All HDLC-NRM (SDLC) terminals must be enabled prior to sending
and receiving of messages.
The enable request will cause the primary station to initialize
the mode for the terminal by sending a Set Normal Response Mode
(SNRM) command to the terminal. After the appropriate (UA) response,
normal polling will commence.
The enable request will cause the secondary terminal to respond
to commands from the primary station, such as SNRM and polls.
The activate request is used to allow the receiving of messages
on a terminal. Normally, an application will issue an activate request
at the same time it is enabled. The initial enabled state of the
terminal will also be activated. A deactivate request can be used
to mark a terminal busy, so that it is unable to receive messages.
When a terminal is busy, it will acknowledge polls and other frames
with a Receiver Not Ready (RNR) frame rather than the usual Receiver
Ready (RR) frame.
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