Siemens SIMATIC TIWAY 1 - Manual (Page 192)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–7
USER MANUAL
The Unnumbered frame format is used to extend the number of supervisory
control functions. Frames transmitted using this format do not increment the
sequence counts at either the transmitting or receiving stations.
Commands/Responses of this type may or may not contain an Information field.
Valid control field values are
hex
value
93 SNRM command
53 DISC command
73 UA response
1F DM response
97 FRMR response
The Poll/Final bit should always be set to a binary one. This bit designates a poll
when set by the host computer and designates the final frame when set by the
adapter. BDLC operates with a window size of one, meaning that only one
outstanding command frame can be issued at any given time. Before the host
computer can send another command frame, it must receive a response frame
with the Final bit set to one. The only exception to this is if the adapter does not
respond to the host computer’s poll within the 30 ms timeout.