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ASCII C
OMMUNICATIONS
A-7
Solutions with Vision
2.
Each new message will have a new sequence number. If a message is
received 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
received: 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 acknowledgement 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 acknowledgement should be acknowledged even if
its sequence 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 acknowledged then discarded. In some systems, the
master will keep track of these duplicate message errors since they would
indicate that an acknowledgement had been lost. A broadcast message (one
sent to unit “00”) must not be acknowledged.
M
ULTIDROP
P
ROTOCOL
E
XAMPLES
Message framing
FFh,
02h,
idhigh, idlow, type,
seq no,
...data..., lrc0, lrc1, 0Dh
(DEL,
STX,
?,
?,
?,
?,
?,
?,
?,
CR)
The DEL character is used as a guard character to make sure that the
transmission line is quiet for one character time before the STX is sent. The
sequence number only appears on data messages. The LRC stands for “linear
redundancy check” and appears on all messages.
Polling sequence:
1. MUX polls slave at address 01 with the following format:
STX, unit id (2 char), 0, 1, lrc (2 char), CR
Example:
STX
0
1
0
1
0
0
CR
HEX:
02h
30h
31h
30h
31h
30h
30h
0Dh
2. SLAVE answers the poll with the data in the following format:
STX, unit id, 0, 2, seq (1 char), ...data..., lrc, CR
Example:
STX
0
1
0
2
1
A
B
C
D
E
7
3
CR
HEX:
02h
30h
31h
30h 32h
31h 41h 42h 43h 44h 45h 37h 33h
0Dh
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