Abstract:
Circuitry allocates requests for demand-shared bus access among a plurality of service requesting ports. During bus contention time, each requesting port synchronously and sequentially applies the digits of its assigned unique priority code to the bus beginning with the most significant digit. After the application of all digits, only the requesting port having the highest code remains in contention and it seizes the bus. Increased flexibility in port preference is provided by the use of a plurality of status flip-flops in each port for generating dynamic port parameter bits. The generated parameter bits are normally applied to the bus as the most significant bits of a dynamic port priority code during contention time. However, the selective application of a mask signal to a mask conductor during contention time causes each requesting port to prevent any parameter bits from being applied to the bus as long as the mask signal remains. This returns control of the port preference to any unmasked parameter bits and to the assigned port priority codes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following concurrently filed applications: 
     J. O. Dimmick, Ser. No. 337,673, entitled &#34;Circuitry for Allocating Access to a Demand-Shared Bus&#34;; 
     G. J. Grimes 2, Ser. No. 337,672, entitled &#34;Circuitry for Allocating Access to a Demand-Shared Bus&#34;; 
     and G. J. Grimes 4, Ser. No. 337,868, entitled &#34;Circuitry for Allocating Access to a Demand-Shared Bus.&#34; 
     TECHNICAL FIELD 
     This invention relates to a shared resource system and, more particularly, to apparatus for assigning access to the resource equitably among a plurality of requesting devices. More particularly, the invention relates to a packet switching system having an allocation facility for controlling access to a common bus by a plurality of requesting devices such as ports. 
     BACKGROUND OF THE INVENTION 
     Systems in which many devices share a common resource typically utilize arrangements for allocating access to the resource under conditions during which a plurality of associated devices may concurrently request access. Many different allocation arrangements are known in the art. In data processing and packet switching systems, it is known to use a centralized allocator or controller for allocating access to a common data bus interconnecting a plurality of units, such as ports, that may concurrently request access to the bus. The controller may be programmed with an appropriate algorithm to allocate bus access in accordance with any priorly determined criterion that may be desired. Although centralized controller allocation arrangements operate suitably to perform their intended function, they are not always desirable because of the inherent system complexity resulting from the many interconnections required between the controller, the bus, and the ports. Also, a reliability problem exists since a malfunction of the controller may remove the whole system from operation. A system having a centralized controller is shown by U.S. Pat. No. 3,983,540 issued Sept. 28, 1976 to Keller et al. 
     It is known to use distributed bus allocation arrangements in which a controller is not used to determine access and instead, the interaction of the requesting ports determines bus allocation in the event of simultaneous requests. Such distributed arrangements are often preferable since the expense of and the reliability problems associated with the centralized controller arrangement are avoided. 
     In accordance with one such distributed allocation arrangement, each port that may request access to a common bus is assigned a fixed priority number comprising a plurality of binary digits. Access is granted by priority number in case of concurrent requests. During bus contention time, when two or more ports concurrently request access, each requesting port applies the corresponding bits of its priority number to an arbitration bus sequentially, bit by bit, in synchronism with the application of corresponding bits by all other concurrently requesting ports. As each bit is applied, each bidding port compares the magnitude of the bit it is currently applying with the logical union of the corresponding bits applied simultaneously to the arbitration bus by all concurrently requesting ports. If a bit that one of the requesting ports currently applies to the arbitration bus has a prescribed relationship (such as equal to or higher) to the bits applied to the bus by the other requesting ports, this operation proceeds and the port applies the next bit of its assigned priority number to the arbitration bus. 
     Each port stays in contention as long as each bit it applies has the prescribed relationship to the logical union of the corresponding bits currently applied by other contending ports. A port removes itself from contention when it determines that a bit it applies has a relationship (such as is lower than) to the bits applied by the other ports indicating that one or more of the other ports has a higher priority number. At that time, each port having a lower priority number removes itself from contention and applies no further bits to the bus. 
     This contention operation continues; the remaining bits of the port priority numbers are applied to the bus by all remaining requesting ports; ports of a lower priority remove themselves from contention; and at the end of the contention interval when the last bit is applied to the bus, only the port having the highest priority remains in contention and it is granted access to the bus. An arrangement of the above described type is shown in U.S. Pat. No. 3,796,992 issued Mar. 12, 1974, to Nakamura et al and in U.S. Pat. No. 3,818,447 issued June 18, 1974 to Craft. 
