US 4,514,824 AGrant
Byte-Oriented Line Adapter System
Issue Date:1985-04-30
•9 Claims
•4 Drawing Sheets
Abstract
A line adapter system for handling byte-oriented data transfers between remote data terminals and I/O subsystem. Working in conjunction with a controlling microprocessor, the line adapter provides input output means for regulating the baud rate of transmission to/from a remote terminal and for the timing and protocols required for both synchronous and asynchronous data transmission. The microprocessor may address and control various of the timing elements and byte to bit transfer means of the input/output circuit means. Likewise, each of the elements of the input/output circuit means may request service from the microprocessor for further detailed instructions.
Metadata
Assignee
- Burroughs Corporation
Inventors
- Richard A. Loskorn
- Philip D. Biehl
- Robert D. Catiller
Application Information
Application Number:US 3551350
Filing Date:1982-03-05
Priority Date:1982-03-05
Art Unit:232
Classifications
IPC:
G06F 1300
Field of Search:
364455179200 MS File;900 MS File1;318 ES;84 R
Patent Drawings (4 sheets)
Description
Field of the Invention
This disclosure relates to the field of data communications and more specifically to the use of line adapters used to interface remote data terminals to an I/O subsystem.
Cross References to Related Applications
This disclosure is also related to an application entitled "Bit Oriented Line Adapter System", inventors Richard A. Loskorn, Philip D. Biehl and Robert D. Catiller, and filed Mar. 5, 1982, as U.S. Ser. No. 355,134.
Incorporated by reference are several patents which form a background and explanation for the use of the line adapters of this disclosure. These patents, which are included by reference, are:
U.S. Pat. No. 4,293,909 entitled "Digital System for Data Transfer Using Universal Input-Output Microprocessor".
U.S. Pat. No. 4,291,372 entitled "Microprocessor System with Specialized Instruction Format".
U.S. Pat. No. 4,292,667 entitled "Microprocessor System Facilitating Repetition of Instructions".
U.S. Pat. No. 4,189,769 entitled "Input-Output Subsystem for Digital Data Processing System".
Summary of the Invention
A line adapter is disclosed which is particularly adapted for interfacing a byte oriented protocol system which uses data bytes to remote terminals which operate on serial bit data. The line adapter may be configured in a single line adapter format or may be in a multiple line adapter format on one card. Working in conjunction with a state machine processor, the line adapter provides means for usage of synchronous mode transmission or asynchronous mode transmission in addition to adaptation for different byte-oriented protocols. Receiver-transmitter means and timer means in the line adapter may be specifically addressed in order to control baud rate of transmission, timing control, length of bits in a character and protocol factors. The receiver-transmitter and timer means may also request service from the state machine processor and receive necessary instructions. Data may be transferred from the state machine processor to the timer and receiver-transmitter for transmission to a remote terminal. Likewise, data from a remote terminal may be packaged into bytes by the receiver-transmitter and timer means and thence routed to a multiplexor means for buffering in a RAM storage means.
Brief Description of the Drawings
FIG. 1 is a schematic drawing of slide-in cards fitting into a base module backplane and having frontplane connectors therebetween.
FIG. 2 is a block diagram of a multiple line adapter card oriented to byte-protocols.
FIG. 3 is a block diagram of the state machine processor which controls operations of the line adapter.
FIG. 4 is a block diagram of a single byte-oriented line adapter.
FIG. 5 is a drawing of logic circuitry used to select memory for a given line adapter.
Description of Preferred Embodiment
The line adapter of the present disclosure is designed to be used as part of a Line Support Processor (also often called a frame recognition-data link processor) as part of a data comm I/O subsystem.
FIG. 1 shows such a data comm I/O subsystem wherein a state machine processor card 600 works in coordination with various types of line adapters. A single line adapter card 700 may be used as well as Quad line adapters such as that shown in cards 400 and 500. These Quad line adapters constitute a unit of four separately addressable line adapters, each line adapter of which may handle a single data communications line through an electrical interface.
FIG. 4 shows a block diagram of a "Single" byte oriented line adapter system. A remote data set or data terminal may be connected to input/output circuit means (single line adapter) which includes timer 507 and USART 508. This input/output circuit connects to a transceiver bus controller 503 which can route the data to a multiplexor means 504 for conveyance along I/O bus 10 to the state machine processor 600 or into a RAM buffer 550.sub.m. Data in the state machine processor 600 can be routed from the state machine's output control register 38, FIG. 3, along bus 17.sub.2 into the transceiver bus controller 503 for transmission to the input/output circuit means. The input/output means, such as 507 and 508, is commonly denoted as a "line adapter". The multiplexor 504 receives control signals from the data link interface unit 700 of FIG. 1 in addition to other control signals which identify units within the line adapter system. Also provided is an automatic calling unit output register 505 which can receive signals useful for dialing remote terminals on telephone lines.
FIG. 2 is a block diagram of a "Quad" Line Adapter used for byte oriented protocol operations. The input/output circuit means which connect to remote terminals is composed of four basic units such as 507, 508 (timer 0 and USART 0). Likewise, input/output service to a remote terminal is provided by timer 509 and USART 1 designated 510. Units 511, 512 constitute an operating unit for another remote terminal as do units 513 and 514 for yet another remote terminal. Each pair of these units, such as 509 and 510, as 511 and 512, as 513 and 514, each constitute a "line adapter".
As discussed under FIG. 4, each of the Line Adapters uses a transceiver bus controller 503 and a set of multiplexors 504 and 506. It should be noted that the multiplexors such as 504 and 506 are in essence "dual operating multiplexor sets" in that they receive control signals from two different input/output units. For example, MUX 504 involves a set of eight multiplexor units (A, A.sub.0) in order to handle line adapter 0 (507 and 508); and a set of eight multiplexor units (LA.sub.1) to handle the line adapter 1 (509 and 510).
Corresponding to the Automatic Calling Unit Output Register 505 of FIG. 4, the Quad Line Adapter of FIG. 2 uses 4 such ACU output registers (505.sub.0, .sub.1, .sub.2, .sub.3). Also in the Quad Line Adapter there is provided an extra set of RAM buffer memories designated 550.sub.m1 and 550.sub.m2, having inputs from the I/O bus and outputs on the MEMOUT bus 12.
