Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 232)

Data Link Layer
Appendix E
E-5
A block check is used to detect errors at the link level. You can select a
CRC instead of this block check. Any device connected to the
asynchronous link must be capable of generating a BCC or a CRC, if
used.
A BCC is added to the end of every message block to help detect errors of
transmission between station interface modules. The command station
generates a BCC by first summing every byte of the text (excluding
control characters), then taking the 2’s complement of that sum. The
result is the BCC. Any final carry-out bit is ignored in the BCC
computations.
The receiving station also sums the text bytes, then adds that sum to the
BCC to produce zero. Any sum other than zero indicates an error has
been made in the transmission and causes the receiving station to respond
with DEL ACK.
The CRC is used at the data link layer to validate messages transmitted on
the Data Highway link.
The station interface module at the sending station appends the CRC to
the message. The CRC is based on the bit pattern of the transmitted
message. The receiving station also computes a CRC based on the
received data and checks this against the CRC value included with the
received message. A discrepancy between the transmitted CRC and the
CRC computed by the receiving station indicates some fault in the
transmission. If the received and computed CRC values do not agree, the
message is not accepted as valid.
Floating master operation continues normally as long as all stations share
mastership of the Data Highway link. However, if any one station
retained continuous control of the communication link due to a fault
condition, floating master operation would not be possible and Data
Highway communication would be disabled. As a preventive measure
against this type of situation, each station interface module has automatic
link disconnect circuitry. If a module detects that it is not relinquishing
mastership of the Data Highway, this circuitry can disconnect the module
from the Data Highway link. The other stations on the Data Highway can
then continue to function normally. Link Disconnect