Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 139)

Half-Duplex Protocol
Chapter 11
11Ć12
compares the SRC, CMD, and both TNS bytes with the corresponding
bytes of the previous message received. If these bytes are the same, the
slave discards the current message and sends a DLE ACK.
If the current message differs from the previous one, the slave next tests
the state of the message sink. If the message sink is full, the transceiver
discards the current message and does not respond. Otherwise, the
transceiver:
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
While waiting to receive a message, a slave station could receive a polling
packet that begins with a DLE ENQ sequence. The slave will ignore the
poll if the polling packet does not contain the slave’s station number or if
the BCC is the polling block is incorrect. If the poll is valid, then one of
three conditions can exist:
The slave is still holding a message that it had transmitted previously
but had not been acknowledged by the master station. There is a limit
on the number of times the slave will attempt to transmit a message. If
this limit has been exceeded, the slave responds to this by writing an
error code into its error word in the PC data table, and then tries to
transmit the next message from the message source. If the NAK limit
is not exceeded, the slave tries to retransmit the current message.
If the slave does not currently have a message to send, it tries to get one
from the message source. If a message is available, the transceiver
initializes its retry counter and transmits the message in response to the
poll.
If not message is available, the transceiver responds to a poll by
transmitting a DLE EOT.
To transmit a message, the slave transceiver uses the same message block
format as the full–duplex format (section 10.3.2). After sending a
message, the transceiver keeps a copy of that message until it receives a
DLE ACK from the master station, or until its retry limit is exceeded.
When the slave transceiver receives a DLE ACK, it discards the current
message. The next time the slave is polled, it will send the next message
available from the message source. If no message is available in the
message source, the slave responds to a poll with DLE EOT.