FIG. 3 is a block diagram of the State Machine Microprocessor which is used to control the single line adapter or multiple configurations of line adapters. The state machine processor (sometimes designated as UIO State Machine) resides on a circuit board of chips which can be inserted as a slide-in card into the base module (FIG. 1) where it connects to the backplane. The State Machine connects to the application dependent logic through the frontplane connectors as seen in FIG. 1.
A detailed description of the elements and use of the UIO State Machine has been the subject of several prior patents which are included herein by reference. These patents are:
U.S. Pat. No. 4,293,909 entitled "Digital System For Data Transfer Using Universal Input-Output Microprocessor", inventors Robert D. Catiller and Brian K. Forbes.
U.S. Pat. No. 4,291,372 entitled "Microprocessor System With Specialized Instruction Format", inventors Brian K. Forbes and Robert D. Catiller.
U.S. Pat. No. 4,292,667 entitled "Microprocessor System Facilitating Repetition of Instructions", inventors Robert D. Catiller and Brian K. Forbes.
The use of a host computer working in conjunction with an I/O subsystem which uses peculiar commands called I/O descriptors, data link descriptors, and result descriptors is shown in U.S. Pat. No. 4,189,769, Feb. 19, 1980, to Darwen J. Cook and Donald A. Millers, II, and entitled "Input-Output Subsystem for Digital Data Processing System" and this patent is also included herein by reference.
FIG. 5 is a diagram of certain logic on the DLI/LA card 700 which is used to select or "Designate" the RAM buffer memory of the single Line Adapter card or a selected memory of a specific one of the four line adapter memories on the Quad line adapter card. Shown as RAM storage means 550.sub.m in FIG. 5 is the particular memory used for the single Line Adapter. However, in the "multiple" Line Adapter situation, each line adapter has a similar selection system for selecting the particular memory associated with that line adapter.
In FIG. 5, address lines from the State Machine Processor (MADDRnn) connect to a Comparator 100.sub.c and also to the RAM buffer 550.sub.m. A chip select signal CS/ is activated to the buffer memory 550.sub.m by means of logic signals from the Comparator 100.sub.c and the Designate Flip-Flop (DESF). A unique jumper bit provides input to the Designate Flip-Flop from the I/O bus 10 in order to particularly identify any given selected buffer memory in the system. The particular bit line to be used is "set" by the State Machine Microprocessor, 600, using one of the bit-lines of I/O bus 10.
"Byte Oriented" Line Adapter
A functional section of the Line Support Processor (also called the Frame Recognition-Data Link Processor and/or Line Support Processor-DLP) is the Line Adapter called the "Byte Oriented Line Adapter". This is sometimes also called a "Character Oriented" Line Adapter.
The data comm Line Adapter is basically a device which interfaces on one end to a data communication line "electrical interface", and on the other end interfaces to a processor which has been designated as the UIO State Machine 600 (UIOSM). The primary function of the Line Adapter is to serialize "bit" information to/from "byte" information, to provide timing, to generate service requests, to supply a RAM memory, to provide automatic calling interfacing and to provide connection to level changers which will match the data communication lines. The Byte-Oriented Line Adapter is also built in two basic configurations designated as (i) Quad Line Adapter and (ii) the Single Line Adapter. The Single Line Adapter is part of the Line Support Processor and the Single Line Adapter shares the same board with the Data Link Interface (DLI) circuitry. The Line Adapter is required regardless of the quantity of lines controlled by the Line Support Processor. The Quad Line Adapter contains essentially four (4) Line Adapters on one board. These boards are typical 10 inch by 13 inch boards which plug into the backplane of the Base Connection Module. Also a smaller version of the Line Adapter may be configured as a "Dual" Line Adapter which consists of two Line Adapters on one board rather than four Line Adapters.
As seen in FIG. 1, each of the Line Adapter cards 400,500 connect both to the State Machine Processor 600 and to the DLI/LA 700 (Data Link Interface-Single Line Adapter).
As seen in FIGS. 2 and 4, connection to the data communications line is through an electrical interface (EI) which connects to the Line Adapter. There are various types of electrical interface boards which exist and which may be mounted in different combinations on the Quad Line Adapters. Thus, depending on the electrical characteristics of the data comm line, the only change required is that of the electrical interface, while the Line Adapter remains as is.
From one to eight Line Adapters may variously be addressed by the State Machine Processor 600; thus, each Line Adapter is jumpered uniquely (as seen in FIG. 5) in order to identify its address. The Line Adapter must be "designated" for the State Machine Processor to communicate with it. Several addressable components are contained on a Line Adapter which the State Machine Processor may communicate with, in the form of Write/Read data or "Status" or "Control" signals.
The addressable components of the Byte Oriented Line Adapter are:
(i) USART (508, 510, 512, 514, FIG. 2)
(ii) Timer (507, 509, 511, 513, FIG. 2)
(iii) Auto Call Output
(iv) Auto Call Status (ACUST)
(v) Component Requestors (CRID)
(vi) Memory (RAM)
The USART (Universal Synchronous/Asynchronous Receiver/Transmitter) accepts data "bytes" from the State Machine Processor 600 and converts them into serial "bits" for transmission; it also receives serial bit data and converts this to parallel data bytes. The USART device is initialized by "writing" into its two internal control registers which then specify the manner in which it operates.
A typical USART preferred for this purpose is manufactured by Western Digital Corporation, 3128 Redhill Avenue, Newport Beach, Calif. 92663, and is designated as UC1671 and described in a Technical Manual dated August 1978 as UC1671 Asynchronous/Synchronous Receiver/Transmitter.
Various bits of the internal control registers of this USART unit specify such things as: synchronous/asynchronous mode; bits per character; parity; baud rate; transparent mode; Echo mode. The Timer used on the Byte Oriented Line Adapter serves two basic functions: (i) as program timers and (ii) as baud rate generators for asynchronous operation. Three independent internal timers are contained in each chip, two of which are used by the software for timing purposes relative to the line operations for "transmit" and for "receive" operation. The third timer is used to generate a square wave clock which is used by the USART for asynchronous operation. Each timer is initialized independently, which indicates the "mode" in which it is to operate. The two program timers are capable of activating a Flag signal to the State Machine Processor 600 when a pre-determined timing value has been reached.
The Auto Call Output Register (ACUOR 505) is a register which is loaded by the State Machine Processor with "dial digit" and control information. The output of this register drives level-changer chips which convert the logic signals to EIA RS-232 voltages. These signals drive an automatic calling unit (ACU) such as a Bell 801, which provides dial-out capabilities.
