Siemens SIMATIC TIWAY 1 - Manual (Page 199)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–14
USER MANUAL
(1) The host computer issues a SNRM to initialize the adapter and the
adapter responds with a UA.
(2) The host computer then sends an I-Frame. The sequence counts are
both zero since the SNRM was issued previously. The adapter does not
have any information to return, so it responds with an RR that
acknowledges the I-Frame.
(3) The host computer sends another I-Frame with the N(S) count
incremented. This time the adapter has information to return and
sends an I-Frame. This I-Frame acknowledges the host computer’s
frame with the N(R) subfield.
(4) The host computer has no further information and polls the adapter
with an RR, which also acknowledges the adapter’s I-Frame. The
adapter also has no information and responds with an RR.
(5) The host computer continues to poll the adapter and this time the
adapter has information to return.
(6) The host computer determines that no further communications are
needed and issues the DISC command. The adapter responds with the
UA and begins a hardware reset. The host computer will have to wait a
minimum of seven seconds before issuing any further BDLC frames.
This allows enough time for the power-up diagnostics to complete.
(7) Once the adapter is disconnected or in the “disconnected mode”, it will
respond to any command other than a SNRM with the DM response.