Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 118)

Full-Duplex Protocol
Chapter 10
10Ć14
overflows, the receiver continues summing the BCC, but it discards the
data.
The receiver also sets an error flag to indicate the occurrence of a parity,
buffer overrun, message framing,or modem handshaking error. If the
receiver receives any control code other than DLE ETX during this time,
it aborts the message and sends a DLE NAK on path 2. When the
receiver gets a DLE ETX BCC, it checks the error flag, the BCC, the
message size, and the destination station number. If any of the tests fail,
the receiver sends a DLE NAK on path 2.
If the current message packet passes the above tests, the receiver next
begins the duplicate message detection process. In this process, the
receiver compares the SRC, CMD, and both TNS bytes of the current
message with the corresponding bytes of the previous message received.
If these bytes are the same, the receiver discards the current message and
sends a DLE ACK.
If the current message differs from the previous one, the receiver next
tests the state of the message sink. If the message sink is full, the receiver
sends a DLE NAK. Otherwise, the receiver:
forwards the current link–level data to the message sink
keeps a copy of the first six bytes of the current link–level data for
purposes of duplicate message detection
sends a DLE ACK
Figure 10.8 is a flowchart which gives a simplified view of an example of
software logic for implementing the receiver. Table 10.B gives a detailed
description of an example of software logic for implementing the receiver
in structured English procedures.
In appendix D are flowcharts which give a detailed view of an example of
software logic for implementing the transmitter.