Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 144)

Half-Duplex Protocol
Chapter 11
11Ć17
If each station is polled only once per cycle, the master must keep a
record of the first 6 link–level data bytes of the last transmission from
each station, since other stations may transfer messages between
retransmissions from a given station.
Figure 11.9
Duplicate Message Transmission
xxxx
xxxx
OK
Not Full
SINK MASTER LINK SLAVE SINK
10059–I
DLE ACK
SOURCE/ SOURCE/
BCC DLE ENQ STN
DLE STX xxxx DLE EXT BCC
BCC DLE ENQ STN
DLE STX xxxx DLE EXT BCC
Not Full
DL ??? CK
Sometime Later ...
(Discard Retransmission)
When a slave station fails to respond to a message from the master, you
should poll the slave to see if it is there. If it answers the poll with a DLE
EOT but consistently fails to ACK the master’s message, the slave’s
message sink is probably full. If the slave answers with DLE EOT to a
poll, you should wait for the slave’s receiver buffers to clear. This
situation is illustrated in figure 11.10.