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State Machines

State Machines

A Token-Ring adapter contains five separate CPU’s in the VLSI (Very Large Scale Integration) chipset on board. Each CPU has its own memory space for program stack, executable code, and data storage. In addition to the state machine memory space the on-board RAM contains the communications buffer memory used by the adapter to copy frames from the ring.

• The REPEAT state:
  Bits that are received on the receiver port are repeated on the transmitter port to the next station on the ring. Certain control bits may be modified as the frame is repeated. The LLC data portion of the frame is never modified. Transition is made to the TRANSMIT state when a frame is queued for transmission by the attached system. In this case the repeat state machine changes the token bit in the access control byte to a one as it is repeated. At this time the Token Holding Timer (THT) is reset to zero allowing 8.9 ms of maximum transmission time.
• The TRANSMIT state:
  The station now transmits a single frame onto the ring. The 802.5 spec implies that a station could transmit more than one frame as long as the THT has not expired. In practice this never occurs. "Transmit one frame then release the token", is the operative practice that is normally seen on a ring. If, during transmission of a frame, the station receives either an invalid EDEL on the token frame, a Beacon, Ring Purge, or a Claim Token MAC frame the transmission ends with the sending of an Abort Delimiter (SDEL+EDEL sequence to end the frame). This is a very rare situation; don’t expect to see it often. Upon completion of the transmission the station transmits idle fill (binary 111..) until it returns to the Repeat state.
• The STRIPPING state:
  When the Transmit state is active on the transmitter port the Stripping state is active on the receiver port. When the receiver port sees an Ending Delimiter it knows that it has successfully stripped its own frame. If an EDEL is not seen before the THT expires (8.9 milliseconds) then the station returns to the Repeat state and records one Lost Frame error for transmission in a Soft Error Report frame. When the EDEL is seen the station returns to the Repeat state.
• The MONITOR and ADAPTER CONTROL machines:
  One of the on-board CPU’s is assigned the task of Active or Standby monitor and another CPU is managing the adapter and controlling the interface to the attached system.

Some of the implementation of ‘state machines’ is very real; in the silicon. Some of the operations are more conceptual. You will never have to interact at the state machine level. This information is for your perspective on card operation and not intended for application in the field.