Auto Call Status is a means of providing the condition or state of input lines from the automatic calling unit (ACU) to the State Machine Processor 600. Lines from the ACU are received by level-changer chips which convert the EIA voltages to TTL logic levels. These logic levels may be read by the State Machine Processor to determine the present status.
The Component Requestors from a Line Adapter are as follows: (i) USART; (ii) Program Timer 1; (iii) Program Timer 2.
These three components are capable of generating "service requests" independently of each other at unique times relative to initialization. The "service requests" activate a flag signal to the State Machine Processor which indicates that Line Adapters require servicing. After the State Machine determines which Line Adapters are requesting service, it must then determine which "component" on a particular Line Adapter is requesting service.
Memory on the Line Adapter consists of 2,048.times.17-bit words of RAM for each line. Therefore, each Quad Line Adapter card actually contains 3,192.times.17-bit words of RAM. The Single Line Adapter card (FIGS. 9, 10) contains 4,096 words of RAM 550.sub.m, one half for the data comm line and the remainder for DLI. The RAM is used by the software for transmit/receive message buffering, for tables and for statements associated with the line operation.
Byte Oriented Line Adapter--Operation
Designate: When the State Machine Processor 600 executes code relative to an addressable component on a Line Adapter, (LA), the LA must be "designated". Each Line Adapter contains a Flip-Flop, DESF, FIG. 5, whose input is jumpered to a specific bit of the I/O bus. In order to "designate" a Line Adapter, the State Machine Processor must execute a PUT OP with Strobe No. 1 (D-FLDOOO) and the corresponding bit of the I/O bus must be equal to 1. Executing the same OP with the I/O bus bit equal to 0 will reset the Designate Flip-Flop shown typically as "DESF" on FIG. 5.
Flag Operation: The various components of a Line Adapter are capable of producing "service requests". These "service requests" are basically ORed together in order to drive a common FLAG line for all Line Adapters. A signal line, FLAG 2/, when being low active, notifies the State Machine Processor that some Line Adapters are requesting service. The State Machine Processor can determine which Line Adapters are requesting service by executing a GET OP with the variant field V-FLD (4:5) equal to 00001. The Line Adapter does not need to be "designated" for execution of this OP.
"Register address" (REGADRn) signals in the Line Adapters are five "V-FLD" signals from the State Machine Processor. The two least significant signals of the V-FLD are shown in Table Y-1.
Data Bus Structures: With the exception of the RAM (FIG. 5), memory 550.sub.m1, .sub.m2, all data sent to addressable components on the Line Adapter originate from the "Second Output Control Register 38 (FIG. 3) in the State Machine Processor. With the exception of RAM, all data "read" by the State Machine Processor from addressable components on the Line Adapter will go to the State Machine Processor via the I/O bus 10.
With reference to FIG. 4 (DLI/LA data bus structure), the Single Line Adapter data bus structure is shown.
As seen in FIG. 4, the Second Output Control Register 38 (FIG. 3) lines (OCREG 20n) connect directly to the inputs of the Auto Call Unit Output Register 505 (ACUOR); and they also connect directly to the Transceiver Bus Controller chip 503 which provides bidirectional bus drivers.
The Auto Call Unit Output Register 505 is a six bit "D" type flip-flop register (DR6n). When the clock input is enabled, data from the Second Output Register 38 will be strobed into ACUOR 505.
Data sent to both the Timer 507 and to the USART 508, FIG. 4, originate from the Second Output Register 38 in the State Machine Processor (FIG. 3) and is sent through the Transceiver bus controller 503; then is sent to the addressed component. The data lines for the Timer component are HI active and for the USART component they are LO active. Being as both components share the same data bus, data to one of the components must be inverted. The Timer 507 is used to receive the "inverted" data, that is, 1=0 and 0=1, while the USART 508 receives the conventional format. Thus, a "one" bit from the Second Output Register 38 in the State Machine Processor (FIG. 3) will appear as a "one" bit to the USART (active low) and as a "zero" bit to the Timer. The Transceiver bus controller 503, although being a three-state device, is not used in its third or high impedance state. It is used for driving either DIN (data in) to DOUT (data out) or DOUT to ROUT (FIG. 4) depending on the state of the RE (Read Enable) signal which originates from bit 4 of the First Output Control Register 37 in the State Machine Processor. When bit 4 of Register 37 is ON, the signal RE is positive and "enables" the DIN to DOUT direction through the Transceiver bus controller 503.
Reading of information (except RAM read) from a Line Adapter is performed by decoded GET OPs, and the read information is available on the least significant 8 bits of the I/O bus 10. The 8-1 multiplexor 504 is the source of the read information.
On the "Single" Line Adapter (FIG. 4) four of the eight inputs to MUX 504 are used by the Line Adapter and the remainder are used by the Data Link Interface (DLI). The multiplexors (MUX's) are chip selected (low level) during a GET OP when the V-FLD (3:2) is equal to "11" and either V-FLD (4:1) equals 0 (DLI GET) or the Designate Flip-Flop (DESF) is "ON" (LA GET).
On the "Quad" Line Adapter cards there are 16 multiplexors, each having an eight-one ratio. There are 8 multiplexors for each pair of Line Adapters.
As seen in FIG. 4, the eight input lines to MUX 504 are divided in half such that four lines connect to the DLI (Data Link Interface), and four lines connect to the Line Adapter. Similarly in FIG. 2, in the Quad Line Adapter, the eight input lines of each group of eight multiplexors is divided in half, similar to the Single Line Adapter, thereby making four groups. Any "group" of "four" input lines is selected by its "Designate Flip-Flop" (DESF, FIG. 5) being ON. The selection of any one of four lines of any such group is performed by the two least significant bits of the V-FLD of a GET OP, (Table Y-1).
Data "to be "written into" RAM memory in a given Line adaptor" is sent via the I/O bus in 16 bits plus parity format. The data "read from" RAM memory in a Line Adapter is placed on the MEMOUT bus 12 with 16 bits plus parity.
Component Addressing: As seen in FIG. 4, the outputs of components to be "read" are routed to the inputs of the 8-1 multiplexor which then drive the I/O bus 10. There are five components on a Line Adapter which may be "read" by the State Machine Processor, these are:
Component Requestor ID (CRID)
USART (508)
Timer (507)
Automatic Calling Unit Status (ACUST)
Adapter Type ID (ADPT.ID)
Although these five components on a Line Adapter may be read, the USART 508 and the Timer 507 share the same input line (ROUT) to the multiplexors. Selection of one of the four inputs in either group (of inputs to the 8-1 multiplexors) is performed by the two least significant bits of the V-FLD of the GET OP. V-FLD (3:4) equals 11XX and selection of one of the four inputs is determined as shown in Table Y-1.
