Chapter 4 Parameters
4-144
3.4 Check sum
ASCII mode:
LRC (Longitudinal Redundancy Check) is calculated by summing up, module 256, the
values of the bytes from ADR1 to last data character then calculating the hexadecimal
representation of the 2’s-complement negation of the sum.
For example, reading 1 word from address 0401H of the AC drive with address 01H.
STX ‘:’
Address 1
Address 0
‘0’
‘1’
Function 1
Function 0
‘0’
‘3’
Starting data address
‘0’
‘4’
‘0’
‘1’
Number of data
‘0’
‘0’
‘0’
‘1’
LRC Check 1
LRC Check 0
‘F’
‘6’
END 1
END 0
CR
LF
01H+03H+04H+01H+00H+01H=0AH, the 2’s-complement negation of 0AH is F6H.
RTU mode:
Address
01H
Function
03H
Starting data address
21H
02H
Number of data
(count by word)
00H
02H
CRC CHK Low
6FH
CRC CHK High
F7H
CRC (Cyclical Redundancy Check) is calculated by the following steps:
Step 1: Load a 16-bit register (called CRC register) with FFFFH.
Step 2: Exclusive OR the first 8-bit byte of the command message with the low order byte of
the 16-bit CRC register, putting the result in the CRC register.
Step 3: Examine the LSB of CRC register.
Step 4: If the LSB of CRC register is 0, shift the CRC register one bit to the right with MSB
zero filling, then repeat step 3. If the LSB of CRC register is 1, shift the CRC register one bit
to the right with MSB zero filling, Exclusive OR the CRC register with the polynomial value
A001H, then repeat step 3.
Step 5: Repeat step 3 and 4 until eight shifts have been performed. When this is done, a
complete 8-bit byte will have been processed.
