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Half-Duplex Protocol

Chapter 11

119

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.

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