Abstract:
A peripheral controller is provided for controlling data transfers between peripheral devices operably on one type of data bus to devices operable on a second data bus. An intermediate buffer is utilized so that data is read into one memory block from the sending data bus and read out of another memory block to the receiving data bus. Controls are provided to prevent overwriting data which has not been transmitted and to prevent data transfers from the buffer until at least one block of memory has been filed.

Description:
FIELD OF THE INVENTION 
     The present invention relates broadly to the field of digital computers and particularly to circuitry which allows peripheral devices that communicate over one type of data bus to be coupled to a computer system which internally has a different data bus. 
     BACKGROUND OF THE INVENTION 
     In the field of digital computers, typically each digital computer system has one or more data buses with particular bus protocols that are used to transmit data between functional elements within the system such as the arithmetic logic unit, the random access memories, the communication channels and the like. The above-mentioned computer system elements also are utilized in a typical configuration to communicate with magnetic tape transports, magnetic disk storage device, printers and communication networks with other computers. With respect to the later mentioned devices, it is not uncommon to have one or more communication channels which couple to controller devices to provide a way to convert the data signals interpretable directly by the device to data signals which can be interpreted by the channel to which the device is coupled. 
     In an effort to overcome some of the difficulties associated with coupling devices such as tape drives, printers, disk drives and networks to buses in computer systems, the industry has attempted to develop a standard interface known as the Small Computer System Interface (SCSI) which is an ANSII standard interface. At the time this application was in preparation, this interface was in the final stage of being adopted as a standard. The SCSI has been widely accepted in the industry and many products have already appeared which utilize this interface. With this occurring, it is apparent that computer manufacturers with proprietary internal buses will have to provide hardware which will convert signals on the SCSI bus to signals on their own proprietary buses to thereby permit such computers to be coupled to the large number of peripheral devices which are being developed to operate over the SCSI bus. 
     The SCSI bus itself is an intelligent bus in that the data transfer hand shaking between elements on opposite ends of the bus is relatively simple and that relatively complex command block information must be transmitted from the sending to the receiving end of the bus so that both ends will, for example, know how many data bytes of information are to be transmitted over the bus in a given data transfer operation. By doing so, the bus itself can be relatively simple but the control functions which must be performed by the hardware coupling to the bus on either end becomes considerably more complicated. In the situation where a controller is being developed to interface between devices on the SCSI bus and devices on another bus, the complexity is even greater. This is especially true in the event that the data transfer rate on one data bus is significantly different from the data transfer rate on the other of the buses. In situations of this sort, it becomes desirable to provide a buffering function within the controller. In the past, relatively simple buffers have been utilized which, for example, are first filled by the data bus initiating the transfer and then emptied by the bus receiving the transfer. In the event that the buffer is not sufficient to retain the whole data transfer, system efficiency is compromised because only one bus is operating at a particular moment in time. 
     Accordingly, it is a primary objective of the present invention to provide an apparatus useful in permitting transfer of data from one type of computer bus to another type of computer bus which may have a different data transfer rate. 
     It is a further objective of the present invention to provide a controller which permits transfer of data between two dissimilar data buses of differing data transfer rates and at the same time maximize the utilization of both buses during the data transfer. 
     SUMMARY OF THE INVENTION 
     In accomplishing these and other objectives of the present invention, the peripheral controller device hereinafter described in greater detail includes a data buffer which is accessible substantially simultaneously by both data buses coupled thereto. When a transfer of data is from the data bus having the higher data transfer rate to that having the slower data transfer rate, a portion of the data buffer is filled by the higher rate bus. Then, as additional portions of the buffer are filled, the lower rate bus begins and continues to remove data from the buffer substantially simultaneously with the higher rate bus filling other portions of the buffer. Accordingly, both data buses are operating substantially simultaneously until such time as, in certain events, the higher speed bus fills the buffer and must come to a halt until a portion is made available by reason of the fact that it has been transferred to the lower speed data bus and can subsequently be filled again by the higher speed bus. By making the buffer substantially large compared to typical data transfers, it is possible to minimize the number of times when the higher speed bus must go into a mode of waiting until buffer space is available. 
     When a data transfer occurs from the slower speed to the higher speed bus, a somewhat different mode for operating the buffer occurs. In this mode, the slower speed bus transfers data into the buffer. This data transfer continues until one segment of the buffer is filled by the lower speed bus. Then, the higher speed bus is actuated to remove data from the filled buffer segment and transmit it over that bus. The data will be transmitted over the high speed bus until the segment has been emptied. Then the higher speed bus waits for a new segment to be filled. In this mode, the slower speed bus is always operating and the higher speed bus intermittently operates at its maximum speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects, advantages and features of the present invention are described below in connection with the drawings which form a part of the original disclosure wherein: 
     FIG. 1 is a block diagram for the peripheral controller of the present invention; 
     FIGS. 2-4 are detailed circuit diagrams for the MUX DMA control, the SCSI DMA control and the dual access buffer of FIG. 1; 
     FIG. 5 is a state diagram for the buffer to SCSI driver circuitry in FIG. 4; 
     FIG. 6 is a state diagram for the SCSI to buffer control circuitry of FIG. 4; 
     FIG. 7 is a state diagram for the MUX control circuits of FIG. 4; 
     FIG. 8 is a state diagram for the DMA address clock generator controls of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, the circuitry of the present invention is illustrated generally at 10 and comprises that circuitry which couples between the P-E MUX bus 12 and the SCSI bus 14. The P-E MUX bus 12 comprises the MUX bus of a typical Perkin-Elmer computer such as a Perkin-Elmer Model 3210 or other computer in the Perkin-Elmer 3200 Series of computers. The SCSI bus 14 is the standard bus by that name which is described, for example, in the article entitled &#34;SCSI Bus Solves Peripheral Interface Problems&#34; which appeared in the magazine &#34;Mini-Micro Systems&#34; issue of May, 1984, pages 241-246. The circuitry of the present invention comprises a specific implementation for the element entitled &#34;DMA host adapter&#34; appearing in FIG. 3 of the Mini-Micro Systems article referred to. 
     The peripheral controller 10, in accordance with the present invention, includes a microprocessor 20, a random access memory (RAM) 22 and a read only (ROM) 24 all coupled together via an address bus 26 and a data bus 28. In the preferred embodiment, the microprocessor 20 is a &#34;68000&#34; microprocessor manufactured, for example, by Motorola or other supplier of this well known and accepted microprocessor. The RAM 22 is any suitable random access memory which can be utilized in the configuration illustrated for storing data to be used by the microprocessor 20. The ROM 24 is any suitable read only memory for storing the control program for operating the microprocessor 20 in the manner desired. 
     A coupler 30 is provided for buffering data as it is transmitted between the MUX bus 12 and the data bus 28. This data path is provided strictly for the purpose of passing information from the system coupled to the MUX bus 12 to the microprocessor 20. It is used, for example, to cause the microprocessor 20 to initialize the hardware so that a data transfer between the MUX bus 12 and the SCSI bus 14 is started. 