     The above described distributed contention arrangement operates satisfactory. However, it suffers from the disadvantage that the port priority numbers are fixed and, since port access is determined by these numbers, the ports may be considered to be functionally arranged in a fixed preference chain with the most preferred port having the highest priority number and the least preferred port having the lowest priority number. This being the case, access to the bus is not equitable since ports having the higher priority numbers are always favored in the event of simultaneous requests. While this unequitable allocation of ports may be tolerable in certain systems, it is a disadvantage in those systems in which more equitable access by all ports is required. 
     SUMMARY OF THE INVENTION 
     My invention is directed to a solution of the foregoing problems and limitations of the prior art. I provide an improved method and structure for allocating a demand-shared bus among one or more requesting units or ports each of which has a unique assigned priority number comprising a plurality of binary coded digits. Further, and in accordance with my invention, each port includes facilities for monitoring dynamically the current state of various operational port parameters and for generating corresponding port priority bits representing these parameters. These generated bits are used in conjunction with the assigned port priority number bits to determine bus access. 
     The port parameter bits generated by the facilities of my invention are entered into the more significant bit positions of a port shift register. The assigned port priority numbered bits are entered into the remainder of the shift register so as to be of lesser significance than the parameter bits. During bus contention times, the bits in the shift register of each requesting port are read out sequentially one at a time beginning with the most significant bit and applied to the arbitration bus. 
     During conditions in which no port parameter bits are generated by the facilities of my invention, the shift register contains 0&#39;s in its corresponding more significant bit positions and contains the regularly assigned port priority number bits in its lesser significant bit positions. Under such conditions, port priority is determined using only the assigned port priority number. However, during operational states of ports in which a 1 is generated for one or more of the parameter bits, these parameter bits are read out of the shift register ahead of the port priority number bits and thus will, by themselves, control bus access. If two or more ports have identical parameter bits set to a 1, and there are no other ports having parameter bits indicating a higher priority, the port priority number bits are then used to break the tie. 
     The facilities provided in accordance with my invention for monitoring port parameters and for generating corresponding parameter bits, for entry into the port&#39;s shift register, include facilities for monitoring the current contents of a packet buffer in each port to determine whether it is less than half-full, at least half full or full of packet information. These facilities can generate respectively, no bits, a buffer half-full bit, and a buffer full bit. 
     The facilities I provide for generating port parameter bits include snapshot facilities which ensure that all ports requesting service at a particular time are served ahead of later requesting ports. These snapshot facilities include a flip-flop which is set at a particular time, termed a snapshot time, in each port that is then requesting bus access. The setting of this flip-flop in each such port applies a snapshot bit of 1 to its associate shift register as a more significant bit ahead of the bits of the assigned port priority number. A snapshot flip-flop is reset as its port is granted bus access and at the end of a given number of contention cycles, all ports having their snapshot flip-flop set will have been served and their snapshot flip-flops will all be in a reset state. A new snapshot time then occurs and the snapshot flip-flop in each then currently requesting port is set to define a new group of ports to be given preference. 
     As before, the corresponding shift register bits of each requesting port are applied concurrently to the arbitration bus during contention time sequentially, bit by bit. This includes the port parameter bits as well as the assigned port priority number bits. The bit values of each contending port are compared in a prescribed order to the corresponding bus digit value. A unit is removed from bus contention if, on any digit comparison, a prescribed result is obtained indicating that another port of higher priority is requesting access. In the disclosed embodiment of the invention, the priority for bus access is based on the magnitude of the number defined by parameter bits and the priority number bits in each port&#39;s shift register. In the disclosed embodiment a wired OR TTL bus is used in which a 1 represents the dominant low voltage state. 
     Further, in accordance with my invention, I provide increased flexibility in determining port preference by the provision of a mask bus. The mask bus is effective when activated by a system controller to cause the arbitration circuitry of each port to ignore, selectively, any or all of the parameter bits during contention time. 
