Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 143)

Half-Duplex Protocol
Chapter 11
11Ć16
Figure 11.8 shows a slave being polled, and answering with a message.
Because a block check error is found, the master does not acknowledge;
instead, it sends the poll to the slave again. Since the slave did not receive
an acknowledgement to its first message transmission, it retransmits the
same message in answer to the second poll. The master receives the
second transmission of the message with no error and responds with DLE
ACK.
Figure 11.8
Poll with Message Returned
xxxx
xxxx
OK
Not Full
SINK MASTER LINK SLAVE SINK
10058–I
DLE ACK
SOURCE/ SOURCE/
BCC DLE ENQ STN
DLE STX xx???xx DLE EXT BCC
BCC DLE ENQ STN
DLE STX xxxx DLE EXT BCC Same Message
Figure 11.9 shows a slave unable to receive the acknowledgement from
the master after the master successfully received the message from the
slave. Sometime later when the master polls that same slave again, the
slave sends the same message again. The master responds with DLE
ACK, but discards the received transmission block because it detects it to
be a duplicate message received from that slave. With multiple slaves, to
implement this duplicate message detection, the master must do either of
the following:
Poll a station repeatedly (without polling any other station until it
receives a DLE EOT to be sure it has detected any retransmissions.