     The microprocessor 20, in initializing a data transfer between the MUX bus 12 and the SCSI bus 14 causes data to be placed into the MUX DMA controller circuit 32 and also into the SCSI DMA controller circuit 34 when such control data is required. The controller circuits 32 and 34 are operative in response to the data received from the microprocessor 20 to control the substantially simultaneously filling of one page in the dual access buffer 36 and removing data from another page in the buffer 36. 
     In controlling data transfer between the SCSI bus 14 and the buffer 36, an SCSI protocol control circuit 38 is utilized. This SCSI protocol circuit, in the preferred embodiment, comprises an NCR 5385 plus supporting circuitry typical of the type shown in engineering application notes for the NCR 5385. The purpose of this circuit, which is known in the prior art, is to provide the necessary control signals for operation on the SCSI bus 14 and to provide an interface which can easily utilized to couple, in the manner illustrated, to some other circuit such as buffer 36. 
     Referring now to FIG. 3, the dual access buffer 36 of the preferred embodiment is illustrated and comprises four 4K×4 RAM circuits such as 1420S-45 circuits. To the left of the buffer 36 is located a plurality of registers 40, 42, 44 and 46 and a pair of line buffers 48 and 50. In the preferred embodiment, the registers 40, 42, 44 and 46 comprise 74F374 circuits and these serve as staging buffers for the data coming from one of the data buses on their way to the buffer 36. Specifically, the register 40 is used in conjunction with the lower byte load signal (LBLD-) to load the lower data byte to be later stored in one location in the buffer 36. This data comes from the SCSI data bus over the lines labeled SCSDAT00-SCSDAT07. The register 42 operates in combination with the upper byte load signal (UBLD-) to store the upper data byte to be later stored in the same location in the buffer 36. Both of the registers 40 and 42 are gated by the SCSI bus unload signal (SCSIULD-). The other control signals which couple to the registers 40 and 42 are generated by the circuitry in FIG. 4 and these will be explained in greater detail later. 
     The registers 44 and 46 couple to the data bit positions of the MUX bus 12 as illustrated in FIG. 1. These inputs comprises the lines labeled DI00-DI15. The registers 44 and 45 are gated by the MUX input load signal (MUXILD-) and by the simultaneous occurrence of the write buffer (WRBUFF) and the synchronized data available signal (SMYDA) indicating that data is good on the MUX bus. When these three control signals are available, the data from the MUX data bus is gated into the registers 44 and 46 and the output thereof thereafter contains signals representative of the data from the MUX bus. 
     The line drivers 48 and 50 couple directly to the data buss 28 coupled to the microprocessor 20. These data bus 28 lines are labeled DB00-DB15. The line drivers 48 and 50 are actuated by the write signal (WRIT-) which comes directly from the microprocessor and the buffer select signal (BUFFER-) which is a signal decoded from the address bus 26 coupled to the microprocessor 20. The data path from the data bus coupled to the microprocessor to the buffer 36 is used primarily for diagnostic purposes thereby permitting selected locations of the buffer to be loaded with predetermined data words. 
     The buffer RAMs 36 are addressed by the address lines generated by the circuitry on FIG. 2 (lines BUFADR00-BUFADR11) and gated by the signal buffer write (BUFFWR-) which is generated in the circuitry on FIG. 3 from either the write RAM signal (WRRAM-) from the circuitry of FIG. 4 or from a delayed decoding of the signals from the microprocessor 20 indicating a transfer from the microprocessor data bus 28 to the buffer 36. 
     The buffer 36 is easily read by placing an address on the address leads (BUFADR00-BUFADR11) and allowing the signal buffer write (BUFFWR-) to go high which will be the case when a right operation is not in process. The data from the buffer 36 will be placed on the buffer data output lines labeled BD00-BD15. The buffer data output lines are coupled to registers 52, 54, 56 and 58 and also to line drivers 60 and 62. The registers 52, 54, 56 and 58 in the preferred embodiment comprise 74F374 circuits. The registers 52 and 54 are utilized to buffer the data from a particular address within buffer 36 and selectively gate one of the bytes to the SCSI data out bus labeled SCSDAT00-SCSDAT07. The registers 52 and 54 are loaded by the SCSI load signal (SCSILD) generated in FIG. 4. The lower byte is placed onto the SCSI data bus by the lower byte send signal (LBSND-) signal and the upper byte is placed onto the SCSI data bus by the upper byte send signal (UBSND-) generated in FIG. 4. 
     Data is unloaded from the buffer to the MUX bus by the registers 56 and 58. The registers 56 and 58 are loaded by the MUX output load signal (MUXOLD-) generated in FIG. 4 and the data in the registers 56 and 58 is placed onto the MUX bus by the concurrence of the read buffer signal (RDBUFF) generated in FIG. 4 and by a synchronized data request signal (SMYDR) from the MUX bus. 
     A data transfer from the buffer 36 to the data bus 28 coupled to the microprocessor 20 is accomplished by gating of the line driver 60 and 62 by the concurrence of the read signal (READ-) and the buffer select signal (BUFFER-). 
     The buffer 36 comprises four 1420S-45 circuits and is arranged in 16 pages with each page comprising 256 two byte long data words. The circuitry of FIG. 2 is operative to address the buffer 36 in this manner. To accomplish the page addressing, the circuitry of FIG. 2 includes two registers 64 and 66 which are coupled via a multiplexer circuit 68 to the four high order buffer address bit positions (BUFADR08-BUFADR11). The microprocessor 20 of the system is operative to load the page address for MUX bus transfers into the register 66 whenever the decoded signal from the decoder 70 on line 72 goes low. When this occurs, the data bus 28 from the microprocessor 20 positions DB08-DB11 are gated into the register 66 and are used for subsequent addressing of a particular page in the buffer 36 whenever a transfer is occurring either from or to the MUX bus. 
     In a similar fashion, the register 64 is utilized to store the page address specified by the microprocessor 20 for use with transfers either to or from the SCSI bus. The register 64 is activated to store the data bus bit positions DB08-DB11 when the decoder 74 causes it&#39;s output line 76 to go low. 
     The decoders 70 and 74 are operative to decode the address bus 26 positions AB01-AB03 coupled to the microprocessor to produce a low signal on the line 76 or 72 when the gating signals DMASCSI- and DMAMUX- are respectively actuated by the microprocessor 20. Accordingly, the microprocessor 20 in accordance with the program running therein can selectively set page addresses into the registers 64 and 66 independently of each other. In this manner, the circuitry of the present invention ensures that an operation occurring on the MUX bus in conjunction with a particular page address in the buffer 36 is different from the page address associated with an operation occurring on the SCSI bus. 
     The remaining low order data bit positions of the address to the buffer 36 is generated either by the DMA address generator 78 or 80 depending on whether activity is occurring on the SCSI bus or the MUX bus respectively. The staring addresses are loaded into the generators 78 and 80 by the microprocessor 20. The DMA address generators 78 and 80, in the preferred embodiment, comprise AMD 2940 circuits. For the DMA address generator 78, whenever the increment SCSI address signal (INSCADR-) goes low, the address appearing on the output lines from the address generator 78 is incremented by one. In a similar fashion, when the DMA address generator 80 receives a signal on the line INCMADE-, the address in the generator 80 is incremented by one. The incrementing of the addresses in generators 78 and 80 makes it possible to easily step from one address to another when accessing the buffer 36 for purposes of either an SCSI bus transfer or a MUX bus transfer. 
