Bus Contention Circuit

This invention relates to a bus contention circuit, and particularly to a circuit for establishing service priority among computer devices sharing the use of a bus channel.

It is desirable to establish priority of use among the several computer units sharing the use of a bus channel. In a typical arrangement, several active units, each capable of seizing use of a communications bus, are connected to the bus to share communications over the bus. However, to avoid conflicts between several units seizing use of the channel simultaneously, a priority is ordinarily established among the users. One technique has been to establish a rigid priority among the several users of the bus so that if two or more users seek use of the bus, priority is always given to that unit having the highest assigned priority. One problem with such a technique, however, is that a few users with relatively high priorities can preclude the ability of a unit with low priority from seizing use of a channel.

It is an object of the present invention to provide a bus contention circuit wherein the several units are assigned a priority, yet providing the opportunity for lower-ordered units to seek access to the bus even if higher-ordered units seek such access.

In accordance with the present invention, the units are arranged in a priority arrangement. The priority resolving apparatus includes register means for registering a request for any unit where either (i) no other requests are so registered, or (ii) the requesting unit has a lower priority than the unit of highest priority whose request is already registered. When the bus becomes free, allocation is accorded to the unit with highest priority whose request is registered.

One feature of the present invention resides in the provision of a pair of J-K flip-flops, clocked with a 16 MHz clock, for each active unit capable of seizing use of the bus. The first flip-flop is a bus request flip-flop which, when set, generates bus request signals to all units with lower priority. The bus request flip-flop is not permitted to set if requests are pending, all of which are lower in priority. However, if a request is pending which is higher in priority, the bus request flip-flop is permitted to set. The second flip-flop, a bus busy flip-flop can be set by the bus request flip-flop one cycle after the bus becomes free if no active request exists from higher priority units.

Another feature of the invention resides in the fact that when the bus is active or busy, a request from a particular unit to seize use of a channel is honored only if no other requests are pending or if the unit seeking the request is of lower priority than at least one other unit whose request is also pending. When the bus subsequently becomes inactive, use of the bus is allocated to that unit having the highest priority and an active request. For example, if there are eight units capable of seizing use of the bus, and an active request exists for the fourth unit, subsequent requests will not be honored from any unit having a higher priority than unit four; requests will be only honored from units having priorities lower than unit four. Consequently, when the channel subsequently becomes inactive, unit four, which has the highest priority of the units having active requests, is allocated use of the channel.

Another feature of the present invention resides in the fact that units of relatively high priority are prevented from monopolizing use of the bus, thereby insuring that all units have an opportunity to access to the bus.

The above and other features of this invention will be more fully understood from the following detailed description and the accompanying drawings, in which:

FIG. 1 is a block circuit diagram of the bus contention circuit in accordance with the presently preferred embodiment of the present invention; and

FIG. 2 is a wiring diagram illustrating the interconnection of the bus contention circuits for the several units connected to the bus.

With reference to the drawings, and particularly to FIG. 1, there is illustrated a block circuit diagram of a bus contention circuit in accordance with the presently perferred embodiment of the present invention, one such circuit being employed for each active unit connected to a bus (not shown). The circuit includes a bus request flip-flop 10 having its set input connected to the output of AND gate 12. AND gate 12 receives a request signal from the unit (not shown) via lead 14, and a second input from the output of EXCLUSIVE OR gate 16. EXCLUSIVE OR gate 16 receives a BREQX signal at one input and a BCREQ signal through inverter 18 at the second input. The set output of flip-flop 10 provides, through inverter 20, the BCREQ signal, and provides a first input to AND gate 22. The reset output of flip-flop 10 is provided through fan out circuit 24 to provide respective BREQ1 through BREQ7 signals. AND gate 22 also has a second input connected to receive the BREQX signal. The output of AND gate 22 is connected to the set input of bus busy flip-flop 26, which in turn provides an output through inverter 28 to provide a BBSY signal output. Also, the output of flip-flop 26 is provided to the reset input of flip-flop 10, and the BBSY signal is provided as a third input to AND gate 22. Flip-flop 26 is reset by signal CLRBSY.

FIG. 2 illustrates the wiring diagram of a back panel for interconnecting the contention circuits illustrated in FIG. 1. As shown in FIG. 2, the BCREQ signals are wired (ORed) together by wire 30 and the BBSY signals are wired (ORed) together by wire 32. The BREQ1 through BREQ7 signals are wired in such a way that the BREQX signal of the contention circuit for each respective slot comprises the BREQ1 through BREQ7 signals from cards or units of higher priority. For example, the contention circuit representing the card or unit in slot 0 provides signals onto each of the leads BREQ1 through BREQ7 to provide a BREQX signal to each of the lower-ordered cards, whereas, for example, the card in slot 4 provides BREQX signals only to the cards in slots 5, 6 and 7.

