Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 71)

Asynchronous Link Protocols
Chapter 4
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2. Some protocol implementations require that the first byte of a
message match the station address. These receivers will ignore
messages that do not contain the correct address. This filtering is not
required, since the message sink can also provide the address
filtering function.
3. As part of the duplicate message detection algorithm, the receiver
checks the second, third, fifth, and sixth bytes of each message.
There must be a difference in at least one of these bytes between a
message and its predecessor for it to be recognized as distinct rather
than a retransmission of the previous message.
Master Polling Responsibilities
The master polling algorithm may vary depending on the expected flow of
traffic through the system.
A simple master that does not expect unsolicited messages continuously
polls each slave once in a round-robin fashion. If a message is received, it
should be handled, then the next station polled. An alternative scheme
would poll each station repeatedly until it was empty and then proceed to
the next. Relaying of peer-to-peer messages is best left to the network
layer, which is the lowest level of the body of software that includes the
message source and sink.
Figure 4.16 is a flowchart of the software logic for implementing
half-duplex protocol from the master station’s point of view.