     The mask signal temporarily disables circuitry internal to each port and causes bus access to be determined by the assigned port priority number plus any of the parameter bits that are not masked. The mask circuitry also prevents any masked bits of 1 from being applied to the arbitration bus. The bus is held in an 0 bit state during mask states. 
     This masking is a desirable feature since under certain system conditions it may be desirable to disregard one or more parameter bits in determining bus access. The invention implements the mask bus feature in a fault tolerant manner which prevents the mask bus from masking the assigned port priority bits. This ensures that a unique port will always be selected in the event of a trouble condition that applies a permanent mask signal to the mask bus. 
     I also provide an enable/disable bus that includes a conductor unique to each port and extending from a system controller to the port. The conductor is connected to circuitry within each port which effectively disables the port and prevents it from bidding for bus access. This feature is useful in connection with circuit malfunctions that might possibly cause a port to consistently apply all 1&#39;s to the bus during arbitration time and therefore monopolize the bus access. 
     In a packet switch, packets are stored in a buffer memory in the ports; the port contention logic competes for access to the common packet resources including typically a common packet transfer bus herein termed a data bus. A packet will be lost if it is transmitted to a port which already has one or more packets in its buffer memory and does not have enough room to store another packet. The rate of packet loss can be made small by providing enough memory in each port so that on a statistical basis the packet loss rate will be acceptable at the traffic limit of the switch. My invention makes it possible to design the packet switch so that smaller memories are required to give the same packet loss rate at a given amount of network traffic; or that with a fixed amount of memory, the packet loss rate will be smaller for a given amount of traffic. Since most of the cost and complexity of the switch is typically contained in the memory or queue of the ports, the port contention logic can be expanded considerably and still reduce the cost and complexity of the system since the capabilities of my invention make possible significant cost and complexity reductions in the memory. This invention provides means for making the switch&#39;s arbitration algorithm sensitive to the dynamic state of the ports as well as means for altering the parameters to which the algorithm is sensitive to on a frame-by-frame basis. This is accomplished by fault tolerant means without sacrificing the distributed nature of the arbitration means. 
     The above described arrangement overcomes the disadvantage of the prior art in that it provides increased flexibility and a more optimal allocation of ports for access to a facility or bus in systems in which each port is assigned a fixed priority number whose magnitude would otherwise determine the bus access priority. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages of my invention will be apparent from the following detailed description of an exemplary embodiment thereof, when read in conjunction with the accompanying drawing, in which; 
     FIG. 1 is a simplified block diagram illustrating the components of a typical system in which my invention may be utilized; 
     FIG. 2 discloses further details of the port circuit of FIG. 1; 
     FIG. 3 is a timing diagram; 
     FIGS. 4 and 5 disclose the circuit details of the arbitration logic of the port of FIG. 2; and 
     FIG. 6 discloses how FIGS. 4 and 5 should be arranged with respect to each other. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 discloses a packet switching system embodying the present invention. On FIG. 1 is disclosed a controller 100, having a polarity generator 122, ports 110-1 through 110-n, switch 107, and a plurality of busses interconnecting the controller 100 with the ports 110. Packet bus 105 receives the data applied from the output 111 of each port directed to another port. Packet bus 106 receives this data after it has been extended through switch 107 and applies it to the input 112 of each port. Clock bus 103 extends the signals shown on FIG. 3 from the controller 100 to ports 110. Arbitration bus 102 concurrently receives the corresponding priority bits applied sequentially by each requesting port during bus contention time. Polarity bus 101 applies a potential from controller 100 to the ports 110 at selected times to cause them to apply to bus 102 the inverse of any digit of their assigned priority number. 
     Enable/disable bus 108 includes a conductor unique to each port and extends from the controller 100 to each port 110. This bus is effective when activated to cause the associated port to be removed from service and to deny it access to the arbitration bus 102 and the packet busses 105 and 106. The mask bus 104 comprises a conductor common to all ports and extends from the controller 100 to the ports 110. It is effective when activated to cause any combination of the port parameter bits to be disregarded during contention time so that bus access is awarded on the basis of the remaining parameter bits, if any, and the priority number assigned to each port. 
     Data processor 120-1 and terminal controller 120-n, together with terminals 121, are illustrative of the type of facilities that may be served by the ports. As is typical in packet switching, a transmitting port that obtains access to the packet busses 105 and 106 transmits whatever data may be desired to the outport 111 over packet bus 105, through switch 107, and over packet bus 106 to the input 112 of the port to which the information is directed. 