     The address bit output of the generators 78 and 80 are coupled via multiplexer circuits 82 and 84 to the eight low order bit positions of the buffer input address lines BUFADR00-BUFADR07. The multiplexers 82 and 84 respond to the line ADRSEL which either gates the output from the generator 78 to the buffer address lines or the output of the generator 80 depending on the level of the ADRSEL. In a similar fashion, the multiplexer 68 gates the page address from the registers 64 or 66 as a function of the level on the line ADRSEL. The page address, as already indicated, appears on the buffer address lines BUFADR08-BUFADR11. 
     The circuitry of FIG. 4 provides most of the control signals for use by the circuitry illustrated in FIGS. 2 and 3. The control signals are primarily generated by four different sets of programmed array logic circuits 86, 88, 90 and 92. The particular manner in which each of these programmed array logic devices is configured will be described hereinafter in conjunction with a more detailed discussion of the manner in which they operate. The programmed array logic circuits 86 is utilized for control of the apparatus with respect to operations on the MUX bus. The programmed array logic circuits 88 is utilized in conjunction with DMA address clock signal generation. The programmed array logic circuits 90 is used in conjunction with transfers from the SCSI bus to the buffer and the programmed array logic circuits 92 is utilized for transfers from the buffer to the SCSI bus. A clock circuit is illustrated generally at 94 which receives a 100 nanosecond clock signal from an external clock circuit (not shown) at its input 96 and generates a 400 nanosecond clock signal at 98. The clock signal is utilized by the circuitry in accordance with the present invention such that for one half of the clock cycle at 98, the circuitry coupled thereto is operative to perform some form of operation relating to transfers between the MUX bus and the buffer. For the remainder of the cycle for the signal at 98, the circuitry coupled thereto is operative to perform operations related to the transfer of data between the buffer and the SCSI bus. In this manner, operations with the MUX bus and the SCSI bus are time multiplexed thereby permitting data transfer on these two buses which is as rapid as possible. 
     As already indicated, the programmed array logic circuit 92 is utilized to control transfers of data from the buffer to the SCSI bus. The programmed array logic circuits 92 sit in a zero state in response to a system reset signal which is generated when power is turned on. The state of the programmed circuits 92 remains in state zero until the enable counters SCSI DMA engine signal (ENCTS-) goes low and the buffer write (BUFWR-) also goes low. In addition, the MUX clock has to be active. When these conditions occur, the state of the program array logic circuit 92 shifts from state 0 to state 1 as illustrated in FIG. 5 and the SCSI read line (SCSIRD-) is activated. The programmed array logic circuits 92 remains in state 1 until the MUX signal goes away, the SCSI clock signal is active and a DMA request signal is received from the SCSI protocol controller on the line DREQ. When these three conditions occur, the state of the programmed array logic circuits 92 changes from state 1 to state 2 and the signal SCSILD- is made active. 
     State 2 is merely a delay state which serves to delay the setting of certain signals thereafter and resetting of other signals thereafter. Once leaving state 2, the programmed array logic circuit 92 cause the signals lower byte send (LBSND-), write DMA acknowledged (WRDACK-) and the DMA write enable signal (DMAWR-) to be activated. At the same time, the SCSI read (SCSIRD-) and the SCSI load (SCSILD-) signals are reset. The programmed logic array circuits 92 then enters state 3 waiting for the DMA request signal from the SCSI protocol controller to go away. When this occurs, the programmed array logic circuits 92 passes from state 3 to state 4 and in this process, the signal lower byte send (LBSND-), the write DMA acknowledge (WRDACK-) and the DMA write (DMAWR-) signals are reset. 
     The programmed array logic circuits 92 remain in state 4 until a new DMA request signal is received from the protocol controller at which time the programmed array logic circuits 92 pass from state 4 to state 5. In that process, the upper byte send (UBSND-) the write DMA acknowledge (WRDACK-) and the DMA write (DMAWR-) signals are made active. This causes the data to be gated from the SCSI bus into the upper byte buffer 42 in FIG. 3. Thereafter, upon the occurrence of the clock line BUFFWR-, the data in registers 40 and 42 are gated into the buffer 36. 
     The programmed array logic circuits 92 remain is state 5 until the DMA request signal from the SCSI protocol controller goes away. The departure from state 5 to state 6 requires that the DMA request from the SCSI protocol controller has gone away and that the data transfer is not complete as determined by the failure of the signal (SDONE-) to be present. When this occurs, the write data acknowledge signal (WRDACK-), the upper byte send (UBSND-) and the DMA write signal (DMAWR-) are reset and the increment storage address signal (INSCADR-) is activated thereby causing the address in generator 78 to be incremented. 
     In state 6, the programmed array logic circuits 92 wait until the multiplex clock cycle turns on and then the state goes to state 1 and the increment storage address signal (INCSCSI-) is reset. While in state 1, waiting for the next transfer request from the SCSI protocol control, if the next address of the buffer to be filled is the last one, the done signal (DONE) is activated. The done signal is utilized when leaving state 5 as no further data transfers are accepted over the SCSI bus until a new address is set into the generator 78 which must be done by the microprocessor 20. Accordingly, on leaving state 5 when the done (DONE) signal is active and the DMA request signal from the SCSI protocol controller is inactive, a write interrupt (WRINT-) signal is activated which interrupts the program in the microprocessor 20 thereby indicating that a transfer from the SCSI bus to the buffer has been completed for the current page of data in the buffer and that if further transfers are necessary, it is responsibility of the microprocessor 20 to reload the address buffers illustrated on FIG. 2 for continuing the data transfer. 
     FIG. 6 is a state diagram showing those states and signal actuations for transfers from the SCSI bus to the buffer. The state diagram is utilized in conjunction with the programmed logic array circuits 90. 
     Upon a system reset, the programmed array logic circuits 90 is at it&#39;s zero state and remains there until such time as the signal buffer read (BUFRD-) and the signal enable counters SCSI DMA engine (ENCTS-) are both active. When this occurs, the state of the programmed array logic circuit 90 changes to state 1. In transferring from state 1 to state 2, the staging register 40 is loaded from the SCSI bus. In state 1, the programmed array logic circuit 90 wait until a DMA request signal from the SCSI protocol controller (DREQ) is actuated. When this occurs, the read DMA acknowledge (RDDACK-) the DMA read (DMARD-) and the lower byte load (LBLD-) signals are actuated and the programmed array logic circuits 90 pass to state 2. In state 2, the programmed array logic circuits 90 wait for the DMA request signal from the SCSI protocol controller to go away. When this occurs, the programmed array logic circuits 90 pass from state 2 to state 3 and in the process reset the signals read DMA acknowledge (RDDACK-) and the lower byte load (LBLD-) signal. 
     Once in state 3, the programmed array logic circuits 90 wait until a subsequent DMA request signal is received from the SCSI protocol controller. When this occurs, the state changes from state 3 to state 4 and in the process, the upper byte load signal (UBLD-) and the read DMA acknowledge (RDDACK-) signal is actuated. At the same time, the data from the SCSI bus is gated into the staging register 42. Thereafter, when the proper gate signal (BUFFWR-) is actuated, the SCSI bus data is loaded into the buffer 36 from registers 40 and 42. 