In FIG. 4 the Single Line Adapter multiplexor 504 allows three components on a Line Adapter to be written into (not including RAM). These are: Automatic Calling Unit Output Register 505 (ACUOR), the USART 508 and the Timer 507. The addressing of these three components occurs in two distinct fashions: (a) decode of the V-FLD of PUT Ops and (b) the decode of bits from the First Output Control Register 37 in the State Machine Processor (FIG. 3).
The ACUOR 505 is addressed when a one-of-eight decoder chip decodes the PUT OP V-FLD (4:5) equal 01111 and the Strobe No. 2 is sent from the State Machine Processor. This decoding is performed only on the Single Line Adapter card and is sent to other Line Adapter cards via the frontplane connector. This decoded signal is received by a three input NOR gate (not shown) in each Line Adapter (whose other inputs are Clock and Designate FF). The output of this gate drives the clock input of the six bit ACU output register.
Data from the Second Output Control Register 38 (FIG. 3) will then be strobed into the ACUOR 505.
Addressing a USART or Timer on a designated Line Adapter is the same as "chip selecting" the component. This is accomplished with bits 0 and 1 of the First Output Control Register 37 in the State Machine Processor along with the Designate Flip-Flop (DESF) in a Line Adapter.
Each Line Adapter will "AND" its Designate FF with bits 0 and 1 from output control register 37 in order to provide a UCS (USART Chip Select) or a TCS (Timer Chip Select) for its USART or Timer.
The use of bits 0 and 1 in the First Output Control Register 37 is as follows:
The remaining bits of Register 37 are used for control signals, primarily for the USART and Timer.
Random Access Memory (550m, FIG. 5): Each data comm line has 2,048 words of RAM available for its use. A word is equal to 16 data bits plus one parity bit. In FIG. 5 the RAM chip 550.sub.m is a 4,096.times.1 bit static RAM with a Read Access time of 180 nanoseconds and is arranged with 17 chips making 4,096 words. On the DLI/LA card 2,048 words are for the "Single" Line Adapter and 2,048 words are for the Data Link Interface. The "Quad" Line Adapter card provides 34 memory chips or 8,192 words of which 2,048 words are available for each line.
The data comm Line Adapter memory (for any line) is "pointed at" by the memory address lines, MADDR (15:5) equal 01110. This can be seen in FIG. 5 which shows the Data Link Interface/Line Adapter RAM. A five bit Comparator 100.sub.c on the DLI/LA card compares (for an "equal" condition) for (i) DLI memory selection; or, for a "greater than" condition, (MADDRnn 01110) provides for (ii) Line Adapter RAM selection. The signal "LARAMSEL" (Line Adapter RAM Select) will go to all Line Adapter cards via the frontplane cable to select the "designated" Line Adapter RAM memory. If memory address lines MADDR (15:5) equals 0111x (DLI or LA Select) then a slow memory flip-flop (SLMF) 100.sub.sf will be set equal to 1. The Flip-Flop 100.sub.sf output drives an open collector NAND gate whose output connects to the WAIT/ frontplane signal line to the State Machine Processor. This signal (WAIT/), when low, will force the State Machine Processor to "wait" until the signal goes "high". Using a RAM chip whose Read Access time is 180 nanoseconds requires the State Machine Processor to wait for one clock time, thereby when the DLI memory (550.sub.m, FIG. 5) or any Line Adapter memory is selected, the SLMF (slow memory flip-flop) will be "on" for one clock and then toggle off.
Selection of the RAM memory 550.sub.m on the DLI/LA card is done via MADDR (15:5) equal to 01110 or else MADDR (15:5) equal 01111 and the Designate Flip-Flop being ON. This logic controls the chip-select input on the RAM chips. The division of RAM for DLI or for Line Adapter memory is handled by controlling the "A-11" (FIG. 5) address pin on the RAM 550.sub.m chip. If MADDR (15:5) is equal to 01111 and the Line Adapter Designate Flip-Flop (DESF) is ON, the RAM is chip selected and the A-11 address input is TRUE.
The "Quad" Line Adapter card (FIG. 2) contains two groups of memory chips (550.sub.m1, 550.sub.m2) wherein Data-Comm (D.C.) lines 0 and 1 on the card share the same group of RAM chips and Data-Comm lines 2 and 3 share the other group of RAM chips. The signal LARAMSEL (Line Adapter RAM Select, FIG. 5) goes to all Line Adapters and is then essentially ANDED with appropriate Designate conditions to allow the desired RAM group to be chip selected. A "division" of RAM for the first or second data-comm line on a Quad Line Adapter is handled by controlling the "A-11" address pin (FIG. 5) on the RAM chip (signal DESn where n=1) and for the third and fourth line the "A-11" pin on the second group of RAM chips is controlled by DESn (FIG. 5) where n=3.
A "Dual" Line Adapter will only contain one group of memory chips (17) and will operate the same as line 0 and line 1 on the Quad Line Adapter. Data to be written into RAM must be placed on the I/O bus 10 by the State Machine Processor and "read data" will be sent to the State Machine Processor on the MEMOUTnn bus 12 (nn equals 00.fwdarw.16).
Clear: There are two methods of clearing used to clear the Line Adapters; these are "Power Up" Clear and "Designate" Clear.
The Power Up Clear is a signal which occurs during the power-up sequence for the cabinet housing the Line Adapters. The signal comes from the backplane of the Base Module cabinet and is active Low.
The Designate Clear is a function controlled by the State Machine Processor, and only the Line Adapters which are designated are the ones that get cleared. The Clear signal originates from bit 7 of the First Output Control Register 37 of the State Machine Processor (FIG. 3). The "Power-up" Clear operates to clear three components on the Line Adapter. These are: the Designate Flip-Flop; the Auto Call Output Register; and the USART.
The "Designate" Clear signal clears two components on the Line Adapter. These are: Auto Call Unit Output Register (ACUOR); and the USART.
USART Organization and Operation: The USART is a MOS/LSI device housed in a 40-pin Dual-in-line package and is TTL compatible on all inputs and outputs. The USART performs the functions of interfacing a "serial" data communications channel to a parallel digital system and is capable of full duplex communications with synchronous or asynchronous systems.
