Abstract:
Disclosed is a token access protocol for a local area network and the architecture for a token access controller to implement this protocol. The protocol provides for network initialization, recovery and addition of nodes to the network. The host of each node has a minimal involvement with maintaining the network and the protocol does not require the host processor to format frames for transmission. Data reception or transmission is facilitated through the use of a Direct Memory Access (DMA) so the host processor is freed to perform other tasks while data is being transmitted or received.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to protocols for access to local area networks (LAN) and more particularly to protocols for token access controllers for local access networks. 
     2. Description of the Prior Art 
     Local Area Networks, which are usually privately owned communication systems with data rates in excess of one hundred thousand bits per second, can have a number of different topologies. A LAN has a number of nodes; each node is some type of device such as a mainframe computer, a printer, a micro-computer or a terminal. A LAN permits nodes to communicate with each other on the network. A node usually has a host processor, some memory and it can have other components such as a printer or a disc drive. 
     Among the more common topologies used for LANs are the star topology, the ring topology, and bus topology (FIG. 1). When a star topology is used (FIG. 1A), all nodes 10 in the network communicate with each other only with the explicit permission of a central node 12. In a ring topology, (FIG. 1B) each node 14 can only communicate directly with one of the two adjacent nodes of the ring. And information that is to be communicate to a non-adjacent node must be transmitted through those nodes between the source of the message and its destination. In a bus topology, (FIG. 1C) each node 16 can communicate directly with any other node through the bus 18. Each topology has its advantages and drawbacks. 
     An advantage of both the star topology and the ring topology is that the protocol for allowing access by a node to the network is relatively simple. The drawbacks of a star topology are a high initial cost, reliability problems, and the difficulty of reconfiguring the network to accept new nodes. The drawbacks with ring topologies are that a ring requires active taps and reliability is not good because if one node of the ring fails, the entire ring ceases to function. 
     On the other hand, a bus topology can be easily reconfigured to accept new nodes, it can be media independent, and the distributed control to the various nodes of the network improves the reliability of the network. 
     A major disadvantage of using a bus is that a complex protocol is required to prevent two nodes from transmitting simultaneously. A number of protocols have been developed to arbitrate access to the bus. Among the more sophisticated of these protocols are token protocols. 
     Token protocols establish a logical loop for the members of the computer network. The four-node bus network shown in FIG. 2A has a token, which is the right to transmit information on the network bus. The token is passed from node to node via a predetermined field. Only the node that currently has the token has access to transmit on the bus except that a node that just received a transmission may transmit a confirmation message called an acknowledgement. Each node has its own unique identity address (MA) and the address of the device it will send the token to after finishing its transmission. For example, if a node having an address set to 19 (MA=19) has the token, all the other devices on the network cannot have access to the bus 16. While node MA=19 (node 19) has the token, it can send a message to any other node in the local network. When node 19 has finished sending its messages, node 19 will then pass the token to the member of the network whose address is loaded in node 19&#39;s next or successor address (NA) register. In this example, node 19&#39;s next address is node 54. When node 54 is finished sending its messages, it will send the token to the device whose address is in device 54&#39;s next address register. In this case, that node is node 87. When node 87 has sent its messages, it will in turn send the token to node 101. Node 101 will return the token to node 19 in this example. Dotted lines 22 show how the token is passed from node to node in a logical loop. This process will repeat itself until the token is lost (e.g., noise on the bus), one of the nodes of this LAN is taken off the network (e.g., its power is turned off), a duplicate token is erroneously created causing two nodes to transmit simultaneously, or a new node is to be added to the network. 
     In the latter case, the addition of a node to the loop requires that the next address (NA) of one of the current nodes must be changed to the address of the device requesting entry into the loop. In turn, this new entrant must have as its next address its predecessor&#39;s former next address. Thus, in FIG. 2B with node 89 entering the logical ring, node 89 will now have as its next address NA=101 and node 87 will have as its next address NA=89. The identity and next address of each node do not have to be in numerical order as as the assignment of next addresses forms the logical loop. 
     A number of implementations of token protocols for bus topologies are available. Generally, the token protocols are implemented by those access controllers. Each node has a token access controller that connects that node to the bus. 
     Most of the bus topology systems that use a token method for allocating access to the bus are machine specific. In other words, these machines are designed to work with each other and with no other machines. One example is the token access controller for the Datapoint Corporation&#39;s computer system. Datapoint has developed a system called ARCNET, which is a LAN using token access controllers for bus arbitration. Furthermore, the current implementation of Datapoint design also requires special additional hardware including an external RAM buffer. ARCNET also requires a special format for its information transmission so only those devices that can use the ARCNET format can be included in nodes on the network. The number of frames that can be put onto the queue for transmission by a node is limited due to the size of the RAM buffer. 
     Accordingly, it is an object of the present invention to provide a token access controller and a token access protocol which interfaces between a host system and any common transmission medium. It is another object of this invention to provide a protocol that easily allows additional nodes to be coupled to the token ring. It is a further object of this invention that the transfer of data to or from the network is accomplished with a minimal amount of involvement with the host system. Furthermore, it is an additional object of this invention to provide a controller that allows recovery of the system in case there are two token on the system or a tokens is lost. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are provided by the token access controller (TAC) and protocol disclosed below. The TAC allows nodes to communicate with each other by transmitting frames over the network bus. Frames are packages of information that include certain sequences for indicating the beginning and the end of the frame and for detecting errors in the transmission of the frame. 
     The preferred embodiment of the TAC is implemented on an LSI integrated circuit. The process of the protocol is controlled in the TAC by a microcontroller and a ROM. Certain registers in the TAC are accessible by the host system, and the TAC contains control circuitry and a direct memory access (DMA) circuit. In addition, there is a transmitter microcontroller (UT) and a receiver microcontroller (UR) controlling a transmitter and receiver respectively. The transmitter microcontroller regulates all transmissions of data to the network bus and certain encryption sequences while the receiver microcontroller directs certain decoding sequences of received data. Generally, the host system can operate independent of the token access controller. 
     The TAC is responsible for initializing the network, rebuilding the network after network failures, performing network diagnostic routines, transmitting data onto the network bus or receiving data from the network bus, sending data received acknowledgements, and passing the token that is used for control of the network. Thus, the host device is free to perform other tasks and only must occasionally interface with the TAC. 
     The TAC performs these functions by having stored within it its identity address (MA) and the address (NA) of the device that it will pass the token to next. During network initialization, each host system loads a unique value into its TAC&#39;s MA to represent its own address. At a certain time, subsequent to entering a network over which no communication is being done, the TAC will start sending inquiries to address MA+1 to see whether a TAC with that address is on the network. If such a node is not on the network, the original TAC will send inquiries to address MA+2, etc., until another node on the network responds. At that point, the TAC that responded has the token and the inquiring TAC will also store that TAC&#39;s address in its next address register. The TAC that has just received the token will repeat this process, until eventually one TAC passes the token back to the first node. At this point the logical loop has been established. Of course, while the network is being established, each host can perform some tasks while waiting for the network to be established. 
     Once the network has been established, then the node with the token will either start transmitting data, or, if it has no data to transmit, it will simply pass the token to the node whose address is in its NA register. If a transmitting node wishes to ensure that the data it has transmitted was correctly received, it will send a special token to the destination node. The destination node, upon receipt of the data, will send a special frame, called an access control frame, back to the transmitting node. While the transmitting TAC is waiting for the access control frame, its host computer can continue to perform other processes. When the TAC is done sending data, it will simply pass the token to the next node. 
     There are certain priority schemes included in this token passing system so that some nodes can send data more frequently than other nodes. This is again largely handled by the TAC and requires only minimal involvement of the host system. 
     The host system has little to do with the transmission or reception of data or formatting data to be transmitted onto the network. Newly received data or data that is about to be sent is stored in the host&#39;s memory in special buffers of a specific size which have been allocated by the host. A link field is created in each buffer so that the link of each buffer points directly to the address of the next buffer, forming a chain of buffers. There is one chain of buffers used for frames that are sent to this TAC and there is another chain of buffers for frames that are to be transmitted by this TAC. The host is responsible for setting up these links in the memory. This allows virtually unlimited amounts of memory to be available to the TAC for reception or transmission of data. 
     The TAC is responsible through its direct memory access (DMA) for writing received data into its host&#39;s memory or reading data to be transmitted from its host&#39;s memory. For transmitting data, the host loads data into the buffers and the next time the TAC receives the token, the TAC will start to use its DMA to read the data from the buffer and send it out onto the network bus. Similarly, if data is being sent to this TAC by another node, then the TAC will read the data from the network bus and directly enter it into the buffers in memory without requiring any assistance from the host (once the buffer linkage has been established). 
     If a node on the network fails, or if a node on the network is simply removed from the network (such as by turning power off), then the TAC that has the removed or failed node&#39;s address in its NA register will then be unable to pass the token to that node. The TAC will then automatically start incrementing its NA register until it can send the token to a node that is still on the network. Thus, the host system is not involved with this recovery process. 
     If the token is lost, no node will be able to send data on the bus. To reestablish the logical ring, each TAC has a TD timer, which monitors the bus for data transmission activity. If there is no activity on the bus after a certain period, this TD timer will time-out and the TAC will then simply arbitrarily assign itself the token. When a TAC&#39;s timer times out, that TAC will then send the token to the next node in its next address register (NA) and the network will be reestablished. Again, the host system has only minimal involvement. Usually, the TD timer of each TAC is set to time out at a different time than the TD timer of the other TAC&#39;s. If two TAC&#39;s TD timers time out simultaneously, they will detect that there are two tokens on the network and remedy the condition. 
     There are three ways to add nodes to a network. These are distributed polling, centralized polling, or central scanning. In distributed polling, each node of the network is responsible for polling a certain address range to determine if there are any new nodes desiring to enter the logical ring. If there are, that node resets its next address to the station that desires to gain entry to the ring and then can either continue to send data or send a token to that station. In centralized polling, one node is charged with the polling task and polls the entire address range. In centralized scanning, each TAC is required to reset its next address register to the address of the next address that a node could possibly have. Each TAC will then poll above its address until it receives a response from another node. Each TAC will then set its NA register to that node&#39;s address. 
     Although the preferred embodiment envisions the use of a custom LSI integrated circuit, this invention could be implemented with a microprocessor and associated hardware and software, or entirely in software. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-C are a drawing of three different Local Access Network (LAN) configurations. 
     FIG. 2A is a drawing of a 4-anode LAN using a token protocol. 
     FIG. 2B is a drawing of the LAN of FIG. 2A after a fifth node has been added. 
     FIG. 3 is a block diagram showing how an integrated circuit embodying this invention is connected to a network. 
     FIG. 4 is a block diagram showing how an integrated circuit embodying this invention can be connected to a network and have a cache memory. 
     FIG. 5 is a block diagram of the logical architecture of a large scale integrated circuit embodying this invention. 
     FIG. 6 is a diagram of the transmitter buffer chain. 
     FIG. 7 is a diagram of the receiver buffer chain. 
     FIGS. 8 through 24 are flow charts of the protocol for this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The novel features of this invention, both as to its architecture and its protocol, together with further objectives and advantages thereof, will be better understood from the following description, considered in connection with the accompanying drawings, in which a presently preferred embodiment of the invention is described by way of example. It is expressly understood, however, that the drawings and description of the preferred embodiment are not intended as a definition of the limits of the invention. 
     The description of this invention is divided into three parts. The first section is the architecture of an integrated circuit embodying the invention. The second section explains the types of frames that may be transmitted on the network and explains the memory buffer structure. The third section explains the token access control protocol for the invention. 
     A. The Architecture 
     FIG. 3 is a block diagram of how a token access controller is typically used to connect a host system to a network. The TAC 30 serves as an interface between the network bus 32 and the host&#39;s 34 internal bus 36. The host must have a processor 38 and memory 40 coupled through the host&#39;s internal bus 36. One or more other devices 42 (such as printers, terminals or disk controllers) may also be connected to the internal bus 36. Generally, a modem 46 or some other interface device is used to connect the TAC to the network bus 32. The network bus can be anyone of several transmission mediums. The TAC/NETWORK control lines 44 and the TAC/HOST control lines 48 connect the TAC to the modem 46 and the host bus 36, respectively. 
     The functions of the control lines (44 and 48) are given in Table 1. These control lines are deliberately chosen because they can directly interface with many different devices in the market. In some applications a simple interface circuit may be needed for the TAC to connect to the host&#39;s bus or the network bus. 
     If the TAC will be used in an application where memory access latency is a drawback, then a two-port cache memory can be used to interface the TAC to the host, as shown in FIG. 4. In FIG. 4, the TAC 30 connects via an interface device 52 to the network bus 32. Because the host memory 40 will often be &#34;dominated&#34; by the disk controller 54, the TAC 30 is provided with a two-port cache memory 56 so adequate memory is available to the TAC for transmitting or receiving data on a timely basis. When the host memory 40 is freed by the disk Controller 54, the host 34 can start writing from the cache 56 into the host memory 40, or vice-versa. Drivers and receivers 58, address decoders 60, host timing 62 and TAC/HOST arbitration 64 circuits are used to interface the TAC and the host&#39;s central processing unit (CPU). These components (58-64) are typical of two-port memories. 
     FIG. 5 is a block diagram of the architecture of the TAC 30. The TAC is controlled by a general purpose microcontroller 70. This microcontroller 70 has a ROM 72 containing the firmware for the TAC, an Arithmetic Logic Unit (ALU) 74, and an accumulator register (ACC) 76. There is a register file 78 with 27 registers. Sixteen of these registers are host visible and eleven registers are invisible to the host. A register is visible if the host can read data directly from the register and/or write data directly into the register. There is also a control logic 80, a direct memory access (DMA) circuit 82 with DMA registers, a First In First Out buffer 86 for received data, a receiver microcontroller (UR) 88 and its associated ROM 90, a transmitter microcontroller (UT) 92 and its associated ROM 94, an acknowledge timer (TA) 96, and a network dead timer (TD) 98. There is also a transmitter 102 and a receiver 100. Internal signal lines coupling these component circuits are shown, with the arrows indicating the direction of information flow. Thus the line from the general microcontroller 70 to the register file 78 is bi-directional. The external signal lines are identified in Table 1. (Symbols marked with an asterisk denote negative logic functions). 
     