     FIG. 2 discloses further details of the ports 110 of FIG. 1. Each port includes an I/O interface 200, an input bus interface 210 and an output bus interface 220. The input bus interface 210 includes arbitration logic 218 and buffer 213 which applies data over output 111 to the packet bus 105. Interface 210 further includes a FIFO buffer 211 and a FIFO controller 214. FIFO 211 receives packet information from interface 200 and temporarily stores it until it is read out and extended through buffer 213 to packet bus 105. The FIFO controller 214 receives information over path 212 from FIFO 211. This information includes bits specifying whether the FIFO is currently at least half-full or full. The FIFO controller 214 passes this information over paths 206 and 207 to the arbitration logic 218 which uses this information as additional parameter bits for contention purposes. 
     Output bus interface 220 contains the circuitry by which the port receives information from packet bus 106. This circuitry includes buffer 221, FIFO 227, FIFO controller 225, and packet recognizer 223. 
     Typically, the data processor 120 served by the port 110-1 of FIG. 2, applies a packet of information to be sent to another port over path 116-1, through I/O interface 200 and over path 201 to FIFO 211. The FIFO controller 214 detects the receipt of a complete packet by FIFO 211, and transmits a request for bus access to arbitration logic 218 which then functions during the next contention interval to attempt to obtain access for the port to bus 105. Upon obtaining such access, FIFO controller 214 causes FIFO 211 to apply the packet information it contains via buffer 213 to packet bus 105. This information includes header information identifying the port to which the packet is being sent. After passing through the switch 107 on FIG. 1, the information is applied over packet bus 106 to input 112 of the receiving port, and via its buffer 221 over path 222 to its FIFO 227 and its packet recognizer 223. Element 223 detects that the information now in FIFO 227 is indeed directed to this port and then, by means of FIFO controller 225, causes FIFO 227 to output the information via path 202, I/O interface 200, and over path 117 to the device served by the receiving port. 
     FIG. 3 discloses the waveforms of the timing and control signals applied over clock bus 103 of FIG. 1 to the ports 110. The top signal is a positive frame pulse and identifies the beginning of each frame. A bus contention interval begins with each frame pulse. This frame is as long as required for an entire packet to be transmitted. The bus contention logic and the packet transmission can occur simultaneously during each frame, with the port which wins a contention cycle controlling the packet bus 105 during the next frame. The lower signal is the bit clock signal and it is used for a number of control purposes during the contention or arbitration interval. 
     The detailed embodiment of the arbitration logic 218 of FIG. 2 is illustrated in FIGS. 4 and 5 arranged as shown in FIG. 6. The circuitry of FIGS. 4 and 5 implements additional priority code bits and enable/disable bus 108 or mask bus 104 causes each requesting port 110 to ignore any status bits on the arbitration bus when the disable potential is applied. 
     During the basic arbitration cycle, the hardwired assigned port number in element 527 is loaded into parallel-in, serial-out shift register 500 over path 528. The FRAME CLOCK pulse is passed to the LOAD input of shift register 500 over path 426. When the FRAME CLOCK goes HI, all bits of the assigned port number are loaded into shift register 500 in parallel. Assume at this time that the signals for the full bit (path 524) half-full bit (path 522) and the snapshot bit (path 423) applied to shift register 500 are LO (This will be discussed later). 
     When the port 110-1 is first powered up, flip-flops 410, 412, 418, 421 and 422 are reset by the MASTER CLEAR signal input to their respective CLR inputs on path 416. Their respective Q outputs are LO when these flip-flops are reset. 
     When a REQUEST PENDING signal 216 is present from FIFO controller 214 as the port requests bus access, a HI is passed over path 216 to the right input of NAND gate 430 and the lower input of AND gate 417. This HI enables these gates. When the next FRAME CLOCK goes HI on path 426, the output of NAND gate 430 goes LO. The LO output of NAND gate 430 is passed over path 431 to the active LO preset input of D flip-flop 410, and to the active LO set input of SR flip-flop 412. This LO input sets both flip-flops and causes their Q outputs to go HI. The HI Q output from SR flip-flop 412 is passed over path 413 to partially enable the 4-input NAND gate 406. This high also enables the D input of D flip-flop 421. 