One preferred embodiment of the USART is that manufactured by Western Digital Corporation, 3128 Redhill Avenue, Newport Beach, Calif. 92663 and is designated as Model UC1671 Asynchronous/Synchronous Receiver/Transmitter and described in their Technical Data Publication of August 1978 which includes a block diagram showing the various registers, controls and components which are briefly described herein below.
(i) Receiver Register (RR): this is an eight bit shift register which inputs the received data at a clock rate determined by an internal control register. The incoming data is assembled to the selected character in length and then transferred to the Receiver Holding Register with logic zeroes filling out any unused high-order bit positions. At this time the INTR (Interrupt) output is made active for informing the State Machine of the LSP that the Receiver Holding Register contains valid data.
(ii) Receiver Holding Register (RHR): this is an 8-bit parallel buffer register which presents assembled receiver characters to the DAL (Data Access Line) bus lines (FIG. 2) when requested through a Read operation.
(iii) Comparator: the 8-bit comparator is used in the Synchronous Mode to compare the assembled contents of the Receiver Register and the SYN register or the DLE register. A "match" between the registers sets up the stripping of the received character (when programmed) by preventing the data from being loaded into the Receiver Holding Register. A bit in an internal Status Register is set when stripping is performed. The comparator output also enables character synchronization of the Receiver on two successive matches with the SYN register.
(iv) SYN Register: this is an 8-bit register which is loaded from the DAL (Data Access Line) lines (FIG. 2) by a Write operation and it holds the synchronization code used to establish receiver character synchronization. It serves as a fill character when no new data is available in the Transmitter Holding Register during transmission. This register cannot be read onto the DAL lines. It must be loaded with logic zeroes in all unused high-order bits.
(v) DLE Register: this is an eight bit register which is loaded from the DAL lines by a Write operation and holds the "DLE" (Delimiter) character used in the Transparent Mode of operation, in which an idle transmit period is filled with the combination DLE/SYN pair of characters rather than a single SYN character. In addition, the USART may be programmed to force a single DLE character prior to any data character transmission while in the "transmitter transparent mode".
(vi) Transmitter Holding Register (THR): this is an eight-bit parallel buffer register which holds parallel transmitted data transferred from the DAL lines by a Write operation. This data is transferred to the Transmitter Register (TR) when the transmitter section is enabled and the Transmitter Register is ready to send new data. During this transfer, the signal interrupt (INTR) is made active for informing the Line Support Processor that the Transmitter Holding Register is empty.
(vii) Transmitter Register: this is an eight-bit shift register which is loaded from the THR (Transmitter Holding Register), the SYN register, or the DLE register. The purpose of this register is to serialize data and present it to the transmitted Data Output lines.
(viii) Control Register: there are two eight-bit control registers in the USART which hold device programming signals such as: mode selection, clock selection, interface signal control, and data format. Each of the control registers can be loaded from the data access lines (DAL) by a Write operation, or else read into the DAL lines by a Read operation.
Operation of USART of Byte Oriented Line Adapter:
ASYNCHRONOUS MODE: The framing of asynchronous characters is provided by a Start Bit (logic low) at the beginning of a character, and by one or more Stop Bits (logic high) at the "end" of a character. Reception of a character is initiated on recognition of the first Start Bit by a positive transition of the receiver clock, right after a proceeding Stop Bit. The Start and Stop bits are "stripped off" while assembling the serial bit input into a parallel character.
The character assembly is completed by the reception of the Stop Bit after the reception of the last character bit. If this bit is a logic "high", the character is determined to have "correct" framing and the USART is prepared to receive the next character. If the Stop Bit is logic "low", the Framing Error Status flag is set and the Receiver assumes this bit to be the Start Bit of the next character. Character assembly continues from this point if the input is still a logic "low" when sampled at the theoretical center of the assumed Start Bit. As long as the Receiver input is "spacing" (i.e. receiving a space rather than a Mark), then all zero characters are assembled, and error flags and data received interrupts are generated so that line breaks can be determined. After a character of all zeroes is assembled along with a zero in the Stop Bit location, the first-received logic "high" is determined as a Stop Bit and this resets the receiver circuit to a "Ready" state for assembly of the next character.
In the Asynchronous Mode the character transmission occurs when information contained in the THR (Transmitter Holding Register) is transferred to the TR (Transmitter Register). Transmission is initiated by the insertion of a Start Bit, followed by the serial output of the character (least significant bit first) with parity, if enabled, following the most significant bit; then there is the insertion of the 1-, 1.5-, or 2-bit length Stop condition. If the THR (Transmitter Holding Register) is full, the next character transmission starts after the transmission of the Stop Bit of the present character in the TR (transmitter register). Otherwise, the "Mark" (logic high) condition is continually transmitted until the THR (Transmitter Holding Register) is loaded.
SYNCHRONOUS MODE: The synchronization of messages is carried out by a special Synchronization Character Code (SYN) transmitted at the beginning of a block of characters. The Receiver, when enabled, searches for two contiguous characters matching the bit pattern contained in the SYN register. During the time that the Receiver is searching, data is not transferred to the THR (Transmitter Holding Register) and status bits are not updated; and the Receiver interrupt is not activated. After the detection of the first SYN character, the Receiver assembles subsequent bits into characters whose length is determined by the contents of the USART internal control register. If, after the first SYN character detection, a second SYN character is present, the Receiver enters the Synchronization Mode until the Receiver Enable Bit is turned "off". If a second successive SYN character is not found, then the Receiver reverts back to the Search Mode.
In the Synchronous Mode, a continuous stream of characters are transmitted once the Transmitter is enabled. If the THR (Transmitter Holding Register) is not loaded at the time the Transmitter Register has completed the transmission of a character, this "idle" time will be filled by a transmission of a character contained in the SYN register in the Non-Transparent Mode, or filled by the characters contained in the DLE and the SYN registers respectively (while in the Transparent Mode of operation).