                       TABLE 1______________________________________EXTERNAL SIGNAL LINESSYMBOL  NAME             FUNCTION______________________________________SQ*     SIGNAL QUALITY   An active low input                    which signals the TAC                    that a frame may be re-                    ceived. The modem or                    similar device may clear                    this signal if the modem                    receives a signal of a                    quality below a relia-                    bility threshold, ensur-                    ing that the TAC will                    not accept the frame.CS*     CHIP SELECT      Active low chip                    select for CPU control                    of I/O registers.RE*     READ ENABLE      The content of the                    selected register is                    placed on the Data                    Access Lines (DAL)                    when CS* and                    RE* are low.WE*     WRITE ENABLE     The data on the Data                    Access Lines (DAL) are                    written into the selected                    register when CS* and                    WE* are low. (RE* and                    WE* must not be low                    at the same time.)CLK     CLOCK            Clock input used for                    internal timing.MR*     MASTER RESET     Initialize on active low.                    All registers reset to                    zero, except control bit                    ISOL (CR00) is                    set to one.DAL0-7- DATA ACCESS      An 8-bit bi-directionalDAL7    LINES            three-state bus for CPU                    and DMA controlled                    data transfers.RD      RECEIVE DATA     Receive serial                    data input.RC*     RECEIVE CLOCK    This is an external clock                    input. RD is sampled on                    the rising edge of RC*.TC*     TRANSMIT CLOCK   An external clock input.                    TD changes on the                    falling edge of TC*.TD      TRANSMIT DATA    Transmitted serial data                    output.RTS*    REQUEST-TO-SEND  An open collector out-                    put which goes low                    when the TAC is ready                    to transmit either data                    or flags.CTS*    CLEAR-TO-SEND    An active low input                    which signals the TAC                    that transmission                    may begin.DRQO*   DMA REQUEST OUT  An active low output                    signal to initiate CPU                    bus requests so that the                    TAC can output data                    onto the host&#39;s bus.DRQI*   DMA REQUEST IN   An active low output                    signal to initiate CPU                    bus requests so that                    data may be input to the                    TAC from the                    host&#39;s bus.DACK*   DMA ACKNOWLEDGE  An active low input                    from the host CPU done                    in response to DRQO*                    or DRQI*. DACK*                    must not be low if                    CS* and RE* are low or if                    CS* and WE* are low.                    DACK* must be set be-                    fore MR* is set.A0-A15  ADDRESS LINES    Sixteen address outputs                    from the TAC to the                    host&#39;s bus for DMA                    operation.INTR*   INTERRUPT        An active low interrupt   REQUEST          service request output.                    Returns high when the                    Interrupt Register (IR0)                    has been read                    by the host.IA0-    ADDRESS          Four address inputs toIA3     LINE IN          the TAC for host con-                    trolled read/write                    operations with regis-                    ters in the TAC.______________________________________ 
    
     A detailed description of the register file is given in Tables 2-14. Table 2 lists the visible registers while Table 3 lists the invisible registers. 
     
                       TABLE 2______________________________________VISIBLE REGISTERSRegister Address(Hexidecimal)        Name               Symbol______________________________________0            Control Register 0 CR01            Control Register 1 CR12            Status Register 0  SR03            Interrupt Register 0                           IR04            Status Register 1  SR15            Status Register    SR26            Counter Register 0 CTR07            Next Address       NA8            Acknowledge Timer Limit                           TAL9            Network Dead Timer Limit                           TDLA            Control Block Pointer (MSB)                           CBPHB            Control Block Pointer (LSB)                           CBPLC            Next Address Request                           NARD            Access Hold Off Limit                           AHOLTE            Transmit Limit     TXLTF            My Address         MA______________________________________ 
    
     
                       TABLE 3______________________________________INVISIBLE REGISTERSRegister Name           Symbol______________________________________Status Register 3       SR3Enable Transmitter Receiver                   ENTRRTransmitter Control Register                   TCRReceiver Control Register                   RCRScratch Pad 1           AX1Scratch Pad 2           AX2Scratch Pad 3           AX3Direct Memory Access (LSB)                   DMALDirect Memory Access (MSB)                   DMAHLast Buffer Address (LSB)                   LASTLLast Buffer Address (MSB)                   LASTHNext Transmit Register (LSB)                   nxttlNext Transmit Register (MSB)                   nxtthNext Receive Register (LSB)                   nxtrlNext Receive Register (MSB)                   nxtrh______________________________________ 
    
     For the purpose of clarity, several of the invisible registers have two different names even though they are the same register. For example, AX2 and DMAL are the same register. 
     Tables 4-10 define the functions of the registers listed in Table 2. Tables 4-9 are for individual registers; the bit number is given in the left hand column, with the least significant bit denoted as &#34;0&#34; and the most significant bit denoted as &#34;7&#34;. Each bit&#39;s symbol is provided in the second column and the description of each bit&#39;s function is in the third column. There are two tables for Control Register 1 (CR1). Table 5 defines CR1 when the DIAGC bit (CR17) is clear. The other table, which defines the diagnostic tests for the TAC, has been omitted. Table 10 defines the functions of CTRO, NA, TAL, TDL, CBPH, CBPL, NAR, AHOLT, TXLT and MA. 
     