     All the bits in the shift register 500 are now serially shifted out of the shift register one bit at a time with each pulse of the bit clock, over path 501 to the lower input of exclusive OR gate 404. Assume at this time that the upper input to exclusive OR gate 404 is LO so that signals on the lower input over path 501 pass unchanged through gate 404 to path 405. The output from exclusive OR gate 404 is passed over path 405 to the left center input of NAND gate 406 and to the lower input of exclusive OR gate 409. The right and left inputs of NAND gate 406 are currently enabled and therefore the bits applied to its center input are inverted and passed over path 114-1 to arbitration bus 102. 
     The logical union of the priority bits applied by all requesting ports to bus 102 is applied from bus 102 via path 114-1 to the top input of exclusive OR gate 409 over path 408 as well as the top input of gate 417. Exclusive OR gate 409 compares the current digit value of arbitration bus 102 with the value of the digit this port puts on the bus. If there is a mismatch, the inputs to exclusive OR gate 409 will match and the output of exclusive OR gate 409 goes LO. A mismatch is said to exist when the port attempts to apply a HI to the bus as a 0 via the output of gate 406 at a time when the bus is driven LO when at least one other port applies a 1 to the bus from the output of its gate 406. Thus, when such a mismatch exists, the lower input of gate 409 is LO from the 0 from the shift register 500 of FIG. 5 of the current port; the upper input of the gate is also LO from the 1 applied in inverted form to the bus 102 by the gate 406 of another port. This causes gate 409 to generate a LO as a mismatch signal. This means that the port applying a 1 has a higher priority and that the port applying an 0 should remove itself from contention. 
     The LO mismatch signal from exclusive OR gate 409 is passed over path 439 to the D input of D flip-flop 410. At the beginning of the next bit clock pulse, the Q output of D flip-flop 410 is made LO and this active LO is passed over path 411 to the LO R input of RS flip-flop 412. This LO resets RS flip-flop 412. The resulting LO output at Q of RS flip-flop 412 is passed over path 413 to the right input of NAND gate 406. This LO effectively removes NAND gate 406 from bus 102 by disabling its right-hand input. Thus, the port of FIGS. 4 and 5 will have failed to win the arbitration under the above assumed conditions. Let it now be assumed that a mismatch is not detected. The output of exclusive OR gate 409 remains HI as each digit is applied since the two inputs to the gate do not match. This HI output is passed over path 439 to the D input of D flip-flop 410. This HI input causes its Q output to remain HI over path 411 to the LO reset input of SR flip-flop 412. This HI signal on the LO reset input of SR flip-flop 412 does not reset this flip-flop and therefore, its Q output remains HI. The Q output from SR flip-flop 412 is passed over path 413 to the right most 3-input of NAND gate 406. This allows the NAND gate to continue to pass the priority bits to arbitration bus 102 to keep the port in contention. 
     A port wins the arbitration for bus 102 when a mismatch condition is not detected by its exclusive OR gate 409. This causes Q outputs of flip-flops 410 and 412 to remain HI. The HI Q output from SR flip-flop 412 is passed to the D input of D flip-flop 421 over path 413. The LO to HI transition of the next FRAME CLOCK pulse sets the Q output of D flip-flop 421 HI. The HI output from Q of D flip-flop 421 is passed as a PORT SELECTED signal on path 217. The PORT SELECTED signal is also passed to the R input of SR flip-flop 422 to reset its Q output LO. This LO Q output from SR flip-flop 422 is passed over path 423 to the shift register 500 of FIG. 5 as a 0 for the SSB bit. 
     The present invention provides for additional priority code bits to be added to the assigned port number bits of element 527 as the most significant bits of the port&#39;s priority code during contention. These are provided by flip-flops 521 and 523 which operate when a &#34;FULL&#34; or &#34;HALF-FULL&#34; condition is detected by FIFO controller 214. When the FIFO controller 214 determines that FIFO 211 is at least one-half full, a HI one-half FULL signal is passed over path 206 from FIFO controller 214 to the D input of D flip-flop 521. The LO to HI transition of the next FRAME CLOCK pulse on the CLK input of flip-flop 521 over path 426 sets the flip-flop to cause its Q output to go HI. The HI Q output of D flip-flop 521 is passed over path 522 to the 2SB input of shift register 500. The delay in flip-flop 521 allows for the same clock edge of the FRAME CLOCK to clock both flip-flop 521 and shift register 500 with no race conditions. 
