ACC HDLC-NRM (SDLC) User's Guide

Using HDLC-NRM (SDLC) Protocols
Timeout Processing
Chapter 3 33
Poll Timer
The secondary station has a timer configured to allow the protocol
software to detect and inform the application when it is no longer
receiving a poll from the primary. This timer is started when the
terminal is enabled, and restarted whenever a poll is received by a
particular secondary station. When this timer expires, the protocol
software will mark the secondary station down and inform the
application program with an unsolicited status message indicating that
the “Host has stopped polling”.
Terminal Busy Timer
When a terminal is busy (responds to polls or other frames with a
Receiver Not Ready or RNR frame and is unable to accept messages), a
timer known as the terminal busy timer is started. The value of this
timer is approximately 30 to 40 seconds. The timer is stopped when the
terminal is no longer busy. If the timer expires, any transmit messages
which have not been transmitted to the terminal and acknowledged will
be rejected with an error “Terminal busy for too long”.