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MCD Modbus Module Instructions
10.4. Calculating the Checksum (LRC)
Each command string sent to and from the starter includes a checksum. The form used is the
longitudinal redundancy check (LRC) in ASCII hex. This is an 8-bit binary number represented and
transmitted as two ASCII hexadecimal characters.
To calculate LRC:
1.
Sum all ASCII bytes
2.
Mod 256
3.
2's complement
4.
ASCII convert
For example Command String (Start):
ASCII
STX
or
02h
ASCII
Hex
STX
02h
B
42h
1
31h
0
30h
A5h
A5h
5Ah
01h
5Bh
ASCII
5
or
35h
The complete command string becomes:
ASCII
STX
or
02h
To verify a received message containing an LRC:
1.
Convert last two bytes of message from ASCII to binary
2.
Left shift 2
3.
Add to last byte to get binary LRC
4.
Remove last two bytes from message
5.
Add remaining bytes of message
6.
Add binary LRC
7.
Round to one byte
8.
The result should be zero
Response or status bytes are sent from the starter as an ASCII string:
STX
[d1]h
d1 =
30h
d2 =
30h
d3 =
30h plus upper nibble of status byte right shifted by four binary places
d4 =
30h plus lower nibble of status byte
For example status byte = 1Fh, response is:
STX
30h
B
1
0
42h
31h
30h
Binary
0000 0010
0100 0010
0011 0001
0011 0000
1010 0101
1010 0101
0101 1010
0000 0001
0101 1011
B
42h
B
1
0
42h
31h
30h
nd
to last byte four bits
[d2]h
[d3]h
[d4]h
30h
31h
46h
MG.17.F5.02 - VLT® is a registered Danfoss trademark
SUM (1)
MOD 256 (2)
1's COMPLEMENT
+ 1 =
2's COMPLEMENT (3)
ASCII CONVERT (4)
LRC CHECKSUM
5
B
ETX
35h
42h
03h
LRC1
LRC2
ETX
LRC1
LRC2
ETX
AP ASCII Protocol
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