With the foregoing in mind, operation of the apparatus illustrated in the drawings may now be explained. Assuming initially that the bus is not busy and that there are no requests for use of the bus, this system is, of course, dormant. Flip-flops 10 and 26 in all contention circuits are both in a reset condition so that the BCREQ signal is high (through inverter 20), the BREQ1 through BREQ7 signals are also high, and consequently the BREQX signal to all circuits is high. Preferably, the BREQX signal in slot 0 is tied to a permanent high signal source. The conditions at EXCLUSIVE OR gate 16 are such that the BCREQ signal through inverter 18 is low whereas the BREQX signal is high. EXCLUSIVE OR gate 16 will provide an output when either, but not both, of its inputs are high. Consequently, EXCLUSIVE OR gate 16 provides a high signal to the lower input of AND gate 12.

If a unit becomes ready to seize use of the bus, a request signal from the unit is imposed on lead 14 to AND gate 12 thereby causing the AND gate to provide a high input to flip-flop 10 thereby setting the flip-flop upon the next clock cycle. The set output of flip-flop 10 provides a high signal output to inverter 20, thereby driving BCREQ low. Also, the setting of flip-flop 10 places a high signal to one input of AND gate 22. Since the BBSY signal is high (because no flip-flop 26 is set) and because the BREQX signal is high, AND gate 22 operates to set flip-flop 26 thereby driving the BBSY signal low, thereby indicating the channel is busy. Preferably, flip-flop 26 is also connected to the unit (not shown) to enable the unit to seize use of the bus. Therefore, the unit which has requested use of the bus has seized use of the bus and the bus is busy. The setting of flip-flop 26 resets the respective flip-flop 10 thereby providing a low signal to fan out circuit 24, driving or releasing the BREQX signals high to all units with a lower priority. Furthermore, with signal BBSY low, the conditions at all of the AND gates 22 are such as to prevent the setting of any further flip-flops 26.

If, during the time that the channel is busy, a request from another unit comes in to its contention circuit, its respective flip-flop 10 is set (assuming no other requests had yet been made). This occurs because all the BREQX signals are high as are the BCREQ signals so that the condition at EXCLUSIVE OR gates 16 are such as to provide a high signal output to AND gates 12. Consequently, a request on a line 14 will set the respective flip-flop 10 as heretofor described. However, because BBSY signal is low, the respective AND gate 22 will not pass a signal to set the respective flip-flop 26. Therefore, the subsequent request will not cause seizure of the bus. Instead, flip-flop 10 is set, thereby driving the BREQX signal in lower-ordered units low and driving BCREQ signals in all units low. The BREQX signals to higher-ordered units remains high.

During the time when the bus is busy and a request is registered in flip-flop 10 of a particular unit, if a lower-ordered unit requests priority on its line 14, the conditions are such that the BREQX and BCREQ signals are low (at the lower-ordered unit). Consequently, EXCLUSIVE OR gate 16 receives a high input from inverter 18 and a low input BREQX signal. Thus, AND gate 12 is operated when a high signal appears on lead 14 to thereby set the flip-flop 10 and provide a continued low output on the respective outputs of fan out 24. However, if a request is made on line 14 of a higher-ordered unit, AND gate 16 is not operated due to the high condition of the BREQX signal. Therefore, if the channel is busy and a request is registered from another unit, additional requests will only be registered in units having a lower priority than the highest one registered. For example, if the bus is busy and an outstanding request exists from unit four, subsequent requests from any of the units in lower-ordered priority units five, six or seven will cause setting of their respective flip-flops 10, but a request from a higher-ordered unit (higher than the unit in slot four) will not set the respective flip-flop 10. When the bus is released by the unit using it, that unit will generate a CLRBSY signal to its flip-flop 26, thereby resetting the flip-flop and driving or releasing the BBSY signal high. Thereafter, the unit with the highest priority and having its flip-flop 10 in a set condition (unit four in the example) will cause its flip-flop 26 to be set one clock cycle after release of the channel. This is because the BBSY signal goes high when the channel becomes free thereby permitting the AND gate 22 to operate. (The middle input of AND gate 22 is already high because the BREQX signal is high due to the reset condition of all flip-flops 10 associated with units of higher priority.) However, any units with yet lower priorities for which requests are outstanding, will not have their flip-flops 26 set because the flip-flop 10 in the now honored unit is initially in a set condition thereby rendering the BREQX signal low in each lower-ordered unit. Flip-flop 10 in the now honored unit is reset one clock cycle after its flip-flop 26 becomes set.

Preferably, for simplicity of circuit design, all output signals are open collector signals, BCREQ and BBSY being held high by one unit and driven low by any unit and BREQX being held high for each unit and driven low by any unit of higher priority. Therefore, when a signal is returned to its high state, it is released high.

The present invention thus provides a bus contention circuit for resolving contention among users of a bus wherein requests for use of the channel are registered only for units having lower priority than the highest ordered unit whose request is registered. Use of the bus is accorded to the highest-ordered unit having a request so registered. Therefore, higher-ordered units are precluded from monopolizing use of the channel. The circuit is rapid in operation in that requests are honored within one clock cycle of the channel becoming free. The circuit is effective in operation, simple in design, and inexpensive to manufacture, operate and maintain.

One feature of the invention resides in the fact that the system may be expanded to accommodate any number of units merely by expanding the back panel pattern (FIG. 2) and the outputs of fan-out circuit 24.

This invention is not to be limited by the embodiment shown in the drawings and described in the description, which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.