Calculating The Checksum (Lrc) - Emerson Digistart CS User Manual

6.4

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
or 02h 42h
ASCII Hex Binary
STX 02h 0000 0010
B 42h 0100 0010
1 31h 0011 0001
0 30h 0011 0000
A5h 1010 0101
A5h 1010 0101
5Ah 0101 1010
01h 0000 0001
5Bh 0101 1011
ASCII 5 B
or 35h 42h
The complete command string becomes:
ASCII STX B
or 02h 42h
To verify a received message containing an LRC:
1. Convert last two bytes of message from ASCII to binary
2. Left shift 2nd 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
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
12
1 0
31h 30h
SUM (1)
MOD 256 (2)
1's COMPLEMENT
+ 1 =
2's COMPLEMENT (3)
ASCII CONVERT (4)
LRC CHECKSUM
1 0 5
31h 30h 35h
[d3]h [d4]h LRC1
31h 46h LRC1
www.controltechniques.com
B ETX
42h 03h
LRC2 ETX
LRC2 ETX
Modbus Module User Guide
Issue 1