signals are actuated by changing from the first to the second state, and wherein said first means of said signal generating means includes means for generating the drive signals and the control signals to change between the first and second states at substantially the same time.

8. Interface apparatus for coupling across integrated circuit chip boundaries by way of a multiple line, parallel signal transfer bus having an inherent electrical capacitance on each line of said bus, said bus sequentially propagating data in parallel by maintaining on said bus lines for a predetermined period of time voltage levels representing the data and said interface apparatus comprising: a plurality of first bus drivers each corresponding to a different line of said bus and being capable of driving the corresponding bus line to either one of two different levels, and each of said first bus drivers including an input terminal for holding first input data to be transmitted to the corresponding line of said bus, a bus interface terminal coupled to the corresponding line of said bus, and an enable terminal for receiving a first drive signal which, in a first state, causes the first bus driver to drive the corresponding bus line to one of the levels according to the first input data and, in a second state, causes said first bus driver to cease driving the corresponding bus line and present a high electrical impedance at its bus interface terminal; a plurality of first latches each corresponding to a different line of said bus and each including an input terminal coupled to the corresponding line of said bus, the first latch presenting a high electrical impedance at its input terminal, and a latch control terminal for receiving a first control signal that causes the first latch to store data corresponding to the voltage level on the corresponding bus line when the first control signal is actuated; a plurality of second bus drivers each corresponding to a different line of said bus and being capable of driving the corresponding bus line to either one of two different voltage levels, and each of said second bus drivers including an input terminal for holding second input data to be transmitted to the corresponding line of said bus, a bus interface terminal coupled to the corresponding line of said bus, and an enable terminal for receiving a second drive signal which, in a first state, causes the second bus driver to drive the corresponding bus line to one of the levels according to the second input data and, in a second state, causes the second bus driver to cease driving the corresponding bus line and present a high electrical impedance at its bus interface terminal; a plurality of second latches each corresponding to a different line of said bus and each including an input terminal coupled to the corresponding line of said bus, the second latch presenting a high electrical impedance at its input terminal, and a latch control terminal for receiving a second control signal that causes the second latch to store data corresponding to the level on the corresponding bus line when the second control signal is actuated; and signal generating means, coupled to said plurality of first and second latches and to said plurality of first and second bus drivers, for generating the first and second drive and control signals to transfer the first input data at the input terminals of said first bus drivers, through said bus, to said first latches, and to transfer the second input data at the input terminals of said second bus drivers, through said bus, to said second latches, said signal generating means including means for switching the first drive signals from the first state into the second state at substantially the same time as the first control signals are actuated, for switching the second drive signals from the first state into the second state at substantially the same time as the second control signals are actuated, and for ensuring that the first and second drive signals are not simultaneously in the first state.

9. The interface apparatus of claim 8 wherein each of said plurality of first and second bus drivers is constructed of CMOS components.

10. The interface apparatus of claim 8 wherein each of said plurality of first and second latches is constructed from CMOS components.

11. The interface apparatus of claim 10 wherein each of said first and second latches includes a CMOS inverter.

12. The interface apparatus of claim 8 wherein the first and second control signals have a first and a second state and wherein the first and second control signals are actuated by changing from the first to the second state, and wherein said signal generating means includes first means for generating the first drive signals and the first control signals to change between the first and second states at substantially the same time and for generating the second drive signals and second control signals to change between the first and second states at substantially the same time.