                       TABLE 4______________________________________CR0 - CONTROL REGISTER 0 DEFINITIONSBIT   SYMBOL    DESCRIPTION______________________________________CR00  ISOL      Isolate. Set true on power up or master           reset. Host clears this bit after the           host memory-based TAC control block and           other TAC registers have been set up.           CR00 may be set by the host at any time           (will be ignored if TAC is in diagnostic           state). There is some delay for the TAC           to respond to any change of state           request. A change of state to the           network mode is acknowledged by clearing           the state confirmation status bit (SR21           STATE). Setting ISOL while the TAC is           in the network state will cause a change           of state to the isolate state. The           change of state is confirmed by an           interrupt event bit (IR00-ITM) and the           STATE status bit (SR21) being set. If           the TAC has the token when ISOL is set,           this transaction will be delayed until           the node does not possess the token.           Any in-progress frame transmission will           be completed normally followed by a           normal token pass sequence.CR01  NOINT     Master Interrupt Suppress. When this           bit is clear, the TAC will generate host           interrupt requests (INTR* low) if any           bit in the TAC interrupt request           register (IR0) is set. When NOINT is           set, the interrupt requests are           suppressed, but the TAC will still set           the bits in IR0. Any interrupt request           that has been set will be cleared by the           TAC when IR0 is read.CR02  COPY      Enable COPY mode. When this bit is set           it causes all received data frames to be           accepted and DMA&#39;ed into memory           regardless of destination address.CR03  ILOOP     This bit instructs the TAC to loop data           internally from transmitter to receiver           during the LOOP diagnostic test. ILOOP           must not be set while the TAC is in the           network mode (CR00-ISOL clear).CR04  ITOKON    When this bit is cleared, the ITOK           interrupt (IR02) is suppressed.CR05  RXEN      Receive Data Enable. When clear, the           TAC still makes normal responses to           supervisory frames (scan, token and           access frames), but the TAC will not DMA           any data frame into memory and ignores           the receiver buffer chain. If any data           frame that has an acknowledgement           request (TC=255) is addressed to this           node the TAC will respond with a           &#34;receiver not enabled&#34; access frame.           When RXEN is set, it allows the receive           to DMA appropriate data frames into           memory. RXEN may be arbitrarily set and           reset while in the Network State but           these changes will not take effect until           the current frame being received by this           TAC, if any, is completed.CR06  TXEN      Master Transmit Enable. When this bit           is clear no transmissions will occur and           the transmit buffer chain will be           ignored. When this bit is set,           transmission activity is further           dependent upon TXDEN (CR07).CR07  TXDEN     Data Transmit Enable. Has no meaning           unless TXEN is set. When TXDEN is set           and in conjunction TXEN is set, normal           TAC transmission of data and supervisory           frames will occur. When TXDEN is           cleared and TXEN is set, only data frame           transmission will be suppressed. Token           pass and access frames will still be           transmitted when appropriate.______________________________________ 
    
     
                       TABLE 5______________________________________CR1 - CONTROL REGISTER 1 DEFINITIONSNETWORK STATE SYM-BIT   BOL      DESCRIPTION______________________________________CR10  NEWNA    When this bit is set the TAC copies the          contents of register NAR into the          register NA. The TAC clears this bit          after the function is complete. This          mechanism allows the host to change the          TAC&#39;s successor in the logical ring.          The node&#39;s next token pass will be to          the new NA node.CR11  TOFF     When set causes TAC to ignore timers.          This is not intended to be used in an          operational network but is provided to          support network diagnosis. When set          this control bit disables all automatic          network error recovery.CR12  --       (Not used, Reserved.)CR13  GIRING   This bit instructs the TAC to gain entry          into the logical loop at the next          opportunity (i.e. respond to a token          pass). The INRING status bit (SR20) is          used as confirmation; when INRING is          set, it indicates that the TAC is          participating in a logical loop of at          least two nodes. If the host clears          GIRING while INRING is set, the TAC will          not accept the next token pass and it          will then clear INRING to confirm the          TAC is no longer in the logical loop.CR14  ADRV     When this bit is set, it enables the          sixteen output address lines (A0-A15).          If this bit is cleared the outputs are          tri-state outputs and only function when          DACK* goes low. If this bit is set, the          outputs are always TTL levels.CR15  INIT     When this bit is clear, the TAC will not          attempt to initialize or reinitialize          the network if the network dead timer          (TD) expires. When this bit is set, the          TAC will enter Scan Mode when its TD          timer expires.CR16  PIGT     If this bit is set, the TAC is          instructed to piggyback the token on the          last data frame transmitted.CR17  DIAGC    This bit causes the TAC to enter the          diagnostic mode when ISOL is set. In          the Network State this bit should be          cleared.______________________________________ 
    
     The definitions in this table apply when the TAC is in not in the diagnostic state (CR17=1). In the diagnostic state, the functions of most of the bits in the control register one are dedicated to specific diagnostic states. 
     
                       TABLE 6______________________________________SR0 - STATUS REGISTER 0 DEFINITIONSBIT  SYMBOL     DESCRIPTION______________________________________SR00 BSIZ       These four bits indicate to the TAC theSR01            size of all of the transmit and receiverSR02            buffers in the host&#39;s memory. BufferSR03            sizes are in multiples of 64 bytes. If           BSIZ=0, the buffers are 64 bytes long.           If BSIZ=15, the buffers are 1024 bytes           long. The value is obtained from the           TAC control block contained in the           host&#39;s memory. The control block is           read when the TAC transitions from the           Isolate State to Network State (see           ISOL-CR00).SR04 --         Not used.SR05 L2         An internal one-bit flag that is set           during frame transmission if the length           of the current frame is equal to eight           For data frame transmission this means           the frame has no data field; otherwise           this flag is set if the frame is an           access control frame.SR06 SENDACK    An internal flag set during data frame           reception to indicate that the incoming           frame should be acknowledged (by sending           the appropriate access frame). This           flag is cleared when the acknowledgement           has been transmitted.SR07 LASTF      An internal flag set during data frame           transmission to indicate that the           current frame will be the last frame to           be transmitted before the TAC passes the           token. Five situations can cause this           to occur: (1) ISOL (CR00) is set, (2)           TXDEN (CR07) is clear, (3) the current           frame is flagged by the host as the last           frame in the buffer&#39;s TFSB, (4) the           current token frame count has reached           the TXLT limit, or (5) the TAC detected a           transmitter under-run error. Also if a           piggy-back token is permitted (CR16 set)           and no acknowledge is requested, the           token will be piggybacked on the last           data frame.______________________________________ 
    
     
                       TABLE 7______________________________________SR1 - STATUS REGISTER 1 DEFINITIONSBIT   SYMBOL     DESCRIPTION______________________________________SR10  --         (Not used, reserved.). .. .SR14SR15  RECIDL     This bit indicates that the receiver is            in the idle condition. The idle            receiver condition occurs when the TAC            has detected 15 continuous &#34;ones.&#34;SR16  IRTS*      This indicates the transmitter is            attempting to send a frame.SR17  TAOUT      The TAC&#39;s TA Timer has timed out if this            bit has been set.______________________________________ 
    
     
                       TABLE 8______________________________________SR2 - STATUS REGISTER 2 DEFINITIONSBIT   SYMBOL    DESCRIPTION______________________________________SR20  INRING    This bit is set to indicate the node had           the token and successfully passed it at           least once. This bit can be read to           indicate the logical loop has at least           two nodes.SR21  STATE     Depending on DIAGC (CR17), the TAC is           either in Isolate or Diagnostic State.           When ISOL (CR00) is set, if STATE is           set, STATE confirms that the TAC is not           in the Network State. When ISOL is           clear, if STATE is clear that confirms           the TAC is in the Network State.SR22  TSENT     TSENT is set when the TAC passes the           token. It does not care whether a           piggyback or explicit token pass frame           is sent. TSENT is cleared when the TAC           receives the next frame.SR23  RETRY     An internal flag which is set when           either a data frame or a token pass           frame must be retransmitted. Data           frames are only retransmitted if they           have an acknowledge request and an           acknowledgement was not received. Token           pass frames are retransmitted if no           network activity was detected unless the           TAC is in the scan mode.SR24  ACKRQ     An internal flag is set during data           frame transmission if the TAC has           requested an acknowledgement for the           specific frame. The TAC pauses to await           the ACK/NAK response frame; if the TA           timer expires before the response, a           single retry will occur (see RETRY           SR23). ACKRQ is not cleared until the           beginning of the next data frame           transmission.SR25  TR        An internal flag is set when the TAC           receives a token passed to it. It is           cleared when the token is passed or if           the token is ignored for any reason.SR26  NXTR0     When this bit is set, it indicates the           TAC has the address of the next buffer           and that all prior frames can be removed           from the chain. When NXTR0 is clear it           indicates that the TAC has advanced to a           zero link (end of chain).SR27  NXTT0     When NXTT0 is set it indicates that the           TAC has the address of the next frame to           transmit in its internal register.           However, when clear, it indicates that           the transmit chain internal register           points to the link field of the last           buffer of the last transmitted frame.______________________________________ 
    
     
                       TABLE 9______________________________________IR0 - INTERRUPT REGISTER DEFINITIONSBIT  SYMBOL     DESCRIPTION______________________________________IR00 ITD/M      Network dead or mode change (dual use).           When the TAC is in Network state this           bit is set to indicate no network           activity has occurred before timer TD           timed out. This bit will be set if the           TAC transitions from the Network or the           Diagnostic State to the Isolate State.           The choice between the ITD and ITM           interpretations is easily made based           upon reading the ISOL (CR00) bit.IR01 ITA        Data Frame Transmission Unsuccessful.           This interrupt indicates that a           transmitted data frame with an           acknowledge request was acknowledged.           Either a NAK or no response after two           transmissions will cause this. The           exact cause can be determined by           inspecting the appropriate FSB.IR02 ITOK       This bit is set when the token has been           received.IR03 IREC       Data Frame Received. This interrupt           signifies that a good data frame has           been properly received and DMA&#39;ed into           the buffer chain.IR04 ITRAN      This bit indicates that at least one           data frame has been transmitted.IR05 INS        New successor. The TAC has identified a           new successor (new address in NA) in the           logical loop. This bit is set if the           previous successor failed to respond to           a token pass or if the TAC received a           frame that caused the TAC to scan for           the successor.IR06 IROR       Receiver over-run. The TAC ran out of           buffers or the TAC was denied access to           the host&#39;s bus by the host for too long           causing a loss of received data.IR07 ITUR       Transmitter under-run. The TAC was           delayed access to the DMA channel by the           host long enough to cause the loss of           transmitted data. The frame is not           retried and the token is passed.______________________________________ 
    
     INTR* goes low if any of these bits are set, causing an interrupt unless NOINT (CR01) has been set. 
     
