Siemens SIMATIC TIWAY 1 - Manual (Page 195)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–10
USER MANUAL
B.5 ERRORS AND ERROR RECOVERY
BDLCs design includes methods for detecting and correcting transmission
errors so that message integrity is assured. Methods are provided for busy
condition, N(S) and N(R) sequence errors, BCS errors, invalid commands, and
timeouts.
B.5.1 Busy Condition
When either the host computer or the adapter temporarily cannot receive or
continue to receive I-Frames or UIs because of internal buffer limitations, that
station alerts the sending station by transmitting an RNR with N(R) set to the
count that it expects to receive on the next frame. The busy station should
continue to respond with RNR until buffer space is available, while the sending
station should respond with RR.
A “busy condition” on an adapter will be forced clear whenever the host
computer sends a SNRM.
B.5.2 N(S) Sequence Error
An N(S) sequence error occurs when a receiving station receives a BCS error
free I-Frame that contains an N(S) sequence number not equal to the internal
receive count. The receiving station does not increment its receive count and
responds with a REJ with the N(R) set to the message it expected to receive. If
the receiving station is the host computer, it can optionally issue a SNRM to
reset the adapter’s sequence counts.
B.5.3 N(R) Sequence Errors
An N(R) sequence error occurs when a receiving station receives a BCS error
free I-Frame that contains an N(R) sequence number not equal to the internal
send count. If the receiving station is the host computer, it should respond with
an RR to acknowledge the last frame received correctly or should send a SNRM
to reset the adapter’s sequence counts.
If the receiving station is the adapter, it will respond with a FRMR indicating
the condition causing the error. FRMR is further described in Paragraph B.5.7.