     When FIFO controller 214 determines that FIFO 211 is full, a HI FULL signal is passed on path 207 from FIFO controller 214 to the D input of D flip-flop 523. The HI transition of the next FRAME CLOCK on the CLK input of flip-flop 523 sets the flip-flop to cause its Q output to go HI. The HI Q output of flip-flop 523 is passed over path 524 to the MSB input of shift register 500. The delay in flip-flop 523 allows for the same clock edge of the FRAME CLOCK to clock both flip-flop 523 and shift register 500 with no race conditions. 
     Bits from flip-flops 521 and 523 change the priority for port arbitration based on a full or one-half full determination. These bits are then loaded into shift register 500 along with the snapshot bit on path 423 and the assigned hard-wired port number 527 over path 528. 
     The snapshot bit is set to a 1 by divide-by-3 counter 514 if no other port is applying a 1 as a snapshot bit to arbitration bus 102 at the time the SSB bit is read out of the shift register and applied to bus 102. The bus is HI at that time since all ports are applying a 0. Divide-by-3 counter 514 is reset by each FRAME pulse and counts the subsequent bit clock pulses that are applied to its CLK input on path 425. 
     In order for the rising edge of the pulse generated by the divide-by-3 counter 514 to correctly clock the snapshot bit into flip-flop 418, care must be taken for the particular combination of parts selected that the snapshot signal will still be stable at the D input of flip-flop 418 when the clock pulse comes from counter 514. A potential race condition exists here because the same rising edge of the bit clock which causes the arbitration logic to put the next bit out on arbitration bus 102 also clocks counter 514. For most implementations, a worst case timing analysis would show that the delay caused by the combination of the delays caused by shift register 500, gate 404, bus driver gate 406, the capacitance of arbitration bus 102, and gate 417 is much greater than the delay through counter 514 and hence no race condition would exist. If for a particular choice of logic parts a race problem exists, then a delay element inserted between gate 417 and the D input of flip-flop 418 would eliminate the problem. The SSB bits present on arbitration bus 102 represent a 0 and are applied as a HI to the upper input of AND gate 417. Since D flip-flop 418 is only clocked by the third bit clock pulse from divide-by-3 counter 514, the Q output of this flip-flop can be set HI only if the output of AND gate 417 is HI at this time. The output of AND gate 417 is HI at bit clock 3 time only if there are no snapshot bits of 1 present on arbitration bus 102 and if this port has an active HI REQUEST PENDING signal on path 216. In this case, a HI signal is applied over path 114-1 through gate 417 to set flip-flop 418 when it is clocked by counter 514. The HI Q output of D flip-flop 418 is passed to the Set input of SR flip-flop 422 over path 419 to set its Q output HI. This HI output is passed as a 1 to the SSB input of shift register 500 over path 423. The snapshot bit is then loaded into shift register 500 by the LO transition of the next FRAME CLOCK. 
     All the bits in parallel-in, serial-out shift register 500 are subsequently shifted out of the shift register one bit at a time with each pulse of the bit clock over path 501 to exclusive OR gate 404. With the upper input of gate 404 LO, the bits are passed unchanged by exclusive OR gate 404. Counter 433 and flip-flop 435 are provided to count bit clock pulses and to prevent a signal on the polarity bus 101 from being effective to invert the first 3 priority bits (full, half-full, and snapshot bits) and to allow a signal on the polarity bus 101 to invert only the assigned port address bits received from element 527. A signal on the mask bus 104 is allowed to mask only the snapshot bits, and full and one-half full bits. 