                       TABLE 10______________________________________OTHER VISIBLE REGISTERS DEFINITIONSSYMBOL  DESCRIPTION______________________________________CTR0    Running Limit Counter. Used by the TAC   for Access Hold-Off Limit (AHOLT) checking   and Transmit Limit (TXLT) checking. When   transmitting data frames, CTR0 is used for   TXLT counting; otherwise it is used for   AHOLT counting. The counter runs from   zero to the 8-bit limit value.NA      Next Address. This register shows the   curent (instantaneous) successor node in   the network logical ring. The successor   node may be changed for a variety of   reasons.1.        Any attempted token pass that     fails twice will cause the TAC to     attempt to locate a new successor     by sequentially trying token     passes to successively higher node     addresses beginning with NA+1.2.        A received Scan frame will cause     NA to be set to MA+1. If the next     token pass fails case one above     applies.3.        The host may arbitrarily redefine     NA by using the NAR register and     the NEWNA (CR10) control bit. At     a convenient point the TAC     recognizes NEWNA, copies NAR into     NA, then clears NEWNA as     confirmation. If the next token     pass fails case one above applies.TAL     Acknowledgement Timer Limit. This   register has the value of the maximum   allowed time between frame transmission   and ACK/NAK (if requested), or between   token sent and network activity. The   delay is in increments of 64 times the   period of the clock CLK. If the clock   rate is 2MHz, then TAL may be set in   increments of 32 microseconds (range of 32   micro seconds to 8.2 milliseconds).TDL     Network Dead Timer. This register has the   value of the maximum time without activity   on the network. Same clock source as TAL   and the range is 1 millisecond to 262   millisecond. TDL must be greater than   TAL.CBP(H,L)   Control Block Pointer. A sixteen-bit   pointer that points to the TAC control   block in the host&#39;s memory. CBP must not   be modified while the TAC is in the   network mode. (Hereafter, when two   registers or addresses are used to access   one sixteen bit address, they will be   denoted by xxx(H,L).)NAR     Next Address, Request. This register is   used in conjunction with the NEWNA (CR10)   control bit to cause the TAC to update the   NA register. NAR is loaded by the host   and the host sets NEWNA. This redefines   the node&#39;s successor in the network&#39;s   logical ring. The acceptance of this   update is confirmed when the NEWNA control   bit is cleared.AHOLT   This register is set at a value indicating   the number of access cycles (tokens   received) that must be skipped before a   data frame may be transmitted. This   register is initialized to zero upon   Master Reset going low.TXLT    This register is set at the maximum number   of consecutive data frames the TAC may   transmit during one access cycle. A value   of zero allows the TAC to transmit all   frames queued up to 256. This register is   initialized to zero upon Master Reset   going low.MA      My Address. The TAC receives only frames   with this destination address (along with   the broadcast address) and inserts this   address into the SA field of any   transmitted frame. My address must be set   by the host (range is one to two hundred   fifty-four 254).______________________________________ 
    
     Hereafter, when two bytes are combined to form a sixteen-bit address, they will be denoted by xxx(H,L) instead of xxxH and xxxL. 
     Tables 11-14 illustrate the functions of the Invisible Registers. The bits are numbered, with the most significant bit being 7 and the least significant bit being 0. Bits that do not presently have a use are omitted. 
     
                       TABLE 11______________________________________SR3 STATUS REGISTER 3BIT    SYMBOL     DESCRIPTION______________________________________SR30   N2Z        This bit is set when either TA or TD             timeout.______________________________________ 
    
     
                       TABLE 12______________________________________ENTRR ENABLE TRANSMITTER RECEIVER REGISTERBIT     SYMBOL     DESCRIPTION______________________________________ENTRR3  ETMI       When this bit is set, the transmitter              will be enabled to increment registers              DMAL and DMAH.ENTRR2  ERMI       When this bit is set, the receiver will              be enabled to increment registers DMAL              and DMAH.ENTRR1  ENR        When this bit is set, the receiver 100              (see FIG. 5) is enabled.ENTRRO  ENX        When this bit is set, the transmitter              102 (see FIG. 5) is enabled.______________________________________ 
    
     
                       TABLE 13______________________________________TCR TRANSMITTER CONTROL REGISTERBIT    SYMBOL     DESCRIPTION______________________________________TCR7   THREX      When this bit is set, it indicates the             transmitter holding register is empty             and the TAC can do another DMA.TCR6   TUR        When this bit is set, it indicates a             transmitter under-run has occurred.TCR5   FLAGX      When this bit is set, the transmitter             will transmit flags.TCR4   ABORT      When this bit is set, the TAC aborts the             transmission.TCR3   NOI        This bit is set to indicate that there             is no information in the field.TCR1   XMIT       When this bit is set, the DMA is             enabled.TCR0   XRDY       This bit is set to inform the             microcontroller the transmitter is ready             to send a frame.______________________________________ 
    
     
                       TABLE 14______________________________________RCR RECEIVER CONTROL REGISTERBIT    SYMBOL     DESCRIPTION______________________________________RCR5   RORUN      When this bit is set, there is a             receiver under-run.RCR4   RIF        This bit is set if there is an             information field in a frame.RCR3   RROV       When this bit has been set, the receiver             overflowed.RCR2   B2         This bit is set if the flag at the end             of a frame is received.RCR0   B0         This bit is set if the flag at the start             of a frame is received.______________________________________ 
    
     B. Transmission Format and Buffers 
     1. Transmission Format 
     Information that is transmitted or received on the bus must have a specified format. Frames are used that are quite similar to the industry standard high-level data link control (HDLC). Each frame begins and ends with an eight bit pattern called a flag. This pattern consists of a zero, followed by six consecutive &#34;ones,&#34; and terminates with a zero. Each frame also has a sixteen-bit Frame Check Sequence (FCS) to allow the TAC to determine whether a frame was correctly transmitted. Currently the industry standard cyclic redundancy check (CRC) procedure is used to create the FCS. 
     There are three different types of frames used with this token protocol. The first is the frame used for sending data from one node to another and this type of frame is called a data frame. The second type of frame is used for adding nodes to the network and confirming whether a node received a transmitted frame. This type of frame is called an Access Control Frame. The third type of frame is used for passing the token from one node to another node. This is called an explicit token control frame. A data frame can also be used for passing the token to any node including the node that receives the data frame. This method for passing the token is called a piggyback token pass. 
     The data frame format is: 
     
         F-TC-DA-SA-I-FCS-F. 
    
     Field Names: 
     F=Flag (binary pattern 01111110) 
     TC=Token Control (8 bit) 
     DA=Destination Address (8 bit) 
     SA=Source Address (8 bit) 
     I=Information Field (0 to 4095 bytes or 16 buffers whichever is less) 
     FCS=Frame Check Sequence (16 bit) 
     The Access Control Format is: 
     
         F-DA-AC-FCS-F 
    
     Field Names: 
     F=Flag (binary pattern 01111110) 
     DA=Destination Address (8 bits) 
     AC=Access Control Field (8 bits) 
     FCS=Frame Check Sequence (16 bits) 
     An explicit token control frame format is: 
     
         F-TC-FCS-F 
    
     Field Names: 
     F=Flag (binary pattern 01111110) 
     TC=Token Control (8-bit) 
     FCS=Frame Check Sequence (16-bit) 
     Table 15 gives the functions of the field descriptions. The field symbol is in the left-hand column and its function is in the right-hand column. 
     
                       TABLE 15______________________________________FIELD DEFINITIONS______________________________________SYMBOL    DESCRIPTION______________________________________TC        The token control byte has the dual     purpose of transferring permission to     transmit between nodes and conveying a     request for an immediate acknowledgement     of a frame by its intended receiver.     There is no interaction between the TC     field and the DA or SA fields. Thus the     token may be transferred to one node and     data sent to the same or a different     node, with one single frame. The value     entered into the TC field is determined     by the TAC and does not appear in the     buffer (except for transparent frames).   TC Value           Meaning   0       Token not passed at this           time.   1-254   After current frame, token           belongs to node TC.   255     Immediate ACK requested;           Token not affected.DA        Destination address. A value of     1 to 254 indicates the destination     address of the frame. The value 255 is     the global (or broadcast) address.SA        Source address. The values of 0 and 255     are reserved. A value of 1 through 254     is the address of the node transmitting     the frame.I         Information Field. The host defines the     format and content.FCS       Frame Check Sequence. The FCS     calculation includes all data between     the opening flag and the start of the     FCS, except for 0&#39;s inserted for     transparency. The sixteen-bit FCS is     compatible with the standard HDLC FCS.AC        Access Control. This field contains     supervisory information. This field may     be sent as a command using transparent     mode or received in response to an     acknowledgement request. Its format is     shown below:AC-   CON-CESS  TROL     FIELDBIT#   8        6       543   210Name  SCANF    WIR-     000   NVAL2-NVAL1-NVAL0          ING______________________________________ 
    
     Table 16 defines the functions of the individual bits in the Access Control Field. The bits are numbered, with the most significant being bit number 7 and the least significant bit being bit number 0. 
     
                       TABLE 16______________________________________ACCESS CONTROL FIELDBIT  SYMBOL    DESCRIPTION______________________________________7    SCANF     Scan Mode (Command). If this bit is          set, it indicates that the addressed          node(s) must redefine NA to equal MA + 1          for use on its next token pass.6    WIRING    Wants in ring. This bit indicates the          node that transmitted that frame is not          in the logical ring but would like to          be. It is the logical function of the          transmitting node&#39;s GIRING AND NOT          INRING. (CR13 AND SR20) The receiving          TAC does not act on this information but          merely passes it to the host via the          received frame&#39;s FSB5-3  --        Reserved.2-0  NVAL      An encoded acknowledgement response.          The receiving node will set one of the          following codes depending upon the state          of the last received frame:          000--No error          001--Insufficient buffers for frame          010--Receiver not enabled at frame start          011--Receiver overrun          100--Frame exceeded 16 receive buffers______________________________________ 
    
     In order to avoid having the host format the messages, certain functions are lodged in the receiver and transmitter. Because the TAC will treat all sequences of six successive &#34;ones&#34; as a flag, the transmitter must insert a zero after five successive &#34;ones.&#34; Corresponding to this zero insertion, when the TAC is receiving a frame, zeros must be deleted from the frame following any sequence of five &#34;ones.&#34; The Frame Check Sequence is generated by the transmitter and it is checked by the receiver. Zero insertion and deletion and the Frame Check Sequence generation and verification are well known in the industry. 
     2. The Memory Buffer Structure 
     The token access controller uses a complex memory buffer architecture to allow it to respond to real time problems with respect to its network obligations (e.g., meeting network data and process delay requirements). These memory structures are managed cooperatively by the host and its associated TAC. Memory management functions requiring a real time response (e.g., traversing a chain of buffers in memory) are handled entirely by the TAC. Other non-time critical operations are the responsibility of the host. The TAC does not store any data that has been received or any data that will be transmitted. Instead, the host creates transmit and receive buffers. Access to these buffers is accomplished through a control block. The control block is set up as shown in Table 17 in the host&#39;s memory. The address location given in Table 17 is the offset from the address stored in the Control Block Pointer, CPB(H,L), by the amount indicated in the left hand column. 
     
