8-17
CRC Check Digit:
RTU Mode uses CRC (Cyclical Redundancy Check) Check Digit.
CRC Check Calculation Method is as follows:
1. Load a 16-bit CRC register with FFFFH;
2. Perform XOR (Exclusive OR) calculation on the first 8-bit byte value of the data content with
the low byte of the CRC Register, and store the result in the CRC Register.
3. Shift the CRC Register one bit (LSB) to the right, then fill 0 to the Most Significant Bit (MSB);
4. Check the value of bit (LSB) shifted to the right.
If it is 0, then save the new value in the CRC Register;
If it is 1, then perform XOR calculation on the new value and A001H, then save the result in the
CRC Register;
5. Repeat Steps 3 ~ 4, until all 8 bit are completed calculation, then perform Step 6;
6. Take the next 8-bits message data of the data content to calculate repeating Steps 2 ~ 5, until all
message data are completed calculations, the CRC Register content at this time is the CRC
Check Digit.
Error Code
If an error occurs during communication connecting process, the Driver will issue an Error Code, add
80H to Command Function Code and then transmit to ModBus master station system altogether.
Error
Code
Name Description
01
Command Instruction
Code Error
The function code received in the query is not an
allowable action for the server (or slave).
02 Data Address Error
The data address received in the query is not an
allowable address for the server (or slave).
03 Data Content Error
A value contained in the query data field is not an
allowable value for server (or slave).
04 Slave Equipment Error
An unrecoverable error occurred while the server (or
slave) was attempting to perform the requested
action.
05
Communication
Command Mode Error
RTU mode: CRC check error
06
Communication
Command Mode Error
ASCII mode: LRC check error or no end
code(CRLF)
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