Timeout Processing

Response Timer

The principal timer used to control the polling cycle is the T1 timer, which is a parameter for each primary station. Its value is configured as described in the next chapter. When the primary station sends a frame with the Poll bit set to the secondary, the T1 timer is started. It is restarted when an I-frame is received without the Final bit. The timer is stopped when a frame is received from the secondary station with the Final bit set. If the T1 timer expires, the primary station will issue another poll (to the same or another secondary station).

If timeouts continue to occur, up to the configured retry limit, without a good response being received, the primary will send a mode setting command (Set Normal Response Mode or SNRM) with the Poll bit set to the secondary to cause a reset. The SNRM command also uses the T1 timer to ensure that it receives a response. If no response is received to the SNRM command for the configured retry limit, the terminal is marked down, and an unsolicited status message is sent to the application.

When down, the primary protocol software will conserve line time by polling the station only once in every slow poll cycle, unless the 'No Slow Poll' option is set for the terminal. The slow poll interval for the HDLC-NRM (SDLC) protocol is 33 seconds. Once the secondary station responds to a poll, it will be reinstated to the normal poll cycle. At the time when it successfully responds to a SNRM command after being down, the protocol software will issue an unsolicited status message to the application with reason code “No error detected”, indicating that the terminal is now operational.

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”.