                       TABLE 17______________________________________CONTROL BLOCKAddress              Function______________________________________0                    NXTRH1                    NXTRL2                    NXTTH3                    NXTTL4                    BSIZE5                    EVT06                    EVT17                    EVT28                    EVT39                    EVT410                   EVT511                   EVT612                   EVT713                   EVT814                   EVT915                   EVT10______________________________________ 
    
     The location of the control block is written into the registers CBPH and CBPL (CBP(H,L)). Because the addresses for the host&#39;s memory are sixteen bits and the internal registers of the TAC are only eight bits, two registers must be used for each memory address. There are sixteen separate memory addresses allotted to the control block. 
     The lowest addresses in the control block (which are CBP(H,L) plus 0 and CBP(H,L) plus 1) give the address for the start of the receive buffer chain NXTR (H,L). NXTR(H,L) is usually stored in the TAC&#39;s registers nxtr(H,L). The next two bytes in the memory are allocated to the address of the transmit buffer chain NXTT(H,L). NXTT(H,L) is usually stored in the TAC&#39;s registers nxtt(H,L). CBP(H,L) plus 4 indicates the size of the buffer (BSIZE). The buffer size is the four least significant bits in BSIZE plus one times sixty four. Thus, the buffer size can range from sixty four bytes to one thousand and twenty-four bytes. The values of NXTR(H,L), NXTT(H,L) and BSIZE are provided by the host. 
     The remaining locations in the control block are allocated to eleven separate event counters. These counters are used for informing the host how many times certain events have occured. The function of these event counters is shown in Table 18. 
     
                       TABLE 18______________________________________EVENT COUNTERSCounter      Description______________________________________0            This counter is incremented        whenever the TAC receives an        access control frame with the        SCANF bit set.1            This counter is incremented        whenever the TAC receives an        acknowledgement indicating        that the first transmission of        the data frame failed but the        second transmission succeeded.2            This counter is incremented        whenever an attempt to        transmit the frame is aborted        due to a transmitter underrun        or the length of the frame        exceeded 16 buffers.3            This counter is incremented        whenever the TD timer times        out.4            This counter is incremented        whenever an access control        frame or a token pass frame is        incorrectly received.5            This counter is incremented        whenever the receiver detects        an error due to an FCS error,        the SQ* line indicates a bad        signal, an incomplete frame        was received, or a data frame        was received when an access        control frame was expected.6            This counter is incremented        whenver the TAC receives a Not        Acknowledge Access Control        Frame (NVAL0- NVAL2 is not set        to 0).7            This event counter is        incremented whenever the TAC        is expecting an        acknowledgement access control        frame and it receives another        type of access control frame        or a token pass frame.8            This counter is incremented        whenever the TAC detects a        duplicate token.9            Reserved10           This counter is incremented        whenever the TAC perceives        that another node has the        identical address. Duplicate        addresses are detected        whenever the TAC reads a        source address that is equal        to the TAC&#39;s MA register.______________________________________ 
    
     The implementation of the transmit buffers, which are set up by the host, is shown in FIG. 6. The first two bytes in these buffers contain the address of the next buffer used as storage of data that should be transmitted (link). The next byte is the transmit frame status byte (TFSB). This byte is written by the TAC. Table 19 shows the functions performed by the transmit frame status byte (TFSB). The next byte in the buffer is the transmit frame control byte (TFCB). The host writes the TFCB. The functions of the byte are shown in Table 20. The next two bytes in the frame give the length of the frame (length). The length includes all the bytes in the frame, including the link size, the frame status byte, the frame control byte, the length bytes and all other bytes. The length bytes are followed by the destination address (DA), which is the node of the logical loop to which the TAC will send the frame contained in the buffer. The source address (SA) is the device sending the data. The remaining space in the buffer contains data bytes if there are any. Those buffers that are shown to the left of the buffer which NXTT(H,L) points to have already been transmitted and can be used by the host unless the link (H) value is 0. 
     The link always points to the next buffer to be transmitted. If a buffer is at the end of the chain, then the contents of that buffer&#39;s link (H) is set to zero. Thus, when the token access controller reads a zero as the content of a link(H), it knows that it has reached the end of the transmission chain. 
     
                       TABLE 19______________________________________TRANSMIT FRAME STATUS BYTEBit    Name        Function______________________________________7      DONE        This bit is set by the TAC              when it is finished transmitting              the frame.6      WIRING      This bit is set to the              corresponding value of the              WIRING bit in the access control              frame.5                  Reserved4                  Reserved3      SELF        When this bit is set, it              indicates that the three least              significant bits were set by              the TAC. Otherwise, the three              least significant bits are              copied from the three least              significant bits of the access              control frame.2-0    VAL         The meaning of these three              bits depends upon the value of              SELF.              When SELF is clear:              000--Frame successfully              transmitted.              001--Insufficient buffers for              transmitted frame.              010--The receiver was not              enabled when transmission              began.              011--The receiver could not              DMA at a sufficient              speed.              100--The frame was longer than              16 buffers in the              receiver&#39;s buffer chain.              When SELF is set:              000--No transmitter error.              010--The transmitter under-run              condition was detected.              011--The end of the              transmit buffer chain was              reached.              100--The frame that was              transmitted was longer              than 16 frames.              001--The transmission failed.______________________________________ 
    
     It should be noted that the VAL bits in the TFSB are determined by the TAC that received this frame when SELF is clear. VAL is determined by this TAC when SELF is set. Thus the receiver mentioned in the description of VAL is the receiver of the destination while the transmitter mentioned when SELF is set is the transmitter of this TAC. 
     
                       TABLE 20______________________________________TRANSMIT FRAME CONTROL BYTEBit     Name        Description______________________________________7       WACK        When this bit is set, the TAC               will not transmit another frame               until it receives an access               control frame or the TA timer               times out.6       FCBLF       The TAC will pass the token at               the end of this frame.5       TRANSP      The TAC will interpret this               buffer to be the exact sequence               of bytes to be transmitted.               The TC and SA generation               functions will be suppressed.4-0                 Reserved.______________________________________ 
    