RECEIVER OPERATION: The Receiver data input is clocked into the Receiver Register by a 1X Receiver clock from a modem Data Set, or by a local 32X bit rate clock (asynchronous) selected from one of four timer chips. When using the 1X Receiver Clock, the Receiver data is sampled on the positive transition of the clock in the Synchronous Modes. When using a 32X clock in the Asynchronous Mode, the Receive Sampling Clock is phased to the "Mark-To-Space" transition of the Received Data Start Bit and defines (through clock counts) the center of each received Data Bit at the positive transition 16 clock periods later. When the complete character has been shifted into the Receiver Register, it is transferred to the RHR (Receiver Holding Register); the unused, higher number bits are filled with zeros. At this time the "Receiver Status bits" (Framing Error/Sync Detect, Parity Error/DLE Detect, Overrun Error, and Data Received) are updated in the Status Register and the Data Received "interrupt" is activated. Parity Error is set if encountered while the Receiver Parity Check is "enabled" in the internal control register. Overrun Error is set if the Data Received Status Bit is not cleared through a Read Operation by an external device when a new character is ready to be transferred to the RHR (Received Holding Register). This error flag indicates that a character has been lost, that is new data is lost, and the old data and its status flags are saved.
The characters assembled in the Receiver Register that match the contents of the SYN or the DLE register are not loaded into the RHR (Receiver Holding Register), and the DR (Data Received) interrupt is not generated if bit 3 of USART control register 2 (CR23=SYN Strip) or Bit 4 of USART control register 1 (CR14=DLE Strip) are set respectively. The SYN-DET and the DLE-DET status bits are set with the next non-SYN or DLE character. When both control register bits CR23 and CR14 are set (Transparent Mode), the DLE-SYN combination is stripped. The SYN comparison occurs only with the character received after the DLE character. If two successive DLE characters are received, only the first DLE character is stripped. No parity check is made in this mode.
TRANSMITTER OPERATIONS: Information is transferred to the THR (Transmitter Holding Register) by a Write operation. Information can be loaded into this THR at any time, even when the Transmitter is not enabled. Transmission of data is initiated only when the Request-to-Send Bit is set to a logic "one" in the USART control register and the Clear-to-Send input is at a logic "low". Information is normally transferred from a THR to the Transmitter Register when the latter has completed transmission of a character. However, information in the DLE register may be transferred prior to the information contained in the THR if the Force-DLE signal condition is enabled (CR15=Force, DLE and CR16=TX Transparent and set to a logic "one"). The control bit CR15 must be "set" prior to loading of a new character in the THR to insure forcing the DLE character prior to transmission of the data character. The Transmitter Register output passes through a Flip-Flop which delays the output by one clock period. When using the 1X clock generated by the modem Data Set, the output data changes state on the negative clock transition and the delay is one bit period.
When the Transmitter is enabled, a Transmitter "interrupt" is generated each time the THR is empty. If the THR is empty when the Transmitter Register is ready for a new character, the Transmitter enters an "idle" state. During this idle time, a logic "high" will be presented to the Transmitted Data Output in the Asynchronous Mode or the contents of the SYN register will be presented in the Synchronous Non-Transparent Mode (CR16=0). In the Synchronous Transmit Transparent Mode (enabled by bit-6 of USART control register 1=Logic 1), the idle state will be filled by a DLE-SYN character transmission in that order. When entering the Transparent Mode, the DLE-SYN fill-in will not occur until the first forced DLE.
If the Transmitter section is disabled by a reset of the Request-to-Send signal (RTS), any partially transmitted character is completed before the Transmitter section of the USART is disabled. As soon as the CTS signal (Clear-to-Send) goes high, the transmitted data output will go high.
When the Transmit Parity is enabled, the selected Odd or Even parity bit is inserted into the last bit of the character in place of the last bit of the Transmitter Register. This limits transfer of character information to a maximum of 7-bits plus parity or 8-bits without parity. Parity cannot be enabled in the Synchronous Transparency Mode.
INPUT/OUTPUT OPERATION OF USART: All Data, Control and Status words are transferred over the Data Access Lines (DALO-7) as seen in FIG. 2, DAL. Additional input lines provide controls for addressing a particular unit and regulating all input and output operations. Other lines provide interrupt capability to indicate to a controller that an input operation is requested by the USART. All input/output terminology is referenced to the Bus Controller-Transceiver 503, FIG. 2, so that a "Read" or Input takes data from the USART and places it on the DAL lines to the Transceiver 503, while a "Write" or an Output places data from the Transceiver 503 onto the DAL lines and into the USART. The following input/output terminology discussed below is referenced to the Bus Controller-Transceiver 503.
(i) READ: A Read operation is initiated by the placement of an 8-bit address from State Machine 600 on the DAL (FIG. 5) by the Bus Controller 503, FIG. 2. When the Chip Select signal goes to a logic "low" state, the USART (as 508) compares bits 7-3 of the DAL with its hard-wired ID code (on Pins 17, 22, 24, 25, 26) and becomes selected on a "Match" condition. The USART then sets its RPLY line "low" to acknowledge its readiness to transfer data. Bits 2-0 of the address are used to select the USART registers to "read from" as follows:
When the Read Enable (RE) line is set to a logic "low" condition by the State Machine 600, the USART gates the contents of the address register onto the DAL bus. The Read operation terminates, and the devices become unselected, and both the Chip Select and Read Enable return to the logic "high" condition. Reading of the Receiver Holding Register clears the DR status bit. Bit zero must be a logic "low" in Read or in Write operations.
(ii) WRITE: A Write operation is initiated by making a Chip Select input go to the logic "low" state. Bits 2-0 of the address are used to select USART registers which are written into as follows:
When the Write Enable (WE) line is set to a logic "low" condition by the State Machine, the USART gates the data from Transceiver 503 onto the DAL bus and into the addressed register. If data is written into the Transmitter Holding Register (THR), the THRE (THR empty) Status Bit is cleared to a logic zero.
The "100" address loads both the SYN and DLE registers. After writing into the SYN register, the device is conditioned to write into the DLE register if followed by another Write pulse which has the "100" address. Any intervening Read or Write operation with another address resets this condition such that the next "100" will address the SYN register.
(iii) INTERRUPTS: The following conditions will generate interrupts:
1. Data Received (DR)--indicates transfer of a new character to the Receiver Holding Register (RHR) while the Receiver is enabled.
2. Transmitter Holding Register Empty (THRE)--indicates that the THR register is empty while the Transmitter is enabled. The first interrupt occurs when the Transmitter becomes enabled if there is an "empty" THR, or after the character is transferred to the Transmitter Register, thus making the THR empty.
