ACC HDLC-NRM (SDLC) User's Guide

SDLC Loop Mode Option
Operation
Appendix B 65
Operation
When one terminal on a port is enabled, then polls (UP frame) with the
go-ahead signal will be transmitted. When all terminals are disabled no
more polls will be transmitted.
Once a terminal is enabled SNRM command frames will be transmitted
to that terminal at 10 second intervals until a UA response is received.
When a terminal is disabled, no more frames will be transmitted with
that terminal’s address (there is no disconnect procedure).
Response Timers
The T1 timer is used as in normal HDLC/NRM operation, to time the
response to a frame transmitted with the P-bit set to 1.
Because in loop mode there are usually no frames transmitted with the
P-bit set to 1 (other than TEST), there is an additional 1 second timer
which is started each time an I-frame is transmitted.
If no acknowledgment has been received when this timer expires, an RR
frame with the P-bit set to 1 is transmitted to solicit a response from the
secondary station.