Danfoss VLT MCD 200, Installation Instructions Manual

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MCD Modbus Module Instructions AP ASCII Protocol
MG.17.F5.02 - VLT
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11
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 B 1 0
or
02h 42h 31h 30h
ASCII Hex Binary
STX 02h 0000 0010
B 42h 0100 0010
1 31h 0011 0001
0 30h 0011 0000
A5h 1010 0101 SUM (1)
A5h 1010 0101 MOD 256 (2)
5Ah 0101 1010 1's COMPLEMENT
01h 0000 0001 + 1 =
5Bh 0101 1011 2's COMPLEMENT (3)
ASCII 5 B ASCII CONVERT (4)
or 35h 42h LRC CHECKSUM
The complete command string becomes:
ASCII STX B 1 0 5 B ETX
or 02h 42h 31h 30h 35h 42h 03h
To verify a received message containing an LRC:
1. Convert last two bytes of message from ASCII to binary
2. Left shift 2
nd
to last byte four bits
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 [d2]h [d3]h [d4]h LRC1 LRC2 ETX
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 30h 31h 46h LRC1 LRC2 ETX