Modbus communication error

Looks from your setup software that the function should be 3 (read hold registers)not 4 ....

I did try 3 (read hold registers) and it did not make any difference:

Serial reading...
5 chars found
INFO CLASSICLADDER- Modbus I/O module received: Lgt=5 -> (Slave address-1 - Function code-83 ) 2 C0 F1
ERROR CLASSICLADDER- MODBUS-EXCEPTION RECEIVED:83 (Excep=2)
ERROR CLASSICLADDER- MODBUS-INCORRECT RESPONSE RECEIVED FROM SLAVE!!!
Serial flush all!
INFO CLASSICLADDER- Modbus I/O module to send: Lgt=8 <- Slave address-1 Function code-3 Data-8 34 0 1 C7 A4
INFO CLASSICLADDER- MODBUS Length we should receive=4 (+3)
Serial config...
Serial writing...
Writing done!
select() for serial reading...
Serial reading...
5 chars found
INFO CLASSICLADDER- Modbus I/O module received: Lgt=5 -> (Slave address-1 - Function code-83 ) 2 C0 F1
ERROR CLASSICLADDER- MODBUS-EXCEPTION RECEIVED:83 (Excep=2)
ERROR CLASSICLADDER- MODBUS-INCORRECT RESPONSE RECEIVED FROM SLAVE!!!
Serial flush all!
INFO CLASSICLADDER- Modbus I/O module to send: Lgt=8 <- Slave address-1 Function code-3 Data-8 34 0 1 C7 A4
INFO CLASSICLADDER- MODBUS Length we should receive=4 (+3)
Serial config...
Serial writing...
Writing done!
select() for serial reading...
Serial reading...
5 chars found
INFO CLASSICLADDER- Modbus I/O module received: Lgt=5 -> (Slave address-1 - