Crc Method - Siemens SICAM P38 Manual

Exception Response
• Exception response frame format
Communication
address (0-247)
• Example
- Address data of illegal register read
Device address
- Response frame
Device address
• Exception code of MODBUS
Code
02 H
03 H
10 H
11 H
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
• 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.
SICAM P38 P39, 3-phase Power Meter with Multi-Tariff function, User Manual
E50417-H8940-C564-4, Release Date 06.2018
Function code
(80 H+03 H) Exception code (XX H)
(80 H+10 H)
03 H XX H XX H
83 H 02 H Low check
Meaning
Illegal data address
Illegal data
No permission
Register length overflow
st
8-bit data and 16-bit CRC register and put the result in CRC register.
Low byte of CRC
00 H 01 H Low check
High check
Communication
6.2.3 CRC Method
High byte of CRC
High check
47