V.27 Signon by the Primary Node

This scenario assumes that the node attempting signon has been identified as the primary node.

Figure 1. V.27 Modem Protocols: Primary Node Signon

START:
      Allow operator intervention to stop this
      process, otherwise continue until 'DLE ACK0' is received.

      Issue  WRITE (cc)   SOH ENQ
             READ

      If ANY SERIOUS ERROR (e.g. device not available)
            -Terminate immediately
      If UNIT EXCEPTION (contention)
            -Issue message
            -Terminate  (contention not allowed)
      If TIMEOUT
            -Retry immediately the same CCW chain up to 5 times
            -If retry works, go to step CKREAD
            -Otherwise, set timer for 2 minutes
            -When timer expires, retry the original CCW
              sequence up to 5 times

CKREAD: (Successful I/O completion)
      See if data read is 'DLE ACK0'
      If so, send 'I' signon record
       ... normal protocols follow
      If not 'DLE ACK0', go back to START