Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 138)

Half-Duplex Protocol
Chapter 11
11Ć11
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 can conceivably happen are listed here:
The message sink can be full, leaving the transceiver with nowhere to
put message.
A message can contain a parity error.
The BCC can be invalid.
The DLE SOH, DLE STX, or DLE ETX BCC may be missing.
The message can be too long or too short.
A spurious control or data code can occur outside a message.
A spurious control code can occur inside a message.
Any combination of the above can occur.
The DLE ACK response can be lost, causing the transceiver to send a
duplicate copy of a message that has already been passed to the
message sink.
Each slave station is in a passive mode until it receives a DLE ENQ or
DLE SOH code. While in a passive mode a slave ignores any
transmission code that is not DLE ENQ or DLE SOH.
When a slave receives a DLE SOH, it resets its BCC accumulator and
message receiving buffer. The next code it receives must be its specific
station number of the global station number 377 (octal). If the packet
does not contain the appropriate station number, the slave ignores it and
waits for the start of a new transmission.
If a slave receives a message packet with the appropriate station number,
it adds the value of that station number to its accumulated BCC. If the
next characters after the station number are DLE STX, then the slave
transceiver starts storing the incoming link–layer data in a buffer. The
transceiver stores all data codes in the buffer and adds these code values
to the accumulated BCC. Even if the storage buffer overflows, the
transceiver continues summing the BCC, while discarding the data.
The slave also sets an error flag to indicate the occurrence of a parity,
buffer overrun, message framing, or MODEM handshaking error. When
the slave gets a DLE ETX BCC, it checks this error flag, the BCC, and the
message size. If any of these tests fail, the slave ignores the message.
If the current message packet passes the above tests, the slave next begins
the duplicate message detection process. In this process, the slave