     The divide-by-3 counter 433 and SR flip-flop 435 are reset each time the FRAME CLOCK pulse goes HI over path 426. This causes a LO output at the Q output of SR flip-flop 435. This LO is passed over path 436 to AND gate 402 and to OR gate 437. With its lower input LO from path 436, the output of OR gate 437 extends the signals received from the mask bus 104 over path 118-1. When the mask bus is LO to effect a MASK operation, the LO on path 118-1 to gate 437 holds the output of gate 437 LO. This LO is extended over path 438 to an input of NAND gate 406. This disables gate 406 and holds the gate output HI making it unable to apply the shift register bits received from gate 404 to the arbitration bus 102. When the mask bus is HI, to represent a non-mask state, the output of OR gate 437 is HI to NAND gate 406. This partially enables gate 406 so that its priority bits can transmit onto arbitration bus 102 when the other three inputs to gate 406 are HI. 
     The LO input on path 436 from flip-flop 435 also goes to the lower input of gate 402 to disable the gate 402 to prevent it from passing any polarity reversal signals that might be received from the polarity bus 101 over path 113-1 during the times of bits MSB, 2SB, and SSB. The resulting LO output from disabled AND gate 402 is applied over path 403 to the upper input of exclusive OR gate 404. This permits gate 404 to receive the shift register 500 output bits over path 501 and to pass these bits unchanged over path 405 to one input of NAND gate 406 and to the lower input of OR gate 409. 
     The divide-by-3 counter 433 is incremented as each bit is shifted out of shift register 500 by the bit clock. When three bit clock pulses have been counted, the first three bits (full, one-half full and snapshot) have been shifted out of shift register 500 and the divide-by-3 counter 433 sets its output HI over path 434 to SR flip-flop 435. SR flip-flop 435 is set HI at its S input and sets its Q output HI. The HI output from SR flip-flop 435 is applied to AND gate 402 and OR gate 437. 
     The HI input to OR gate 437 forces its output HI. This prevents any LO mask bus signal that may subsequently be received on path 118-1 from being passed through OR gate 437. The HI output from OR gate 437 is passed to NAND gate 406 on path 438 to partially enable it. This prevents any possibility of a masking of the assigned port number priority bits from bus 102. 
     The HI input to AND gate 402 from SR flip-flop 435 over path 436 partially enables AND gate 402. This allows the polarity bus 101 to pass polarity reversal signals over path 113-1, through AND gate 402, to exclusive OR gate 404 on its upper input. When the polarity bus reversal signal and the port number bits from the shift register 500 are both HI at the input to exclusive OR gate 404, the gate&#39;s output will be LO. If the input signals from the polarity bus and the port numbers are different, the output will be HI. Thus a LO signal from polarity bus 101 allows any port number bit to pass through exclusive OR gate 404 unchanged. A HI signal input from the polarity bus inverts any port number bit through exclusive OR gate 404. The output from exclusive OR gate 404 is again passed to one input of NAND gate 406 and to exclusive OR gate 409 over path 405. 
     The left input of NAND gate 406 is normally enabled over path 443 by flip-flop 442. Flip-flop 442 can either enable or disable the port arbitration logic synchronously with the frame clock at the beginning of a frame. With a HI on path 119-1, D flip-flop 442 is set HI by the FRAME CLOCK pulse over path 426. The HI output at Q is passed to the left input of AND gate 406 on path 443 to partially enable the gate. Flip-flop 442 remains set with its Q output HI until a low PORT DISABLE signal is applied from the controller 100 to the D input of the flip-flop over path 119-1 to disable this port. 
     The right-hand input to 4-input NAND gate 406 is enabled over path 413 as discussed previously. NAND gate 406 is thus enabled on its 4 inputs and the input signal from gate 404 is inverted and passed over path 114-1 as priority bits to the arbitration bus 102. The priority bits on bus 102 are applied to OR gate 409 and to gate 417. OR gate 409 compares the digit value of what is on arbitration bus 102 with the value of each digit this port puts on the bus. If there is a mismatch, both inputs to OR gate 409 are LO, and the output of OR gate 409 goes LO. The LO output from OR gate 409 is passed to the D input of D flip-flop 410. This resets flip-flops 410 and 412 and removes the port from contention as elsewhere described. If there is not a mismatch, one of the inputs to OR gate 409 is HI and the gate output is HI. This HI output is passed to D of D flip-flop 410 via path 439. With a HI on the D input of D flip-flop 410, the basic arbitration cycle continues to completion as discussed previously.