Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 73)

Asynchronous Link Protocols
Chapter 4
4-30
Slave Transceiver Actions
Since the transceiver receives “dirty” input from the physical world, it
must be capable of responding to many adverse situations. Some of the
things that could conceivably happen are listed below:
1. The message sink can be full, leaving nowhere to put a message.
2. A message can contain a parity error.
3. The BCC can be invalid.
4. The DLE SOH, DLE STX, or DLE ETX BCC may be missing.
5. The message can be too long or too short.
6. A spurious control or data code can occur outside a message.
7. A spurious control code can occur inside a message.
8. The DLE ACK response can be lost, causing the transmitter to send a
duplicate copy of a message that has already been passed to the
message sink.
The slave is always in a passive mode until it receives a message. While
waiting for a message, anything besides the DLE SOH or DLE ENQ is
ignored. Note that in a single circuit system the slaves must be able to
safely ignore everything sent by other slaves.
If a DLE SOH is received, the BCC and the message buffer are reset. The
next code received must be a data code and must equal the station address
or 255 (if the station can receive broadcast messages). If there is no
match, the station ignores the rest of the message and continues waiting
for the start of the message.
If the station address matches, it is added to the BCC. The next code is
received and must match DLE STX. If it doesn’t, the station ignores the
rest of the message. Otherwise it starts building a message.
While building a message, all data codes are stored in the message buffer
and added to the BCC. If the buffer overflows, the receiver continues