Omron V550-A20, Operation Manual

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8
V550-A20 Operation Manual
Section 5-
8
Appendices
Error: The slave does not understand the master’s acknowledgment of
the slave’s data message.
Recovery:
The slave will retransmit the same message in response to the
next poll. The master will see that it is a duplicate message, acknowledge
it, and discard it.
Error: The slave does not understand a broadcast message.
Recovery: None. The message will be lost.
The general rules are as follows:
1. Each data message will be acknowledged by the recipient. If a data message
is not acknowledged, the transmitter should retransmit it again up to three
retries. After the third retry, a communications error message should be dis-
played and the message discarded (in some systems the message may be re-
corded in a disk file or on a printer to prevent data loss).
2. Each new message will have a new sequence number. If a message is re-
ceived that has the same message number as the last message received, the
recipient should acknowledge the message and then discard it. The sequence
number should only be checked for equality to the last sequence number re-
ceived: there is no requirement that the sequence number must be the next
number expected (although in some systems the master will keep track of “out of
sequence” errors since they would indicate that messages had been lost).
The sequence number zero is a special case, since it indicates that the data
message is the first data message sent since the device sending it has powered
up. Messages with a sequence number of zero should always be processed as
required, regardless of whether or not they are repeated “back to back”.
3. Any message that contains parity errors, LRC errors or an unrecognized
message type should be discarded
. No acknowledgment should be sent. In some
systems, the master will keep track of these transmission errors.
4. Any message that contains a correct LRC, has no errors, is of a correct type,
and requires an acknowledgment should be acknowledged even if its se-
quence number indicates that it is a duplicate message (the sequence number
is the same as the last message)
. If it is a duplicate message, it should be ac-
knowledged then discarded. In some systems, the master will keep track of these
duplicate message errors since they would indicate that an acknowledgment had
been lost. A broadcast message (one sent to unit “00”) must not be
acknowledged.