13. Interface apparatus for coupling across integrated circuit chip boundaries by way of a multiple line, parallel signal transfer bus having an inherent electrical capacitance on each line of said bus, said bus sequentially progagating data in parallel by maintaining on said bus lines for a predetermined period of time voltage levels representing the data and said interface apparatus comprising a plurality of first CMOS bus drivers each corresponding to a different line of said bus and being capable of driving the corresponding bus line to either one of two different levels, and each of said first CMOS bus drivers including an input terminal for holding first data to be transmitted to the corresponding line of said bus, a bus interface terminal coupled to the corresponding line of said bus, and an enable terminal for receiving a first drive signal which, in a first state, causes the first bus driver to drive the corresponding bus line to one of the levels according to the first input data and, in a second state, causes the first bus driver to cease driving the corresponding bus line and present a high electrical impedance at its bus interface terminal; a plurality of first CMOS latches each corresponding to a different line of said bus and each including an input terminal coupled to the corresponding line of said bus, the first CMOS latch presenting a high electrical impedance at its input terminal, and a latch control terminal for receiving a first control signal that causes the first latch to store data corresponding to the voltage level on the corresponding bus line when the first control signal changes from a first level to a second level; a plurality of second CMOS bus drivers each corresponding to a different line of said bus and being capable of driving the corresponding bus line to either one of two different voltage levels, and each of said second CMOS bus drivers including an input terminal for holding second input data to be transmitted to the corresponding line of said bus, a bus interface terminal coupled to the corresponding line of said bus, and an enable terminal for receiving a second drive signal which, in a first state, causes the second bus driver to drive the corresponding bus line to one of the levels according to the second input data, and, in a second state, causes the second bus driver to cease driving the corresponding bus line and present a high electrical impedance at its bus interface terminal; a plurality of second CMOS latches each corresponding to a different line of said bus and each including an input terminal coupled to the corresponding line of said bus, the second CMOS latch presenting a high electrical impedance at its input terminal, and a latch control terminal for receiving a second control signal that causes the second latch to store data corresponding to the voltage level on the corresponding bus line when the second control signal changes from a first to a second level; and signal generating means, coupled to said plurality of first and second CMOS latches and to said plurality of first and second CMOS bus drivers, for generating the first and second drive and control signals to transfer the first input data at the input terminals of said first CMOS bus drivers, through said bus, to said first CMOS latches, and to transfer the second input data at the input terminals of said second CMOS bus drivers, to said bus to said second CMOS latches, said signal generating means including means for switching the first drive signals and the first control signals into the first and second states at substantially the same time, for switching the second drive signals and the second control signals into the first and second states at substantially the same time, and for ensuring that the first and second drive signals are not in the first state simultaneously.

14. A method for transmitting data across integrated circuit boundaries from an input terminal of a bus driver to a latch by way of a bus including lines having an inherent electrical capacitance, the method comprising the steps of: enabling said bus driver in order to drive said bus actively to one of two levels corresponding to the input data; disabling said bus driver; temporarily maintaining the level on said bus after said bus driver is disabled by keeping, at a high impedance, discharge paths of said bus capacitance through said disabled bus driver and through said latch; and actuating said latch to store said level on said bus substantially simultaneously with the disabling of said bus driver, whereby said latch correctly stores data corresponding to said level held on said bus after said bus driver is disabled.

15. The method of claim 14 wherein in said enabling and disabling steps, said bus driver receives a drive signal which in a first state enables said bus driver and in a second state disables said bus driver, and wherein said latch actuating step includes the substeps of generating a latch control signal that changes between a first and a second state at substantially the same time as said drive signal changes between said first and said second states, and actuating said latch when said latch control signal changes from the first to the second state.

16. A method for transmitting across integrated circuit boundaries first input data from a first input terminal to a first latch by way of a bus having an inherent electrical capacitance, and for transmitting second input data from a second input terminal to a second latch by said bus, the method comprising the steps of: enabling a first bus driver connected between said first input terminal and said bus in order to drive said bus actively to one of two levels corresponding to the first input data; disabling said first bus driver; temporarily maintaining the level on said bus after said first bus driver is disabled by keeping, at a high impedance, discharge paths of said bus capacitance through said disabled first bus driver and through said first latch; actuating said first latch to store said level on said bus substantially simultaneously with the disabling of said first bus driver, whereby said first latch correctly stores data corresponding to said level after said first bus driver disabled; enabling a second bus driver connected between said second input terminal and said bus in order to drive said bus actively to one of two levels corresponding to the second input data; disabling said second bus driver; temporarily maintaining said level on said bus after said second bus driver is disabled a by keeping, at a high impedance, discharge paths of said bus through said disabled second bus driver and through said second latch; and actuating said second latch to store said level on said bus substantially simultaneously with the disabling of said second bus driver, whereby said second latch correctly stores data corresponding to said level after said second bus drive is disabled.

17. The method of claim 16 wherein in the steps of enabling and disabling said first bus driver, said first bus driver receives a first drive signal which enables said first bus driver in a first state and disables said first bus driver in a second state, wherein said first latch actuating step includes the substeps of generating a two state first latch control signal that changes between a first and a second state at substantially the same time as said first drive signal changes between the first and second states, and activating said first latch when said first latch control signal changes from the first to the second state, wherein in the steps of enabling and disabling said second bus driver, said second bus driver receives a second drive signal which enables said second bus driver in a first state and disables said second bus driver in a second state, wherein said second latch actuating step includes the substeps of generating a two state second latch control signal that changes between a first and a second state at substantially the same time as the second drive signal changes between the first and second states, and activating said second latch when said second latch control signal changes from the first to the second state, and wherein said method further includes the step of generating the first drive signal so that it is not in the first state while the second drive signal is in the first state.

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