     State 4 is a wait state for the programmed array logic circuits 90 wherein the state does not change until the beginning of a MUX cycle and the DMA request signal from the SCSI protocol controller (DREQ) is no longer present. When this occurs, the read DMA acknowledge (RDDACK-) the DMA read signal (DMARD-) and the upper load byte signal (ULBD-) are all reset. 
     Once in state 5, the programmed array logic circuits 90 wait for the next SCSI clock and thereafter sets the SCSI unload signal (SCSIULD-) and the SCSI write signal (SCSIWR-) to active thereby causing the staging registers 40 and 42 to be gated to the input of the buffer 36 thereby causing two bytes of data from the SCSI bus to be gated into the addressed location in the register 36. The buffer address is specified by the data in register 64 and generator 78. 
     Once in state 6, the programmed array logic circuits 90 pass immediately to state 7 and in the process the SCSI unload signal (SCSIULD-) and the SCSI write (SCSIWR-) signals are reset. Once in state 7 and if the data transfer is not done as indicated by the presence of the signal (SDONE-), then the programmed array logic circuits 90 pass to state 8 in the process of which the increment SCSI address signal (INSADR-) is activated. State 8 immediately goes to state 1 and in the process the signal increment SCSI address (INCSADR-) is reset. 
     Data transfers continue as defined by states 1-8 until, while in state 1, the signal SCSI DMA interrupt (SDMAINT-) is active. Then the done signal (SDONE-) is activated. Thereafter, a data transfer will occur in accordance with the operations described above in states 2-6. In state 7, the done signal (SDONE-) being active causes control to pass to state 0. In the process of doing so, the read interrupt signal (RDINT-) to the microprocessor is activated to indicate to the microprocessor 20 that the final data for the previously specified page has been stored into the buffer and that further instructions were necessary from the microprocessor 20 to permit further data transfer between the SCSI bus and the buffer. 
     Referring now to FIG. 7, a state diagram is shown for the MUX DMA control program array logic circuits 86. As with the other programmed array logic circuits, a system reset will cause this logic array 86 to go to it&#39;s 0 state. Thereafter, the state changes from state 0 to either state 1 or state 4 depending on the receipt of the enable counters SCSI DMA signals (ENCTM-) and either the read signal (BUFRD-) or the write signal (BUFWR-). When the read signal is received, the state changes from state 0 to state 1 and in the process the multiplex read (MUXRD-) signal is set. Once in state 1, the programmed array logic circuits 86 remain there until the data request signal (MYDR-) is received from the MUX bus. Then the state changes to state A wherein the programmed array logic circuit 86 waits until the next SCSI clock cycle. Thereat, the state changes from state A to state 2 and remains there until the beginning of the next MUX cycle clock. At that time, the state changes from state 2 to state 8 and in the process, the signals MUX outload (MUXOLD-) and the read buffer signal (RDBUFF-) are actuated. These signals cause the data in the location addressed by register 66 and generator 80 to appear at the output of the buffer 36 and to be gated to the output staging registers 56 and 58. Thereafter, the registers 56 and 58 are gated to the MUX bus. 
     State 8 is simply a delay state and the state immediately changes to state 3 for the programmed array logic circuits 86. In the process of going from state 8 to state 3, the buffer data request synchronize line (BUFDRSYN-) is set which is a signal to the MUX bus indicating that the data is good on the bus. At the same time, the MUX outload (MUXOLD-) and the read buffer (RDBUFF-) signals are reset. Once in state 3, the programmed array logic circuits 86 remain there until the data request from the MUX bus (SMYDR-) goes away. Then, if the MUX done signal (MDONE-) is not present, the state changes from state 3 to state 6 and in the process the increment address signal (INCMADR-) is set and the read buffer (RDBUFF-), MUX read (MUXRD-), buffer data request synchronize signal (BUFDRSYN-) and the MUX outload signal (MUXOLD-) is reset. 
     Once being in state 6, if the DMA buffer read signal is not present, the programmed array logic circuit 86 changes from state 6 to state 1. In the process of this state change, the MUX read signal (MUXRD-) is set and the increment multiplexer address signal (INCMADR-) is reset. 
     The data transfer will continue from the buffer to the MUX bus until the MUX DMA address generator 80 produces the done signal (MDONE-) when the last address is placed onto the data output bus therefrom. Then, when the programmed array logic circuits 86 reach state 3 and the synchronized data request (SMYDR-) from the MUX bus goes away, the state will change from state 3 to state 0. In the process of doing this, the MUX DMA interrupt signal (MDMAINT-) is set to interrupt the microprocessor 20 to advise that all the data in the current page has been transmitted from the buffer to the MUX bus and that the MUX DMA Control 32 must be reinitialized if further data transfers are desired. In addition, the buffer read synchronized signal (BUFDRSYN-), the read buffer signal (RDBUFF-) and the MUX outload (MUXOLD-) signals are reset and the DMA busy signal (DMABSY-) is set. 
     If the programmed logic array circuits 86 are in state 0 and a buffer write operation is indicated by the presence of the signal (BUFWR-) and the enable counters signal (ENCTM-) is present, the state changes from state 0 to state 4. The programmed array logic circuits 86 remain in state 4 until the SCSI clock occurs and the data acknowledge signal (MYDA-) is received from the MUX bus. when this occurs, the programmed array logic circuits 86 change from state 4 to state 7 and in the process, the write buffer signal (WRBUFF-) is reset. The state remains in state 7 until the beginning of the next MUX clock cycle whereat the state changes from 7 to 5. In this state change, the buffer data acknowledge synchronized signal (BUFDASYN-), the MUX inload signal (MUXILD-) and the MUX write (MUXWR-) are set. The setting of these signals causes the data on the MUX bus to be gated into the buffer 36. 
     The programmed array logic circuits 86 remain in state 5 until the data acknowledge signal (MYDA-) goes away. Then, depending on the status of the MUX done (MDONE-) signal the state changes either from state 5 to state 6 or from state 5 to state 0. The state change from 5 to 6 occurs when more data remains to be filled into the page in the buffer which is currently being filled from the MUX bus. In this case, the MUX done signal (MDONE-) is not present and accordingly, the state change is from 5 to 6. In this state change, the MUX address is incremented by the increment signal (INCMADR-) being set and the signals BUFDASYN-, MUXILD- and MUXWR- are reset. From state 6, the state change is back to state 4 as the signal BUFWR- is not present and in the process of this state change the write buffer signal (WRBUFF-) is set and the increment multiplex address signal (INCMADR-) is reset. 
     Data transfers will continue from the MUX bus to the buffer until the last address position of the current page is filled whereat the MUX done (MDONE-) signal becomes active and the state change from state 5 is to state 0. When this occurs, the signals BUFDASYN-, MUXILD-, MUXWR- are reset and the signals DMABSY- and MDMAINT- are set. The programmed array logic circuits 86 then return to state 0 and remains there until a further multiplex bus operation is initiated by the microprocessor 20. 