     The receive buffer chain structure is quite similar to the transmitter buffer chain. See FIG. 7. NXTR(H,L) points to the next buffer available for frames received by the TAC. The first two bytes of the buffer link(H) and link(L) contain the starting address of the next buffer. There is a receiver frame status byte RFSB. When the most significant bit, which is called DONE, has been set by the TAC, that means that a valid frame has been written into the memory (DONE=1). The other bits are not currently used. There is also a receive frame control byte RFCB, which currently has no function. The next two bytes length(H) and length(L) indicate the length of the buffer. Next is the DA (destination address) which should always be either equal to MA of the TAC that received this frame or equal to the global broadcast address two hundred fifty-five unless COPY (CR2) is set. After the DA is the source address (SA), which is the address of the node that sent this frame. The remainder of the frame is information. Those buffers to the left of the buffer that NXTR(H,L) points to are already filled with information and will be evaluated by the host. 
     C. The Flow Charts 
     FIGS. 8 through 24 are the flow charts for the TAC protocol. FIGS. 8 through 10 are the executive flow charts. FIGS. 11 through 15 are the flow charts for transmission of frames. FIGS. 17 through 21 are the flow charts for reception of frames. FIGS. 23 and 24 are flow charts for various subroutines. The TAC has three states: Isolated, Diagnostic, and Network. 
     When power is turned on the TAC is automatically in the Isolated state. In step 110, upon powering up, the microcontroller 70 checks to determine whether the TAC is on the network or is in the Isolated state, by checking to see if CR00 is set. If CR00 is set, the TAC is in the Isolated state 111 and STATE (SR21) is then set to &#34;one&#34; in step 112. In step 114, the microcontroller 70 then goes to see whether DIAGC=1 (CR17). This bit is set by the host if it desires the TAC to check itself by performing one of several diagnostic states. If DIAGC has not been set, the TAC will stay in the Isolated state and the microcontroller 70 will loop back to wait for a change in either ISOL (CR00) (Step 110) or DIAGC (CR17) (Step 114). 
     If DIAGC (CR17) is set, the TAC enters the diagnostic state (115) and the microcontroller 70 performs the diagnostic test or tests that have been selected by the host and stays in the Diagnostic state until DIAGC is cleared. If DIAGC is cleared, (118) ITM is set to inform the host that the TAC has changed states (120). 
     When ISOL (CR00) is cleared by the host, the TAC will enter the Network state (128). Upon entering the Network mode, the first five bytes of the control block (Table 17) are loaded into the appropriate registers 78 of the TAC (130). The four least significant bits of BSIZE must be loaded into the four least significant bits of SR0 (BSZ3-BSZ0) so the TAC knows how long each transmitter and receiver buffer is. The contents of NXTT(H,L) and NXTR(H,L) are loaded into nxtt(H,L) and nxtr(H,L) respectively (130). Next, status register 2 (SR2) and counter register 0 (CTR0) are initialized (130). 
     Next, the TAC must determine whether the transmit and receiver buffers are already set up (134). If the receiver buffer are established (nxtrh is not equal to zero), then NXTRO is set. If the transmitter buffers are established (nxtth is not equal to 0), then NXTO is set. Next TR (SR25) is cleared to indicate that no token has been received, (144) and also STATE (SR21), RETRY (SR23) and TSENT (SR22) are all cleared (146). STATE must be cleared to confirm that the TAC is now in network mode. Next, ACKRQ must be cleared to indicate no acknowledgements have been requested (148). There are also &#34;jump to&#34; points represented by IDLE (150), IDLE1 (152), IDLE2 (154), and IDLE3 (156). These &#34;jump to&#34; locations are points in the flow chart that the microcontroller will jump to upon certain conditions. After these steps, the TA and TD timers must be started and N2Z* must be set to indicate that none of the timers has timed out (158). 
     There are two timers on the TAC. One is used as a timer for acknowledgements (TA 96) and the other is used to determine whether the network is dead (TD 98). If there is no activity on the network (including the passing of tokens) before TD 98 reaches the limit set in the TDL register, this means the network has ceased functioning. As will be explained later, if the TD timer times out, then the TAC tries to reestablish the network. The TA timer is used to time for acknowledgements. If an acknowledgement is requested in the transmit frame control byte (WACK=1) the TAC will not transmit another message until either TA times out or an acknowledgement is received. In addition, N2Z (SR30) is set to indicate neither time has timed out yet. Of course, if TOFF (CR04) is set, the TAC ignores the timers. 
     The TAC now enters the main idle loop (160). (FIG. 9). In this loop, the TAC waits for activity to be detected on the network or for one of the timers to time out. 
     The first thing that the TAC does is check to see if the host has supplied a new successor node. Therefore, it examines NEWNA (CR10). If a new successor node has been provided by the host processor, the TAC then loads NAR into NA and clears NEWNA (162). After clearing NEWNA or if NEWNA was not set, the TAC looks to see if the first byte of a new frame is detected by the receiver (166). The microcontroller does this by polling B0 (RCR0). If B0 is set, this means the first byte of a new frame has been detected, and the microcontroller 70 will jump to the Receiver Flowcharts (FIGS. 17-21). 
     If B0 is not set, the TAC will determine whether TOFF (CR04) has been set (168). If TOFF has been set, the TAC ignores its internal timers (TA and TD). Therefore the TAC will loop back to the start of the main idle loop (160). The only exception to this is if the host has requested the TAC to return to the Isolate state (ISOL=1) and this TAC does not have the token (172). Then, if ISOL is set and TR is cleared, the TAC will immediately enter the isolate state 111 at A (116) (FIG. 8). 
     If TOFF has not been set (168), the TAC will poll its TD timer to see if the network is dead (174); if TD has timed out, the TAC will try to reestablish the network. If TD times out, the microcontroller 70 sets ITD (IR00), which automatically causes INTR* to go low unless NOINT (CR01) has been set (176). INRING (SR20) is cleared to indicate that the TAC is no longer in an operative logical loop (177). Event Counter No. 3 will be incremented by one (178). 
     The TAC whose TD has expired will poll INIT (CR15) to determine whether it is allowed to reestablish the network (180). If the device is not allowed to reestablish the network, then the microcontroller 70 jumps to IDLE3 (156) and restarts TA and TD timers. If the device is allowed to initialize the network (INIT=1), the next step (186) the TAC does is check to make sure that its transmitter is enabled by polling TXEN (CR06). If TXEN is not set, that means that this TAC is not allowed to transmit so it cannot restart the network. Therefore, the microcontroller 70 will jump to IDLE3 (156) and reset the TA and TD timers. If TXEN is set, then the TAC will attempt to reestablish the network by entering the scan mode shown in FIG. 10. 
     The scan mode causes the TAC to keep sending the token control frame onto the network to another node until the TAC receives a response. In scan mode, the microcontroller 70 loads the TAC&#39;s next address NA into the ACC 76 (Step 220) and determines whether NA is set to the maximum possible node address (222). If NA is set to the maximum possible node address (which is equal to &#34;254&#34;), then NA is set to one. If the address in the NA register is the same as the TAC&#39;s address (224), then that means the TAC has been unable to find a successor node. Therefore NA is set to MA+1 (228) and the microcontroller 70 jumps to IDLE (150) (FIG. 8). If NA is not equal to MA, the TAC will then send a token to the node whose address is in ACC 76 by using the SEND TOKEN subroutine (FIG. 24) (230). The TA timer is started (232) to await an acknowledgement. Then the TAC waits for any responding activity on the network. If the receiver detects any activity before TA times out (236) NA is set to the current contents of ACC, TSENT is set, and INS is set (238). The TAC jumps into the receiver mode (FIG. 17) to receive this next frame. 
     The scan mode can be used for polling or scanning for new nodes desiring to enter the logical loop. One or more of the host processors must be programed to perform these functions. 
     If no activity has been seen, then the TAC will check if the TA timer has timed out yet (236). If not, the TAC will continue to monitor the bus until either activity is seen (234) or the TA timer expires (236). If the TA timer expires, the ACC will be incremented by one and the TAC will jump to step 222. 
     Referring back to FIG. 9, if a response is being waited for after a token transmission (184), then the microcontroller 70 will poll RETRY to determine whether this is the first or the second time the token has been transmitted (186). If RETRY is set, that means that the token has been sent twice, which means the token has been lost. If RETRY equals &#34;one,&#34; the microcontroller will jump to Step 182. If RETRY is not equal to one, that means the token has been passed only once. In that case, RETRY will be set (188), and a token frame will be transmitted to the node address identified in NA by the SEND TOKEN subroutine (190). The TAC will then go to IDLE3 (156) and restart the TA and TD timers. 
     If the TSENT is not set (184), then the microcontroller 70 will poll ACKRQ (SR24) to determine whether a response is expected after the last transmission of a data frame (192). If not, the TA timer will simply be restarted (194) and the microcontroller will reenter the main idle loop unless the host requested an exit from the network mode (172). If an acknowledgement request was requested (192), the microcontroller 70 will poll RETRY to determine whether the data frame transmission that is to be acknowledged was transmitted once or twice (195). If the second transmission has already been done (RETRY=1), then the TAC sets the transmit frame status byte (TFSB) to indicate the transmission failed after two attempts (196). An ITA interrupt is triggered by setting IR01 so the host knows that a data frame transmission was unsuccessful (198). The microcontroller jumps to the FRMEND (200) shown in FIG. 15. 
     If RETRY has not been set (195), then there was only one transmission of the data frame. Therefore, the microcontroller will set RETRY 202 and it will retransmit the frame by jumping to RETXM (204) to retransmit the data frame. The procedure for retransmitting is shown on FIG. 11. 
     The receiver flowcharts are shown in FIG. 16 through 21. After receiving the first byte, the receiver microcontroller 88 sets B0 to inform the microcontroller 70 that the first byte has been received (600). The receiver microcontroller 88 handles all of the handshaking with the bus. Next, TSENT is polled to determine whether the token has just been previously passed (602). If TSENT is set, that means the token has just been successfully passed and TSENT will then be cleared, TR will be cleared and INRING will be set (604). If TSENT is not set, this step is skipped. Next, the receiver microcontroller (UR) 88 determines whether this is the frame end (606). The UR 88 can determine if the frame end has been reached by looking for the flag pattern. If the first byte of data is followed by two additional bytes and then the flag, that means that the first byte is a token and the next two bytes are the FCS because the only three-byte frame is a token control frame. If the receiver microcontroller 88 detects the flag at the end of the frame, it sets B2. The microcontroller 70 will examine the token to determine whether the token has been passed to this TAC at TOKEN FRAME (608) (FIG. 17). 
     The procedure for checking whether this token frame is passing the token to this TAC involves first checking to make sure that a good frame has been received (610). This is done by the receiver 100 checking the FCS using standard techniques and by examining the SQ* signal to determine whether the external bus&#39;s signal met a specified signal quality. If the frame is bad, Event Counter No. 4 is incremented to indicate a bad frame was received (612) and the microcontroller 70 jumps to IDLE3 (156) on FIG. 8. 
     If the frame is good, the TAC determines whether there is a duplicate token in the network. This is done by seeing if TR has been set (614). If TR is already set, there are duplicate tokens on the network and Event Counter No. 8 will be incremented to indicate this to the host (616). The microcontroller 70 will then jump to IDLE (150) on FIG. 8. 
     If the token is not a duplicate, it must be examined to see if it was addressed to this TAC. This is done by comparing the value of the received token with the contents of MA (618). If the token was not addressed to this TAC, then the TAC will jump to IDLE1 (152). If the token is addressed to this TAC, then GIRING is polled to see if it has been set (620). If GIRING is set, that means the TAC is allowed to access the logical loop and it will accept the token by setting TR (622). Then this TAC will jump to the transmit mode (250) (FIG. 11). If GIRING is not set, then INRING will be cleared to indicate that this TAC is not in the logical loop (624). The microcontroller will then jump to IDLE1 (152). 
