Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 136)

Half-Duplex Protocol
Chapter 11
11Ć9
The master should poll each slave repeatedly until that slave has
transmitted all of its messages. The master should then send any
messages it has for that slave. Then the master can poll the next slave in
the same way.
If a slave station fails to respond to a poll, the master should remove that
slave from the list of active slaves. To save time, the master should poll
only the active slaves on a regular basis. The master should poll the
inactive slaves occasionally to see whether they will respond.
It is best not to allow the master station’s transceiver to relay messages
directly from one slave station to another. Instead, the transceiver should
funnel all received messages to the message sink (network layer). The
network layer can then analyze the messages and retransmit any that are
addressed to a slave station.
Figure 11.3 is a flowchart which gives a simplified view of an example of
software logic for implementing half–duplex protocol from the master
station’s point of view.