     The DMA address clock generator programmed array logic circuits 88 are configured in accordance with the state diagram of FIG. 8. When the system reset occurs, the circuits 88 are in state 0 where they remain until the enable for MUX DMA registers (DMAMUX-) occurs. When this happens, the state changes from state 0 to state 1 and in the process the clock MUX signal (CLKMUX-) is set. The circuits 88 remain in state 1 until the address strobe signal (AS-) is received from the microprocessor 20. Then, depending on the state of the enable MUX counter signal, the state changes either from 1 to 2 or from 1 to 5. When the enable counters signal (ENCTM-) is not present, the state change is to state 2 and in the process the DMA acknowledge (DMACKM-) is set and the clock MUX (CLKMUX-) signal is reset. The circuits 88 remain in state 2 until the address strobe (AS-) goes away and then the circuits 88 return to state 0. In the process, the signal DMACKM- is reset. 
     When the circuits 88 are in state 1 and the address strobe is present and the enable counter signal (ENCTM-) is present, the circuits 88 change from state 1 to state 5 and in the process the DMA acknowledge signal (DMACKM-) is set and the clock MUX signal (CLKMUX-) is reset. The circuits 88 remain in state 5 until the address strobe (AS-) goes away and then the state changes from 5 to 3. In the process of this state change, the clock MUX signal (CLKMUX-) is set and the DMA acknowledge (DMACKM-) is reset. The circuits 88 thereafter remain in state 3 until the increment multiplexer address signal (INCMADR-) goes active and the enable MUX DMA registers (DMAMUX-) is not present. Then, the state change is from state 3 to state 4. In this state change, the clock MUX signal (CLKMUX-) is reset. 
     The exit from state 4 depends on the status of the enable bus DMA register (DMAMUX-) and the DMA interrupt signal (MDMAINT-). If both signals are present then the state change is from state 4 to state 0. If neither signals are present then the state change is from state 4 to state 3 and in the process the clock MUX signal (CLKMUX-) is set. 
     In the event that the circuits 88 are in state 3 and the DMA MUX signal (DMAMUX-) goes active, the state change is from state 3 to state 1 and, in the process of this state change, the clock MUX signal (CLKMUX-) is set. 
     The following table is a listing of the logic equations for each of the programmed array logic circuits: 
     
         __________________________________________________________________________PAL 100 EQUATIONS__________________________________________________________________________SCSIRD = /SWS0*/SWS1*/SWS2*BUFWR*ENCTS*/CLR                            ; SO→S1 + SWS0*/SWS1*/SWS2*/CLR    ; S1→S2 + /SWS0*SWS1*/SWS2*/DREQ*/CLR                            ; STAY S2 + /SWS0*SWS1*SWS2*/CLR     ; S6→S1WRDACK = /SWS0*SWS1*/SWS2*DREQ*/CLR                            ; S2→S3 + SWS0*SWS1*/SWS2*DREQ*/CLR                            ; STAY S3 + /SWS0*/SWS1*SWS2*DREQ*/CLR                            ; S4→S5 + SWS0*/SWS1*SWS2*DREQ*/CLR                            ; STAY S5DMAWR = /SWS0*SWS1*/SWS2*DREQ*/CLR                            ; S2→S3 + SWS*SWS1*/SWS2*DREQ8/CLR ; STAY S3 + /SWS0*/SWS1*SWS2*DREQ*/CLR                            ; S4→S5 + SWS0*/SWS1*SWS2*DREQ*/CLR                            ; STAY S5LBSND = /SWS0*SWS1*/SWS2*DREQ*/CLR                            ; S2→S3 + SWS0*SWS1*/SWS2*DREQ*/CLR                            ; STAY S3UBSND = /SWS0*/SWS1*SWS2*DREQ*/CLR                            ; S4→S5 + SWS0*SWS1*SWS2*DREQ*/CLR ; STAY S5SCSILD = SWS0*/SWS1*/SWS2*SCSI*DREQ*CLR*SCSIRD                            ; S1→S2 + /SWS0*SWS1*/SWS2*/CLR*SCSIRD                            ; STAY S2INCSADR + SWS0*/SWS1*SWS2*/DREQ*/CLR*/SDMAINT                            ; S5→S6__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 102 EQUATIONS__________________________________________________________________________SWS0    = /SWS0*/SWS1*/SWS2*BUFWR*ENCTS*/CLR*MUX                           ; S0→S1    + SWS0*/SWS1*/SWS2*/DREQ*/CLR*/BUFRD                           ; STAY S1    + SWS0*/SWS1*/SWS2*MUX*/CLR*/BUFRD*DREQ                           ; STAY S1    + /SWS0*SWS1*/SWS2*DREQ*/CLR                           ; S2→S3    + SWS0*SWS1*/SWS2*/DREQ8/CLR                           ; STAY S3    + /SWS0*/SWS1*SWS2*DREQ*/CLR                           ; S4→S5    + SWS0*/SWS1*SWS2*DREQ*/CLR ; STAY S5    + /SWS0*SWS1*SWS2*/CLR*MUX  ; S6→S1SWS1    = SWS0*/SWS1*/SWS2*/MUX*/CLR*/BUFRD*DREQ                           ; S1→S2    + /SWS0*SWS1*/SWS2*DREQ*/CLR                           ; S2→S3    + /SWS0*SWS1*/SWS2*/DREQ*/CLR*/BUFRD                           ; STAY S2    + SWS0*SWS1*/SWS2*DREQ*/CLR ; STAY S3    + SWS*/ SWS1*SWS2*/DREQ*/CLR*/DONE                           ; S5→S6    + /SWS0*SWS1*SWS2*/MUX*/CLR ; STAY S6SWS2    = SWS0*SWS1*/SWS2*/DREQ*/CLR                           ; S3→S4    + /SWS0*/SWS1*SWS2*/DREQ*/CLR                           ; STAY S4    + /SWS0*/SWS1*SWS2*DREQ*/CLR                           ; S4→S5    + SWS0*/SWS1*SWS2*DREQ*/CLR ; STAY S5    + SWS0*/SWS1*SWS2*/DREQ*/CLR*/DONE                           ; S5→S6    + /SWS0*SWS1*SWS2*/MUX*/CLR ; STAY S6__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 104 EQUATIONS__________________________________________________________________________WRINT  = SWS0*/SWS1*SWS2*DONE*/DREQ*/CLR                             ; S5  + /SWS0*/SWS1*/SWS2*SDMAINT*BUFWR*/CLR                             ; S0SCSIWR = SRS0*/SRS1*SRS2*/SRS3*SCSI*/CLR                             ; S5→S6RDDACK = SRS0*/SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; S1→S2  + /SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; STAY S2  + SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; S3→S4  + /SRS0*/SRS1*SRS2*/SRS3*DREQ*/CLR                             ; STAY S4DMARD  = SRS0*/SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; S1→S2  + /SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; S2  + SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; S3  + /SRS0*/SRS1*SRS2*/SRS3*DREQ*/CLR                             ; STAY S4LBLD   = SRS0*/SRS1*/SRS2*SRS3*DREQ*/CLR                             ; S1→S2  + /SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; STAY S2UBLD   = SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                             ; S3→S4  + /SRS0*/SRS1*SRS2*/SRS3*DREQ*/CLR                             ; STAY S4SCSIULD  = SRS0*/SRS1*SRS2*/SRS3*SCSI*/CLR                             ; S5→S6 SCSIRINCSADR  = SRS0*SRS1*SRS2*/SRS3*/SDMAINT*/CLR                             ; S7→S8__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 106 EQUATIONS__________________________________________________________________________SRS0    = /SRS0*/SRS1*/SRS2*/SRS3*BUFRD*ENCTS*/CLR                            ; S0→S1    + SRS0*/SRS1*/SS2*/SRS3*/DREQ*CLR*/BUFWR                            ; STAY S1    + /SRS0*SRS1*/SRS2*/SRS3*/DREQ*/CLR                            ; S2→S3    + SRS0*/SRS1*SRS2*/SRS3*/DREQ*/CLR*MUX                            ; S4→S5    + SRS0*/SRS1*SRS2*/SRS3*MUX/8/CLR                            ; STAY S5    + /SRS0*SRS1*SRS2*/SRS3*/CLR ; S6→S7    + /SRS0*/SRS1*/SRS2*SRS3*/CLR                            ; S8→S1SRS1    = SRS0*/SRS1*/SRS2*/SRS3*DREQ*/CLR                            ; S1→S2    + /SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                            ; STAY S2    + /SRS0*SRS1*/SRS2*/SRS3*/DREQ*/CLR                            ; S2→S3    + SRS0*SRS1*/SRS2*/SRS3*/DREQ*/CLR                            ; STAY S3    + SRS0*/SRS1*SRS2*/SRS3*SCSI*/CLR                            ; S5→S6    + /SRS0*SRS1*SRS2*/SRS3*/CLR ; S6→S7SRS2    = SRS0*SRS1*/SRS2*/SRS3*DREQ*/CLR                            ; S3→S4    + /SRS0*/SRS1*SRS2*/SRS3*DREQ*/CLR                            ; STAY S4    + /SRS0*/SRS1*SRS2*/SRS3*/DREQ*/CLR*MUX                            ; S4→S5    + SRS0*/SRS1*SRS2*/SRS3*MUX*/CLR                            ; STAT S5    + SRS0*/SRS1*SRS2*/SRS3*SCSI*/CLR                            ; S5→S6    + /SRS0*SRS1*SRS2*/SRS3*/CLR ; S6→S7    + /SRS0*/SRS1*SRS2*/SRS3*/DREQ*SCSI                            ; STAY S4SRS3    = SRS0*SRS1*SRS2*/SRS3*/CLR*/DONE                            ; S7→S8RDINT    = SRS0*SRS1*SRS2*/SRS3*DONE*/DREQ*/CLR                            ; S7    + /SRS0*/SRS1*/SRS2*/SRS3*SDMAINT*BUFRD*/CLR                            ; S0__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 108 EQUATIONS__________________________________________________________________________MUXRD  = /MS0*MS1*/MS2*/MS3*/CLR   ; S2→S8  + /MS0*MS1*/MS2*MS3*/CLR    ; SAMUXWR  = MS0*MS1*MS2*/MS3*MUX*.CLR ; S4→S7MUXOLD = /MS0*MS1*/MS2*/MS3*MUX*MYDR*/CLR                              ; S2→S5                              DR*MUXMUXILD = MS0*MS1*MS2*/MS3*MUX*/CLR ; S4→S7                              DA*MUXBFDASYN  = MS0*MS1*MS2*/MS3*/CLR     ; S7→S5  + MS0*/MS1*MS2*/MS3*MYDA*/CLR                              ; STAY 5  + /MS0*/MS1*/MS2*/MS3*MYDA*MDMAINT*/                              ; S0  CLR*BUFWRBFDRSYN  = /MS0*/MS1*/MS2*MS3*/CLR   ; S8→S3  + MS0*MS1*/MS2*/MS3*MYDR*/CLR                              ; STAY 3                              DR ACTIVE  + /MS0*/MS1*/MS2*/MS3*MYDR*MDMAINT*/                              ; S0  CLR*BUFRDRDBUFF = /MS0*MS1*/MS2*/MS3*/CLR   ; S2→S8  + MS0*MS1*/MS2*/MS3*MYDR*/CLR                              ; STAY 3  + /MS0*/MS1*/MS2*MS3*/CLR   ; S8→S3  + /MS0*/MS1*/MS2*/MS3*MYDR*MDMAINT*/                              ; S0  CLR*BUFRDINCMADR  = MS0*MS1*/MS2*/MS3*/MYDR*/MDMAINT*/CLR                              ; S3→S6  + MS0*/MS1*MS2*/MS3*/MYDA*/MDMAINT*/CLR                              ; S5→S6__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 110 EQUATIONS__________________________________________________________________________MS0    = /MS0*/MS1*/MS2*/MS3*BUFRD*/CLR*/                              ; S0→S1  BUFWR*ENCTM  + MS0*/MS1*/MS2*/MS3*/MYDR*/CLR*/BUFWR                              ; STAY 1                              /MYDR  + /MS0*/MS1*/MS2*MS3*/CLR   ; S8→S3  + MS0*MS1*/MS2*/MS3*MYDR*/CLR*/MDMAINT                              ; STAY 3  + /MS)*/MS1*MS2*/MS3*/MUX*MYDA*/CLR                              ; S4→S7                              MUX*DA  + MS0*MS1*MS2*/MS3*/CLR     ; S7-S5  + MS0*/MS1*MS2*/MS3*MYDA*/CLR*/MDMAINT                              ; STAY 5  + /MS0*MS1*MS2*/MS3*BUFRD*/CLR                              ; S6→S1MS1    = MS0*/MS1*/MS2*/MS3*MYDR*/CLR                              ; S1→SA  + /MS0*/MS1*/MS2*MS3*/CLR   ; S8→S3  + MS0*MS1*/MS2*/MS3*/CLR*/MDMAINT                              ; S3  + MS0*/MS1*MS2*/MS3*/MYDA*/CLR*MDMAINT                              ; S5→S6  + /MS0*/MS1*MS2*/MS3*/MUX*MYDA*/CLR                              ; S4-S7  + /MS0*MS1*/MS2*MS3*/CLR    ; STAY SA,S2  + /MS0*MS1*/MS2*/MS3*/MUX*/CLR                              ; STAY S2  + MS0*MS1*MS2*/MS3*/MUX*/CLR                              ; STAY S7MS2    = MS0*MS1*/MS2*/MS3*/MYDR*/CLR*/MDMAINT                              ; S3→S6  + /MS0*/MS1*/MS2*/MS3*BUFWR*/CLR*/                              ; S0→S4  BUFRD*ENCTM  + /MS0*/MS1*MS2*/MS3*/MUC*MYDA*/CLR                              ; S4-S7  + /MS0*/MS1*MS2*/MS3*/MYDA*/CLR*/BUFRD                              ; STAY S4  + /MS0*/MS1*MS2*/MS3* MYDA*MUX/CLR                              ; STAY S4  + MS0*MS1*MS2*/MS3*/CLR     ; S7-S5  + /MS0*MS1*MS2*/MS3*BUFWR*/CLR                              ; S6→S4  + MS0*/MS1*MS2*/MS3*/MDMAINT*/CLR                              ; S5→S6                              STAY S5MS3    = /MS0*MS1*/MS2*/MS3*/CLR*MUX                              ; S2→S8  + MS0*/MS1*/MS2*/MS3*MYDR*/CLR                              ; S1-SA  + /MS0*MS1*/MS2*MS3*MUX*/CLR                              ; STAY SAWRBUFF = /MS0*/MS1*/MS2*/MS3*BUFWR*/CLR*/                              ; S0→S4  BUFRD*ENCTM°  + /MS0*/MS2*MS2*/MS3*/MYDA*/CLR                              ; STAY S4 /DA  + /MS0*/MS1*MS2*/MS3*/MUX*/CLR*MYDA                              ; STAY S4 SCSI  + /MS0*MS1*MS2*/MS3*/CLR*BUFWR                              ; S6→S4DMABSU = MS0*MS1*/MS2*/MS3*MDMAINT*/CLR*/MYDR                              ; S3→S0  + MS0*/MS1*MS2*/MS3*MDMAINT*/CLR*/MYDA                              ; S5→S0  + /MS0*/MS1*/MS2*/MS3*/MYDA*MDONE*                              ; S0  BUFWR*/CLR  + /MS0*/MS1*/MS2*/MS3*/MYDR*MDONE*                              ; S0  BUFRD*/CLRMDMAINT  = MS0*MS1*/MS2*/MS3*MDMAINT*/CLR                              ; S3→S0  + MS0*/MS1*MS2*/MS3*MDMAINT*/CLR                              ; S5→S0  + /MS0*/MS1*/MS2*/MS3*MDONE*MDMAINT*/                              ; S0  CLR__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 112 EQUATIONS__________________________________________________________________________CLKMUX = /MUXSTAO*/MUXSTA1*/MUXSTA2*DMAMUX*/CLR                                ; S0-S1 + MUXSTA0*/MUXSTA1*/MUXSTA2*/AS*/CLR                                ; S1 + MUXSTA0*/MUXSTA1*MUXSTA2*/AS*/CLR                                ; S5-S3 + MUXSTA0*MUXSTA1*/MUXSTA2*/INCMADR*/CLR                                ; S3 + MUXSTA0*MUXSTA1*/MUXSTA2*DMAMUX*/CLR                                ; S3-S1 + /MUXSTA0*/MUXSTA1*MUXSTA2*/MDMAINT*/CLR                                ; S4-S3 + /MUXSTA0*/MUXSTA1*MUXSTA2*/DMAMUX*/CLR                                ; S4-S3DMACKM = MUXSTA0*/MUXSTA1*/MUXSTA2*AS*/ENCTM*/CLR                                ; S1-S2 + MUXSTA0*/MUXSTA1*/MUXSTA2*AS*ENCTM*/CLR                                ; S1-S5 + /MUXSTA0*MUXSTA1*/MUXSTA2*AS*/CLR                                ; S2 + MUXSTA0*/MUXSTA1*MUXSTA2*AS*/CLR                                ; S5MUXSTA0 = /MUXSTA0*/MUXSTA1*/MUXSTA2*DMAMUX*/CLR                                ; S0-S1 + MUXSTA0*/MUXSTA1*/MUXSTA2*/AS*/CLR                                ; S1 + MUXSTA0*/MUXSTA1*/MUXSTA2*ENCTM*AS*/CLR                                ; S1-S5 + MUXSTA0*/MUXSTA1*MUXSTA2*/CLR                                ; S5 + MUXSTA0*MUXSTA1*/MUXSTA2*/INCMADR*/CLR                                ; S3 + /MUXSTA0*/MUXSTA1*MUXSTA2*/MDMAINT*/CLR                                ; S4-S3 + /MUXSTA0*/MUXSTA1*MUXSTA2*/DMAMUX*/CLR                                ; S4-S3 + MUXSTA0*MUXSTA1*/MUXSTA2*DMAMUX*/CLR                                ; S3-S1MUXSTA1 = MUXSTA0*/MUXSTA1*/MUXSTA2*AS*/ENCTM*/CLR                                ; S1-S2 + /MUXSTA0*MUXSTA1*/MUXSTA2*AS/CLR                                ; S2 + /MUXSTA0*/MUXSTA1*MUXSTA2*/MDMAINT*/CLR                                ; S4-S3 + /MUXSTA0*/MUXSTA1*MUXSTA2*/DMAMUX*/CLR                                ; S4-S3 + MUXSTA0*/MUXSTA1*MUXSTA2*/AS*/CLR                                ; S5-S3 + MUXSTA0*MUXSTA1*/MUXSTA2*/INCMADR*/                                ; S3 DMAMUX*/CLRMUXSTA2 = MUXSTA0*MUXSTA1*/MUXSTA2*INCMADR*/                                ; S3-S4 DMAMUX*/CLR + MUXSTA0*/MUXSTA1*/MUXSTA2*ENCTM*AS*CLR                                ; S1-S5 + MUXSTA0*/MUXSTA1*MUXSTA2*AS*/CLR                                ; S5__________________________________________________________________________ 
    
     
         __________________________________________________________________________PAL 114 EQUATIONS__________________________________________________________________________CLKSCSI = /STATE0*/STATE1*/STATE2*DMASCHSI*/CLR                                ; S0-S1 + STATE0*/STATE1*/STATE2*/AS*/CLR                                ; S1 + STATE0*/STATE1*STATE2*/AS*/CLR                                ; S5-S3 + STATE0*STATE1*/STATE2*/INSCADR*/CLR                                ; S3 + STATE0*STATE1*/STATE2*DMASCSI*/CLR                                ; S3-S0 + /STATE0*/STATE1*STATE2*/SDMAINT*/CLR                                ; S4-S3 + /STATE0*/STATE1*STATE2*/DMASCSI*/CLR                                ; S4-S3DMACKS = STATE0*/STATE1*/STATE2*AS*/ENCTS*/CLR                                ; S1-S2 + STATE0*/STATE1*/STATE2*AS*ENCTS*/CLR                                ; S1-S5 + /STATE0*/STATE1*/STATE2*AS*/CLR                                ; S2 + STATE0*/STATE1*STATE2*AS*/CLR                                ; S5STATE0 = /STATE0*/STATE1*/STATE2*DMASCI*/CLR                                ; S0-S1 + STATE0*/STATE1*/STATE2*/AS*/CLR                                ; S1 + STATE0*/STATE1*/STATE2*ENCTS*AS*/CLR                                ; S1-S5 + STATE0*/STATE1*STATE2*/CLR   ; S5 + STATE0*STATE1*/STATE2*/INCSADR*/CLR                                ; S3 + /STATE0*/STATE1*STATE2*/SDMAINT*/CLR                                ; S4-S3 + /STATE0*/STATE1*STATE2*/DMASCSI*/CLR                                ; S4-S3 + STATE0*STATE1*/STATE2*DMASCSI*/CLR                                ; S3-S1STATE1 = STATE0*/STATE1*/STATE2*AS*/ENCTS*/CLR                                ; S1-S2 + /STATE0*STATE1*/STATE2*AS*/CLR                                ; S2 + /STATE0*/STATE1*STATE2*/SDMAINT*/CLR                                ; S4-S3 + /STATE0*/STATE1*STATE2*/DMASCSI*/CLR                                ; S4-S3 + STATE0*/STATE1*STATE2*/AS*/CLR                                ; S5-S3 + STATE0*STATE1*/STATE2*/INCSADR*/                                ; S3 DMASCSI*/CLRSTATE2 = STATE0*STATE1*/STATE2*INCSADR*/DMASCI*/CLR                                ; S3-S4 + STATE0*/STATE1*/STATE2*ENCTS*AS*CLR                                ; S1-S5 + STATE0*/STATE1*STATE2*AS/CLR ; S5__________________________________________________________________________ 
    
     The foregoing description has completely described the hardware and the states of the programmed array logic circuits for generating various of the control signals utilized by the hardware illustrated in FIG. 1. In addition to this hardware and it&#39;s operation, the microprocessor 20 of FIG. 1 executes a program which is operative to oversee the operation of the transfer of data between the MUX bus 12 and the SCSI bus 14. The most pertinent functions performed by this program will now be described in greater detail. 
