Error: Modbus RTU - CRC error


Thread Starter


I thank you for your support in the following consultation:

A problem in the communication of the RTU with the SCADA has appeared, and verifying with Protocol Analyzer gives me the following message:

X: Modbus function code 128 (0x80) - Unknown modbus function code - Exception Response
Remote address 128 (0x80)
X: Modbus function code 128 (0x80) - Unknown modbus function code - Exception Response
Remote address 128 (0x80)
Q: Modbus function code 3 (0x03) - Read Output Registers
Remote address 1 (0x01)
Request holding registers 4001 to 4012

What do these errors mean, and can I can solve it? Thank you for your comments.

Dear Nair,

Holding Register address is 400001 onwards. Please check node address, parity, baud rate 9600 KBPS.



Vasu, Bangalore, India

Hi Nair,

You can solve these problems.

RTU acts as Modbus Slave and SCADA Software acts as Modbus Master.

03H function code is used to read multiple register from slave to the master.

On any error from the slave, slave will send the function code with MSB Bit set. e.g. In case of 03 function code, on any error, it gives 83h.

In your case, SCADA software requesting with 0h function code. On error the slave sends with msb bit set, i.e. 80h.

If the slave gives 80h response, it means there is a exception. Next byte of the 80h will give the exception code. Based on the code, you can solve the problem. e.g. 01- for illegal function, 02 for illegal data address.

Please note that, as per MODBUS SPEC, there Function code 0h is not existing. Refer the spec to use appropriate function code.

For 03h function code, you need to use addresses from 40,001.

Start analyse thisway, so you can correct yur self.

All the best,

Conzerv systems pvt.Ltd.
Dear Friends,

Thanks for your answers on my consultation. I would like to add more about Error 80 80 80 80 MODBUS RTU CRC Error.

Like reference, the comment that the RTU would be formed as Masterful and the SCADA would be the slave, this RTU is used like mass media of a SCADA to another SCADA. Exists a SCADA Master (protocol MODBUS) that by means of the RTU (gateway) communicates with my SCADA (protocol IEC 101)? In the analyzer of protocol “X” it would come to make the Master SCADA and Q the slave SCADA.

Thank you for your comments.