     If the frame end has not yet been received (606), (FIG. 16) the TAC will then determine whether the second byte has been received (624). The receiver microcontroller 88 will continue to look for a flag and if the end of the frame is reached without a second byte, that means a token frame was transmitted. The TAC will jump to TOKEN FRAME subroutine (608) on FIG. 17. 
     If a second byte arrives, (632) the microcontroller 70 will then determine whether this is the frame end by polling B2. If B2 has been set, then the frame is an access control frame and the microcontroller 70 will jump to the ACCESS FRAME routine that starts at step 650 in FIG. 18. If a third byte is received (636) the microcontroller 70 gets the third byte, and indicates to the receiver microcontroller that the byte has been received 88 (Step 642). If the receiver 100 has received the third byte, the received frame is a data frame. 
     If an access control frame has been received (Step 638) (FIG. 18), the TAC will determine whether the access frame is good or bad. This is accomplished by examining the FCS and SQ*. If it is a bad frame, then the processor will jump to BADFRME (611) on FIG. 17, increment the appropriate event counter 612 and jump to IDLE3 (156). 
     If it is a good frame, the TAC will check to see whether this TAC has been waiting for an acknowledgement by polling ACKRQ (654). If ACKRQ is clear then the TAC has not been waiting for an acknowledgement. The TAC then checks to see if the destination address (DA) is equal to MA (656). If DA is not equal to MA, then the microcontroller 70 checks (657) to see if the DA is equal to the broadcast address (which is equal to two hundred fifty-five). If DA is not equal to the broadcast address, then the TAC will go back to IDLE3 (156) on FIG. 8 ignoring the frame. 
     If DA is equal to MA or DA equals the broadcast address, the access control byte will have its most significant bit polled to determine whether the access control frame commands this TAC to enter the scan mode (is SCANF set) when the TAC receives the token (658). If this bit is set, this indicates that the addressed node or nodes must redefine their NA to equal MA+1 (660) and enter the scan mode upon receipt of the token. Then Event Counter No. 0 is incremented (662). The microcontroller 70 then jumps to IDLE3 (156). 
     If DA=MA or if DA equals the broadcast address, and the SCANF bit of the access control byte has not been set, this means that an invalid frame was received. The microcontroller 70 jumps to P (664) and Event Counter No. 7 is incremented (666) and the microcontroller 70 then jumps to IDLE (150) on FIG. 8. 
     If ACKRQ has been set (654), this means that this TAC has been waiting for an acknowledgement. If an invalid frame is received, (663) Event Counter No. 7 will be incremented (668) and the TAC will jump to IDLE (150). 
     After determining that the frame is valid, the TAC polls RETRY to determine whether RETRY has been set. If RETRY has been set, then Event Counter No. 1 is incremented to indicate that the first transmsmission attempt of the data frame failed but the second transmission attempt succeeded (670). After incrementing Event Counter No. 1 or if RETRY had not been set, the Transmit Frame Status Byte (TFSB) value is calculated based upon the Access Control byte and the TFSB is written into the first buffer for the transmitted frame (674). The TAC then analyzes the access control byte to see whether there has been a transmission error. If there is an error, the ITA bit (IRO1) is set generating an interrupt to alert the host that the frame was improperly received by the destination node (674). At this point, the microcontroller 70 will then jump to FRMEND (680). (FIG. 15) 
     If a data frame has been sent, which means a third byte was found at step 642 (FIG. 16), the microcontroller 70 tests to determine whether it is expecting an acknowledgement frame (ACKRQ=1) (690) on FIG. 19. If ACKRQ is set, that means an invalid or bad frame was received and the TAC will wait to receive the end of the frame (692). If a good frame is received, 694 the TAC will jump to L (696) on FIG. 17 to inform the host that two tokens are on the network. If the frame is bad due to an error detected by the FCS or by SQ* (694) then Event Counter No. 5 will be incremented 698 to indicate a bad frame was received. The microcontroller 70 will then jump to IDLE3 (156). 
     If ACRKQ has not been set (690), the TAC will then examine the received data frame to determine whether the destination address (DA) equals MA (700), whether the TAC is in the copy mode (702) which mean this node copies all data frames on the buffer (CR02=1), or whether this is a broadcast frame (DA=255) to be received by all nodes on the network (704). If the answer to any of these is &#34;yes,&#34; the TAC will then take further steps to receive the frame. 
     If the frame is not intended for this node, then the TAC will check to see whether it has received a token through a piggyback pass (706). After testing for the token, the microcontroller 70 will keep looking for the end of the frame, 708 and upon receipt, the microcontroller 70 will determine whether there is a receiver overrun by polling RORUN (RCR5) (710). If there is a receiver overrun, then the IROR (IR06) bit is set to cause an interrupt to the host to indicate that there was a receiver overrun (712). TR is then cleared to indicate that the TAC does not have the token (714). The microcontroller 70 then jumps to IDLE3 (156) on FIG. 8. 
     If there is no receiver overrun, the FCS and SQ* are checked to determine whether it is a good frame or a bad frame. If a bad frame is received, then the microcontroller 70 jumps to Z (718) on FIG. 21. Otherwise TR is polled (720) and if TR is set indicating a token was passed to this node, then the microcontroller will jump to transmit (250). If TR has not been set, then the microcontroller 70 will jump to IDLE3 (156) on FIG. 8. 
     If the TAC is supposed to receive the data frame (DA=MA, COPY=1, or DA=225), then the TAC will check RXEN (CR05) to determine whether the receiver is enabled (722). If RXEN is not set, the microcontroller 70 does the TEST FOR TOKEN subroutine (810). Then AX3 is loaded with the appropriate access control byte (812) to be sent to the node with the token if an Immediate ACK was requested. The microcontroller then performs the RCRSTOP routine (762). 
     In the RCRSTOP routine, the microcontroller waits for the frame end. RORUN is polled (816) and if it is set, the microcontroller jumps to ROR (818). If RORUN is clear, the FCS and SQ* are checked to determine if a bad frame has been received (820). If a bad frame has been sent, then the microcontroller jumps to Y (822) on FIG. 21. Otherwise SDACK is polled (824). If SDACK is set, then Event Counter No. 6 is incremented (826) and the microcontroller jumps to Send Ack/Nak FRM (804) to send the appropriate access control frame. Otherwise the microcontroller jumps to S (800) to see if the node has just been passed the token. 
     If the receiver is enabled (RXEN was set) (722), then the microcontroller 70 will test NXTRO, to ensure that the receiver chain is properly established (726). If the receiver chain has not been established, the microcontroller does the TEST FOR TOKEN routine (728), which is shown in FIG. 22 and then jumps to U (730) on FIG. 20. 
     If the receiver buffers are properly established (726) then the receiver will start inputting the data by using the DMA 82. To set up the DMA, DMA(H,L) is loaded with nxtr(H,L)+8. An internal receiver register LIM is loaded with the value of the buffer size minus 8, CTR0 is set to 8, and the receiver is enabled (ENR=1) (729). The TEST FOR TOKEN subroutine is then preformed (731). Then DA and SA are outputted to the receiver buffer so they can be saved for a possible acknowledgement. Next the contents of nxtr(H,L) is loaded into Last(H,L) (734). SR3, which is used as the register for the buffer count, is cleared (736). 
     Next, the microcontroller 70 looks for the frame end (738) (FIG. 20). If a frame end is received, the microcontroller jumps to ENDRCFRM (740) on FIG. 21. However, if this is not the frame end, the microcontroller 70 examines the internal registers to determine whether the TAC was unable to DMA received data into the host&#39;s memory at a sufficient rate. (RORUN=1) (742). If there are not enough buffers, the receiver 100 is shut down (744), ITOK is triggered (746) and TR is cleared (748). The microcontroller 70 then jumps to IDLE3 (156) on FIG. 8. 
     If there is no overrun (742) the TAC remains in the Receive Wait Loop (750) and checks to see whether ENR is cleared (752). If ENR is cleared, that means that the end of the current buffer has not been reached by the DMA 82, so the TAC will continue to allow a DMA by the receiver microcontroller 88 until either the current buffer is finished or there is a frame end. If the end of the current buffer has been reached, then the microcontroller 70 tests for a frame end (754). If frame end has been reached, then the microcontroller 70 will jump to ENDRCFRM (740) on FIG. 21. 
     If the end of the current buffer has been reached (752) and this is not the end of the frame (754), the internal registers must be updated to get the next receiver buffer. This is done by re-loading the buffer size limit into an invisible register 78, and loading the address of Last(H,L) into DMA(H,L). The current NXTR(H,L) is loaded into nxtr(H,L) (756). If nxtr(H) is &#34;0&#34;, there are no more buffers available (758). In tha case, the microcontroller 70 branches to set the Frame Status Byte (RFSB) to indicate insufficient buffers were available (760). The microcontroller then jumps to RCVRSTOP (762). If nxtr(H) is not equal to &#34;0&#34;, that means other buffers are still available, so internal register SR3, which counts the number of buffers that have been used, will be incremented by one (764). 
     If the value of SR3 does not exceed fifteen, then the receiver 100 will be allowed to continue to DMA data until the new buffer is filled or a frame end is reached. The Last(H,L) is loaded with the contents of nxtr(H,L) (768). The DMA(H,L) register is set to the address contained in the link field of the buffer that was just filled and the receiver is enabled (770). The microcontroller 70 then jumps to the Receive Wait Loop (750) and tests for the Frame End (738). 
     As noted earlier, if a frame end is found while in the Receive Wait Loop (750), the microcontroller 70 jumps to ENDRCFRM (740) (FIG. 21). The microcontroller 70, checks to see if the frame is good by checking the FCS and the SQ* (772). Next, if the frame is bad, SDACK is cleared (774), Event Counter No. 5 is incremented (776) and the microcontroller jumps to IDLE 150 (FIG. 8). 
     If the FCS and SQ* indicate a good frame was received, the Frame Length is calculated by adding the number of bytes in the received frames information field, and the DA, SA and TC fields to 6 plus two times the number of buffers used for that frame (CTR0) (Step 775). The number of bytes in the received information field, and the DA, SA and TC fields is counted by the DMA and is stored in an invisible register. Next, Last(H,L) is loaded into the DMA(H,L) (776) the Receiver Frame Status Byte is read (778), and the last buffer&#39;s link field is loaded into nxtr(H,L) (780). Then nxtrh is tested to determine whether it is zero (782). If nxtrh is zero, then Last(H,L) is loaded into nxtr(H,L) (784) and NXTR0 is cleared (786). If nxtrh is not zero (782) then step 784 and step 786 are skipped. The RFSB is written into the first receiver buffer used for this frame to indicate a valid data frame was received (788). IREC (IR03) is set to indicate a frame has been received (790). 
     In addition, the TAC checks to see if the source address SA is equal to MA (792). If SA=MA, this indicates two nodes having the same address exist on the network. If SA does equal MA, then SDACK is cleared to indicate not to send an acknowledgement (794) and Event Counter No. 10 is incremented so that the host will know about the duplicate node addresses (796). Control then goes to IDLE 150 on FIG. 8. 
     If SA is not equal to MA, the TAC then polls SDAK (798). If SDAK has not been set, that means no acknowledgement need be sent and the microcontroller will then jump to S (800) on FIG. 19. If SDACK is set, an acknowledgement frame is sent by using the SEND ACK/NACK FRM subroutine (804). Before the subroutine is called, the &#34;no error&#34; access control byte is loaded into AX3 (802). 
     The transmitter flow charts are shown in FIGS. 11-15. Upon entering the transmit mode (250), the TAC clears TSENT, RETRY, and STATE (252). Next, the TAC polls ITOKON (CR04) to determine whether to trigger an ITOK interrupt (254). If set, an ITOK interrupt is triggered (256) to indicate to the host that a token has been received by the TAC. Then the TAC checks to see if TXEN (CR06) is set (258). If TXEN is not set, the transmitter is disabled so the microcontroller 70 jumps to IDLE (150) (FIG. 8). Otherwise, if the transmitter is enabled, the TAC polls TXDEN (260). If TXDEN is set (CR07) normal transmissions are allowed; however, if TXDEN is clear, that means only access frames and token frames may be sent and data frame transmissions are suppressed. If TXDEN is clear, the microcontroller 70 will jump to J (262) on FIG. 15 and simply send a token. 