     The software for controlling the microprocessor 20 is located within the ROM 24. For convenience and for speed of operation, during system power up, the program contained in the ROM 24 is transferred to the RAM 22 and thereafter executed out of RAM. This is done because the RAM access speed is faster than that for ROM and, accordingly, the microprocessor can operate faster when operating on a program stored in RAM 22. 
     The program is set forth below in the C programming language. This program listing is compiled to produce native code for the particular microrpcessor which is utilized and this code is then permanently recorded into the ROM 24 so that on power up it can be transferred to the RAM 22. 
     Once a data transfer has been initiated, for example, to transfer data from a device coupled to the SCSI bus to some computer system facility coupled to the MUX bus 12, the microprocessor 20 performs the necessary functions to control the MUX DMA control 32 and the SCSI DMA control 34 which in turn control the operation of the dual access buffer 36. In such a transfer from the SCSI bus 14 to the MUX bus 12, the microprocessor 20 first causes the SCSI DMA control 34 to be set up for the purpose of transferring a page of data into the buffer 36. In accomplishing this function, the microprocesor 20 loads a page address into the SCSI DMA control circuit 34. This address constitutes the first address in the buffer 36 to which two bytes of data are transmitted from the SCSI bus 14. The data transfer is controlled by the SCSI DMA control circuit 34 in a manner which is evident from the hardware description above until such time as the last available address in the page is filled. When this occurs, as has already been indicated, the SCSI DMA control circuit 34 produces an interrupt to the microprocessor 20 which informs the micro-processor that the page in the buffer 36 has been filled. Assuming that further data is to be transmitted from the SCSI bus 14, the microprocessor 20 then initiates a new command to the SCSI control 34 specifying a new page address for receiving the additional data from the SCSI bus into data locations within that page. The microprocessor 20 also sends a command to the MUX DMA control 32 to cause it to initiate a data transfer from the buffer 36 to the MUX bus 12. The page address sent to the MUX DMA control circuit 32 from the microprocesor 20 is the page address which was previously utilized by the SCSI DMA control circuit 34 as that page has been completely filled. The microprocessor 20 also initiates the data transfer request on the MUX bus 12 in the same manner any other device initiates a transfer on that bus. Thereafter, when the MUX bus is ready for data, the MUX DMA control 32 cooperates in a manner described above with the buffer 36 to place the content of one addressable storage location within the buffer 36 onto the bus 12. This transfer will occur interleaved with transfers from the SCSI to the buffer 36 until the page being emptied by the MUX bus 12 has been completely transmitted. Then, the MUX DMA control 32 initiates an interrupt to the microprocessor 20 advising it that all the data in the page currently being transmitted has been transmitted and then the MUX DMA control 32 waits for a further command from the microprocessor 20. 
     As the SCSI bus is slower than the MUX bus, normally the MUX bus will empty the page it is supposed to empty prior to the SCSI bus filling the next page to be emptied. Accordingly, the MUX DMA control 32 will generally have to wait for a further command until a subsequent interrupt occurs from the SCSI DMA control circuit 34 indicates that the next page to be transmitted is now ready. As already noted, when this interrupt occurs, the MUX DMA control 32 is advised to begin transmitting data from the buffer 36 to the MUX bus 12 and the SCSI DMA control circuit 34 is advised to continue receiving data from the SCSI bus 14 and placing it into a different page in the buffer 36. This mode of operation continues until all the data desired to be transmitted has been placed on the next bus. 
     When a data transmission occurs in the opposite direction, i.e., from the MUX bus 12 to the SCDI bus 14, the MUX DMA control 32 and the SCSI DMA control 34 operate in a slightly different manner. When data is first transmitted by the MUX bus to the buffer, the microprocessor 20 sends control information to the MUX DMA control 32 indicating the page address into which data is first to be transmitted. Thereafter, when the MUX DMA control 32 determines that the page has been filled, it initiates a interrupt to the microprocessor 20. In response thereto, the microprocessor 20 changes the page address of the MUX DMA control 32 to the next available and empty page within the buffer 36 and permits further data transfer from the MUX bus 12 to the buffer 36 into this additional page. The microprocessor 20 also initiates a transfer from the buffer 36 to the SCSI bus 14 by sending control information to the SCSI DMA control circuit 34 indicating the page address of the next page to be transmitted. The SCSI DMA control circuit 34 then causes data to be transmitted from that page in the buffer 36 via the SCSI protocol control 38 to the bus 14. This data transfer will occur until the SCSI DMA control 34 determines that all of the data in that page has been transmitted and then an interrupt is issued to the microprocessor 20. 
     In response to this interrupt from the SCSI DMA control circuit 34, the microprocessor 20 sends an address to the SCSI DMA control circuit 34 indicating the next page to be transmitted. Thereafter, the SCSI DMA control 34 controls the transfer of the data from the next page in the buffer 36 to the SCSI bus 14. 
     Asynchronous to the interrupts of the SCSI DMA control 34 are interrupts from the MUX control 32 because the MUX bus is faster than the SCSI bus. As a result, each time an interrupt is received from the MUX DMA control 32 by the microprocessor 20, the microprocessor has to determine whether there is an available page in the buffer 36 for receiving further data from the MUX bus 12. If such a page is available, then the microprocessor 20 sends the page address to the MUX DMA control 32 permitting it to continue the transfer of data from the MUX bus to the buffer 36. In the event that the buffer 36 is filled, however, the microprocessor 20 discontinues further operation of the MUX DMA control 32 until such time as an interrupt is received from the SCSI DMA control circuit 34 indicating that a page is now available and can be filled. Data transfer continues in this manner until such time as all the data has been transmitted from the MUX bus to the SCSI bus. 
     The following program in the C programming language implements the above described functions in conjunction with the hardware described above. ##SPC1## 
     The foregoing description has been made with particular emphasis on the preferred embodiment as illustrated in the drawings. Those of skill in the art will readily recognize, however, that numerous changes may be made to the embodiment illustrated and described above without departing from the spirit and scope of the present invention as defined by the following claims.