3. Carrier On--this indicates the Carrier Detector input has gone "low" when DTR is "on". (DTR=Data Terminal Ready).
4. Carrier Off--indicates that the Carrier Detector input has gone "high" when DTR is "on".
5. DSR On--indicates the Data Set Ready input has gone "low" when DTR is "on".
6. DSR Off--indicates the Data Set Ready input has gone "high" when DTR is "on".
7. Ring On--indicates the Ring Indicator input has gone "low" when DTR is off.
Each time an Interrupt Condition exists, the INTR output from the USART is made a logic "low". The State Machine then acknowledges the Interrupt Request by setting the CS (Chip Select) and Interrupt Acknowledge Input (IACK) to the USART to a "low" state, otherwise the Interrupt Condition (INTR) would never get reset.
Auto Call Operation: (For Operations Using an 801 Auto Call Unit)
The 801 ACU has a 4-bit interface for receiving digits of the call number to be dialed. This interface is defined by the EIA Standard RS-366 and involves the following signals:
The dialing sequence operates as follows:
The Line Adapter turns CRQ "on" provided that the DLO is "off". After detection of the dial tone, which is done by the 801, the digits are transferred one at a time to the 801. The 801 converts the digits to signals which duplicate the function of a rotating dial-pulse or a touch-tone frequency compatible signal. These signals are transmitted to the phone line. At call completion, DSS comes "on" to signify receipt of answer tone from the called Data Set. Receipt of DSS allows the line to be transferred to the ACU associated data set. If DSS fails to come "on", the Abandon Call and Retry (ACR) timer begins timing out.
With pulse dialing, a typical 10 digit number takes 15 seconds to dial; for touch-tone dialing the same number requires approximately one second. The answer sequence begins sometime after the last digit has been sent by the 801.
Interface Operation (Data-Comm Line-Adapter/State Machine):
The UIO Data-Communication Line-Adapter is an application dependent device which is controlled by the UIO State Machine Processor 600. The basic types of Line Adapters are available--these are the "Character Oriented" Line Adapter and the "Bit Oriented" Line Adapter, each of which may have a variety of electrical interfaces to the data communication lines.
One to eight Line Adapters may be serviced by one State Machine Processor on an individual basis. Each Line Adapter contains components which are addressable and are serviced by the State Machine Processor with PUT or GET instructions. The components on the Line Adapter are serviced with one or a series of instructions which, in some cases, provide sequential control of the component. The "communication" between the State Machine Processor and the Line Adapter can be separated into two basic groups:
(i) Undesignated
(ii) Designated
The "Undesignated" operations do not require the Line Adapter to be designated to execute those instructions. "Designated" type operations require the Line Adapter to be designated or identified to execute those instructions or series of instructions.
The following operations (except for ACUOR), in addition to requiring the Line Adapter to be "Designated", will use the First Control Register 37 in the State Machine Processor 600 for control purposes to components on a Line Adapter. With the exception of the Clear OP, all other operations will be a series of PUT/GET OPs to provide the necessary sequential control.
"Data" outputted to the Line Adapter for these operations will originate from the Second Output Control Register 38 of the State Machine in FIG. 3.
The bits of the First Output Control Register 37 of the State Machine (FIG. 6) are organized for control functions as follows:
USART Interfacing: Three procedures are used when communicating with a USART on a Line Adapter; these are:
(i) Interrupt Acknowledge Procedure
(ii) Read Procedure
(iii) Write Procedure
Timer/Baud Rate Generator Interfacing:
Two basic procedures are used when communicating with the timer components; these are: (i) Write Procedure and (ii) Read Procedure.
Five control signals originating from unique bits of the First Output Control Register 37 are used for the Timer. These are:
TCS--Timer Chip Select
A0, A1--Register Addressing Lines
WE--Write Enable
RE--Read Enable
Baud Rate Generator: The "character" oriented UIO Data Comm Line Adapters will use a USART which requires an input clock that is 32 times faster than the bit-time of the Asynchronous line it is communicating with. To obtain this X32 clock it is preferred to use an Intel 8253 programmable timer chip, whose squarewave output is connected to the USART. This timer is driven by a crystal controlled clock whose frequency is 1.2288 megahertz. After initializing the Timer, a divisor value must be loaded which will produce the necessary X32 clock.
Claims
Certificate of Correction present
The claims shown below may not include correction changes. Use the corrected PDF for authoritative text.
What is claimed is:
1. In a line support processor for controlling data transfer operations between a host computer and data terminal equipment connected via data communication lines, wherein said line support processor includes a plurality of line adapters, a data link interface unit and a microprocessor means, said microprocessor means having an I/O bus for data transfers and output control registers providing control data to each of said line adapters, a byte oriented line adapter system comprising: (a) a plurality of input/output circuit means connecting said data terminal equipment to a first driver switching means, and wherein said input/output circuit means can execute instructions from said microprocessor to operate synchronously or asynchronously during data transfer operations, each said input/output circuit means including: (a1) a USART receiver-transmitter means controlled by said microprocessor means for receiving serial bit data from said data terminal equipment and converting said serial data to parallel character-byte data for transmission to said microprocessor means, and/or receiving character-byte data from said microprocessor means and converting it to serial bit data for transmission to said data terminal equipment, said USART receiver-transmitter means, being connected to respond to control data from said microprocessor means, for selecting the character-byte data size from five bits-per-byte up to eight bits-per-byte; (a2) timing means, set by the said microprocessor means connected to said input/output circuit means, said timing means including: (a2a) first timing means for generating the baud rate of data transfer in said USART receiver-transmitter means; (a2b) second timing means connected to said microprocessor means for establishing programmed time-out periods and delayed response time for accepting data coming in on said data communication lines to said line adapter; (b) first driver switching means, controlled by said microprocessor means, for transferring data from said microprocessor means to a selected one of said input/output circuit means, or transferring data from said selected input/output circuit means to a multiplexor means; (c) multiplexor means, having an output to said I/O bus of said microprocessor means, for conveying data bytes to a buffer memory means or to said microprocessor means for subsequent transfer to a main host computer, said multiplexor means including: (c1) a group of n multiplexors, each multiplexor connected for conveying data from one bit-line of a data transfer bus means from said first driver switching means, said group of n multiplexors providing for parallel transfer of character bytes of up to eight bits; (c2) said data transfer bus means connecting said first driver switching means to said multiplexor means; (c3) a plurality of input information-data means connected to the input of each of said multiplexors, said information-data means being transferrable to said microprocessor means upon control instructions from said microprocessor means; (c4) input instruction means connected to said multiplexor means from said microprocessor means for selecting which of said information-data means shall be transferred to said microprocessor means; (d) buffer memory means, connected to said I/O bus, for temporarily buffering data being transferred between said microprocessor means and said data terminal equipment, said buffer memory means including: (d1) address input lines connected from said microprocessor means to said memory means to receive addresses from said microprocessor means; and (d2) a memory output bus connected to convey addressed data from said memory means to said microprocessor means.
2. The line adapter system of claim 1, wherein said USART receiver-transmitter means includes: (a) internal control registers, receiving control data from said microprocessor means, for enabling operation in either synchronous mode or asynchronous mode; (b) connection means, connecting said internal control registers to said microprocessor means, for conveyance of program instructions to said internal control registers.
3. The line adapter system of claim 2, wherein said internal control registers are loaded with control data by said microprocessor means to control the number of bits to be assembled into a byte.
4. In a data communication subsystem wherein a line support processor controls data transfer operations via data communication lines between a main host computer and remote data terminals, and said line support processor includes a plurality of line adapters, a data link interface unit, and a microprocessor means, said microprocessor means including program data in an internal PROM and in external RAM storage means and having an I/O bus and output control registers for providing control data and information data to said line adapters, a system of multiple byte-oriented line adapters comprising: (a) a plurality of line adapters connected to said microprocessor means for receipt of programmed control data and for control of data transfers to and from said remote data terminals, wherein each such line adapter includes: (a1) USART control component means connected to data communications line for transmission or reception of data between said remote data terminal and said microprocessor means, and wherein said USART control component means includes: (a1a) control register means for receiving programmed control words to set the bit size of each byte of data to be handled and to receive clocking signals from a timing component means for setting the baud rate of data transfer; (a2) first timing component means for setting the baud rate of data transfers, said first timing means connected to said USART control component means; (a3) second timing component means for establishing protocol timing signals to said microprocessor means for enabling said microprocessor to establish time-out periods for polling and for reception of data; (a4) gating means, connected to said USART control components means and said timing means, for signalling said microprocessor means for service and for activating a discrete bit line of said I/O bus to identify a particular USART/timing component means; (a5) logic means, receiving control signals from said gating means and from said microprocessor means for selecting a particular line adapter of the plurality of said line adapters; (b) said microprocessor means connected to enable a data transfer path from a requesting USART/timing component means to said I/O bus for transfer of data to said microprocessor, and including: (b1) means to scan said I/O bus lines to identify the said activated bit line which designates a particular line adapter; (b2) means to select which input line of each of a plurality of multiplexors will be connected for throughput; (c) said enabled data transfer path including: (c1) driver-controller bus means for connecting a selected USART/timing component means via a data transfer bus to a selected group of multiplexors for subsequent data transfer to said microprocessor means; (c2) a data transfer bus connected to each group of n multiplexors for parallel-transfer of n bits of data; (c3) a plurality of groups of multiplexors wherein each group has n multiplexors and is dedicated to a particular line adapter, and connects n bits from said data transfer bus to said I/O bus for data transfer to said microprocessor means, wherein n represents the number of bits in a byte of data, said number n being settable by instructions from said microprocessor means.
5. A byte-oriented line adapter system connected to receive instructions from a microprocessor means having an I/O bus means, and to transfer data, on a data communication line, between a remote terminal and a line adapter, said system comprising: (a) a plurality of line adapters, each line adapter of which includes: (a1) means to send or receive data from a remote terminal via a data communication line, said means to send/receive including: (a1a) first output connection means to a gating logic means; (a1b) second output/input connection means to a data communication line; (a1c) third input connection means from a designate logic means; (a1d) fourth input connection means from a component select logic means; (a1e) fifth input connection means from said microprocessor means via a bus driver means; (a1f) USART component means for converting bits to bytes and bytes to bits and including internal control registers; (a1g) timing component means for controlling the baud data transfer rate and for generating protocol timing signals for said microprocessor means; (b) said gating logic means connected to receive service request signals from each line adapter and to transfer said service request signals: to said designate logic means, to a bit line of said I/O bus means and to said microprocessor means; (c) a plurality of data communication lines wherein each line transfers data between said remote terminal and said line adapter; (d) said designate logic means for generating select signals to select a group of multiplexors servicing a designated line adapter and to select a specific line adapter, said designate logic means including: (d1) sixth output connection means for conveying multiplexor selection signals to a designated multiplexor group; (d2) seventh output connection means for conveying line adapter selection signals to a selected line adapter; (d3) eighth output connection means for conveying line adapter selection signals to said component selection logic means; (e) said component selection logic means connected to receive instruction control data from said microprocessor means and said designate logic means and including: (e1) ninth output connection means to convey USART/timer component select signals to a designated line adapter; (f) microprocessor means for transferring instruction and control signals to a designated line adapter and its USART/timing component means, said microprocessor means including: (f1) I/O bus means for receiving data transferred from a selected line adapter via a selected multiplexor group, said bus means comprising a plurality of bit lines; (f2) means to scan each bit line of said I/O bus means to identify a line adapter that requests service; (f3) means to convey output control signals to a selected line adapter; (g) bus driver means for conveying data from said microprocessor means to a selected line adapter or for conveying data from a selected line adapter to a selected multiplexor group, said driver means including: (g1) a data bus having multiple-bit lines for simultaneous transfer of a full byte of data from said bus driver means to said selected line adapter or said selected multiplexor group; (h) said multiplexor group connected to transfer simultaneously a full character-byte of data from said bus driver means and a selected group of multiplexors, said multiplexor group including: (h1) a plurality of multiplexors formed into groups of "n" multiplexors where "n" represents the number of bits in a character-byte; (h2) tenth connection means from said microprocessor means for conveying said multiplexor selection signals to select the multiplexor group which services the designated line adapter; (h3) multiplexor output bus means from each group of multiplexors for transferring data to said I/O bus means.
6. The system of claim 5, wherein each of said line adapters can be instructed by said microprocessor means to transfer character-bytes of "n-bits" where n equals 2 or 3.
7. The system of claim 6, wherein each of said line adapters can be individually instructed to execute one or more different line protocols and one or more different baud transfer rates.
8. The system of claim 6, which includes: (a) buffer memory means for each line adapter, said memory means connected to said I/O bus means; (b) means to store output data from said multiplexor means in said buffer memory means; (c) means to retrieve data in said buffer memory means; (d) eleventh output connection means from said buffer memory means to convey said retrieved data to said microprocessor means for transfer to a main host computer or to another remote terminal.
9. The system of claim 7, wherein said microprocessor means can instruct a designated line adapter to execute either synchronous or asynchronous data transfers.