     If TXDEN is set, the next thing that is checked is whether CTRO equals the access hold-off limit (AHOLT) (264). Register AHOLT contains the number of times this TAC must pass the token before it is allowed to send data on the network. Thus the network has a priority system since some members of the network can transmit every time they have the token while others can only transmit once every nth reception of the token, where n is equal to the number in AHOLT. If the CTR0 is not equal to AHOLT, this means that this TAC cannot transmit a data frame. Therefore, the token must be passed and its CTR0 must be incremented by one (266). The microcontroller must then jump to J (262) (FIG. 15). 
     If AHOLT=CTR0, CTR0 is cleared (267), NXTT0 is polled (268) to determine whether the transmit buffer chain is already established. If NXTT0 is set, it indicates that the TAC has the address of the start of the next frame to transmit in nxtt(H,L). If NXTT0 is clear, then the transmit chain must be established (270) in the same manner that the receiver buffer chain was established. If Last(H) equals zero (272), that means there are no frames to be sent and the microcontroller branches to J (262) on FIG. 15 to send a token. If Last(H) is not equal to zero, that means the there are data frames to be transmitted and the last link is loaded into nxtt(H,L) to establish the transmit buffer chain (276). If NXTT0 is set (268), then nxtt(H,L) is stored in Last(H,L) 274. 
     Next, NXTT0 is set to indicate that the transmission chain buffers are established and LASTF is cleared (278). The CTR0 is incremented by one (280), SR3 (which will count the number of buffers transmitted) is cleared, and DMA(H,L) is set to NXTT(H,L)+2, which is the address for the TFSB. The TFSB is discarded and TFCB and length are read from the buffer. The length and TFCB are set up and stored in the appropriate locations in the invisible registers 78 (Step 282). The transmitter count TXCNT which is an invisible register is set to the length of the buffer minus six. 
     Next, L2 is cleared and ACKRQ is cleared (284) (FIG. 12). The TAC then determines whether the next frame is the last one to be sent (286). 
     The TAC must poll several bits to determine whether the next frame is the last to be transmitted. The TAC determines whether it should drop out of the Network state. It does this by checking ISOL. If ISOL is set, this will be the last frame. Next, TXDEN is polled to see if it is set. If TXDEN is set, data transmissions are still permitted, but if TXDEN is not set, then this means that this will be the last data frame transmitted. Next, FCBLF (bit six of TFCB) is polled to see if it has been set. If FCBLF is set, that means the host has alerted the TAC that this is the last frame to be sent. Next, CTR0 is compared with the TXLT to determine whether the maximum number of frames that this TAC can transmit on one possession of the token has been reached (296). If the maximum number has been reached, then this again means that this is the last frame. 
     If any of these conditions occurs, the &#34;last frame&#34; routine must be performed. First the last frame flag LASTF must be set (298). Next, the most significant bit of the TFCB (WACK) is polled to decide if the host wants the TAC to wait for an acknowledgement (300). If so, then the token control field is set to &#34;255&#34; and ACKRQ is set (302). If the host does not indicate an acknowledgement is desired then the TAC will poll the PIGT to determine whether a piggyback is desired (304). If PIGT is set, then AX3 is set equal to NA and TSENT is set (306). 
     If the maximum number of frames has not been reached, then the WACK bit of TFCB is polled to determine whether the host wants an acknowledgement (297). If an acknowledgement is desired, then the token control field is set to &#34;255&#34; and ACKRQ is set (302). If no acknowledgement is desired, then TC is set to &#34;0&#34; to indicate that no token pass or acknowledgement has been requested (303). 
     Finally, after Steps 306, 302, or 303, the transmitter must be set up (308). Next, the transmitter must be enabled (310). 
     Next, the TAC must examine the TFCB to determine whether a transparent frame is to be sent. This is done by polling TRANSP (312) (FIG. 13). 
     If a normal data frame is to be transmitted, then the TC field which is stored in AX3 is transmitted out onto the LAN (318). Next, the destination address byte is sent onto the network (322). Then MA is transmitted onto the network (324). This will be the source address for the frame. At this point the transmit loop is entered (326) (FIG. 14). 
     If a transparent frame is to be sent (TRANSP=1) (312), ACKRQ and TSENT are cleared (332). Then L2 is polled to see how long the frame is. If the length of the frame is two bytes (334), this frame is then transmitted (366) and the microcontroller jumps to TXLOOP (326). 
     If the frame is longer than two bytes, the DMA is used to get the first byte (the token control field) and this token control byte is transmitted (344). This byte is examined to determine whether an acknowledgement request is expected (348). If the first byte has been set to &#34;255&#34;, ACKRQ will be set (350). If this byte is not equal to &#34;255&#34;, then the microcontroller will look to see if this byte equals zero (352). If TC equals zero, that means the transparent frame does not affect the token. If TC is not equal to zero, that means the token is being sent as a piggyback and TSENT and LASTF are set (354). 
     After transmitting a two-byte access control frame (366), or after an acknowledgement request is sent (350) or after the TSENT LASTF is set (354), the flowchart branches to TX LOOP on FIG. 14 to enter the transmitter loop (TX LOOP) (326). The first step of the transmitter loop is get the next buffer address from the current link field (326). If the next buffer address most significant byte (Link(H)) is equal to zero (360), this means the end of the buffer chain has been reached and NXTT0 must be cleared (362) and LASTF must be set (364) to cause a token pass at the end of the transmission of this frame. Otherwise the transmitter buffer addresses must be updated (366) and SR3 must be incremented (368). After either of these two processes, the flowchart enters the TSTTUR part of the TXLOOP. 
     The first thing that is done in TSTTUR is the TUR is polled (370). This bit is set when the host denies the DMA access to the memory for a long enough period that the TAC cannot get data from the transmit buffers quickly enough to transmit the data out to the network. 
     If an underrun condition exists, Event Counter No. 2 is incremented (372). As soon as the transmitter is ready, the TAC selects the transmit frame status byte to indicate a transmitter underrun by setting SELF and by setting the three least significant bits to &#34;010&#34; (376). This value is then entered into the host&#39;s memory at the TFSB address (378). Next, ITUR (IR07) is set causing an interrupt to the host to indicate that there was a transmitter underrun (380). The microcontroller 70 then jumps to the Frame End (200) (FIG. 15). 
     If there was no transmitter underrun error (370), then the microcontroller 70 will check to see whether the transmitter is ready (382). If it is, that means a frame is still being transmitted so the TX WAIT loop is entered (384) and ENX is polled to determine whether the DMA is still in process (386). IF DMA is still being done (ENX=1), the flowchart branches back again to check for an underrun condition (370). If the transmitter DMA has stopped (386), then the microcontroller 70 will wait six clock cycles (388). Next, XRDY is polled to determine whether the end of the frame has been reached (390). If the end has been reached, TAC branches back over to test for whether an acknowledgement is expected (393). 
     If the end of a frame has been detected at either step 382 or step 390, ACKRQ is polled to determine whether an acknowledgement is expected (393). If an acknowledgement is expected, then the microcontroller 70 will jump to IDLE3 (156) on FIG. 8 to wait for the acknowledgement. If ACKRQ is clear, the microcontroller writes the Transmit Frame Status Byte into the appropriate transmit buffer (401) and jumps to FRMEND (200). 
     If XRDY is clear (390), NXTT0 is polled to determine whether there is an additional buffer to be sent (392). If NXTT0 is still set, then the buffer count (SR3) is polled to determine whether sixteen or fewer buffers have been sent (394). If more than sixteen buffers have been sent, then there is an error and ABORT is set (396), Event Counter No. 2 is incremented to indicate too many buffers wer transmitted (398), and the TFSB is set to indicate too many buffers were sent (400). Then the program branches to write this TFSB (378). If less than sixteen buffers have been sent, then the DMA 82 is reset to transmit another buffer (396). ENX is then set (397) and the microcontroller 70 branches to TXLOOP (326). 
     If NXTT0 is clear (392), SELF (TFSB) will be set and the three least significant bits of TFSB will be set to &#34;010&#34; to indicate the end of chain has been reached (399) and ABORT is then set (395). The flowchart then branches to Write the TFSB (378). 
     In the FRMEND rountine, (200) (FIG. 15), RETRY is cleared (402). Next, the TFSB is tested to determine whether a transmitter underrun has occurred, or more than sixteen buffers have been transmitted (404). If either error occurred, then TSENT must be cleared and NXTR0 must be set (406). If neither of these errors occurred, then the address of the next transmit buffer is loaded into DMA(H,L) (408). LASTF is polled to determine whether this is the last frame to be sent before passing the token (410). If it is not the last frame, then the microcontroller 70 jumps to I (412) on FIG. 11. If LASTF was set or the TFSB indicated a transmitter underrun or too many buffers were sent, then a ITRAN interrupt will be triggered by setting ITRAN (414). CTR0 is cleared (416) and TSENT is polled to determine whether the token has been passed (418). If the token has been passed, the microcontroller 70 jumps to IDLE3 (156) (FIG. 8). Otherwise TSENT is set (422) the SENT TOKEN frame subroutine is called (424) (FIG. 23), and upon returning the flow chart jumps to IDLE2 (154). These last three steps cause an explicit token pass. 
     The SEND ACK/NAK frame subroutine (FIG. 22) is the routine required to send an access control frame to acknowledge receipt of a message. First SENDACK (SR06) is cleared (850). Then the transmitter determines whether it is enabled by polling TXEN (CR06) (852). If the transmitter is not enabled, then microcontroller 70 jumps to IDLE1 (152). (See FIG. 8) If the transmitter has been enabled, then the microcontroller 70 polls GIRING (CR13) (Step 854). If GIRING is set, then INRING is polled (856). If GIRING is set and INRING is clear, then WIRING is set (858). 
     If GIRING is not set (854), or if INRING is set (856), or after setting WIRING (858) XMIT is set, NOI is set and FLAGX is set to enable the TAC to transmit (890). The TAC then waits for THREX to be set (892). Next the transmitter gets the SA of the data frame that was just received to use as the destination address code for the acknowledgement (894). Next, (896) and the TAC waits until THREX is one (896) and then transmits the appropriate Access Control Field which has already been setup by the microcontroller before this subroutine head been called (898). XMIT is then cleared, NOI is set and FLAGX is cleared (900). The TAC then waits until XRDY is cleared (902). When XRDY is clear, the microcontroller tests TR to see if the TAC has the token (904). If it has been set the microcontroller will continue to transmit (250) but if the TAC does not have the token then control will go to IDLE1 (152) on FIG. 8. 
     The TEST TOKEN routine is used for determining what type of token the TAC has received (970) (FIG. 23). If the TC field is equal to &#34;255&#34; (972), that means an acknowledgement request is desired. Therefore, the microcontroller 70 will compare DA to this TAC&#39;s MA to determine whether the frame is addressed to this node (974). If it is, then the TAC will set SDACK (976). If it is not, SDAK will be cleared (978). The microcontroller 70 returns by jumping back to the point of the flowchart where the subroutine was called. If the token control is not equal to &#34;255&#34;, then SDACK is automatically cleared (980). At this point, the subroutine called &#34;test for piggyback token,&#34; TST4PIGYTC, is entered (982). Here the TC is checked to see if it the token has been sent to this node (984). If the token is not equal to MA, the microcontroller returns. Otherwise, the microcontroller 70 determines whether it is allowed to accept the token by looking at GIRING (986). If GIRING is set, that means the TAC is allowed to accept the token and TR is set (988). If GIRING is not set, then INRING is cleared to show that the TAC is not in the ring (990). 
     The SEND TOKEN routine (1010) involves using NA as the TC byte (FIG. 24). NA is stored in AX2 (1011). There is also an ALTERNATE SEND TOKEN procedure (1012), which is used if TC is provided in the accumulator. Then XMIT is set, FLAGX is set and NOI is set (1014). The transmitter waits for the holding register to empty by polling THREX (1016). When the transmitter is ready, the transmitter transmits the TC (1018). XMIT is then cleared, FLAGX is cleared (1020) and the microcontroller 70 waits for XREADY to become clear (1022). When XREADY is clear, the microcontroller returns. 
     In summary, the present invention provides a system which permits several nodes to communicate with each other over a common external bus through a token protocol. The architecture for a large scale integrated circuit token access controller is also disclosed. The protocol provides for network initialization, network recovery and for the addition of nodes to the network.