Communication
6.2.3 CRC Method
SICAM P38 P39, 3-phase Power Meter with Multi-Tariff function, User Manual
E50417-H8940-C564-
4
, Release Date
06.2018
47
Exception Response
• Exception response frame format
• Example
- Address data of illegal register read
- Response frame
• Exception code of MODBUS
6.2.3
CRC Method
CRC contains 2 bytes, namely 16-bit binary system. CRC is calculated with transmitting device and placed at
the tail of transmitted message. Receiving device recalculates CRC of message received and compares
whether CRC derived matches the received one. If not, error may occur.
To calculate CRC, preset all 16-bit registers to 1. Then, process each 8-bit data message one by one. Only 8-
bit data bit is used for CRC calculation and start bit and stop bit and parity check bit (if any) are not involved.
Steps of calculating CRC:
• Preset 16-bit register to hexadecimal FFFF (namely all to 1), and this register is called CRC register.
• Execute exclusive OR of low bytes of 1
st
8-bit data and 16-bit CRC register and put the result in CRC register.
• Shift the content of register rightwards by one bit (towards lower bit) and fill the highest bit with 0. Check the
lowest bit.
• If the lowest bit is 0, repeat step 3 (shift again); if it is 1, execute exclusive OR of CRC register and polynomial
A001 (1010 0000 0000 0001).
• Repeat step 3 and step 4 until shift rightwards 8 times. Now, all 8-bit data are processed.
• Repeat step 2 to step 5 to process next 8-bit data.
• The CRC register derived finally is CRC.
Communication
address (0-247)
Function code
(80 H+03 H)
(80 H+10 H)
Exception code (XX H)
Low byte of CRC
High byte of CRC
Device address
03 H
XX H
XX H
00 H
01 H
Low check
High check
Device address
83 H
02 H
Low check
High check
Code Meaning
02 H
Illegal data address
03 H
Illegal data
10 H
No permission
11 H
Register length overflow