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9
V550-A20 Operation Manual
Section
5-
9
Appendices
Multidrop Protocol Examples
Message framing:
FFh,
02h, idhigh,
idlow, type,
seq no,
... data...,
lrc0,
lrc1,
0Dh
(DEL,
STX,
?,
?,
?,
?,
?,
?,
?,
CR)
NOTE:
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
If no data is available:
STX, unit id, 0, 5, lrc, CR
Example:
STX
0
1
0
5
0
4
CR
HEX:
02h
30h 31h
30h 35h
30h 34h
0Dh
NOTE:
It is normally faster to allow the master to time out (which takes three
character times) than to use the "no data" response.
3. MUX acknowledges data in the following format:
STX, unit id, 0, 3, lrc, CR
Example: STX
0
1
0
3
0
2
CR
HEX:
02h
30h
31h
30h
31h
30h
32h
0Dh
4. MUX polls the next unit . . .
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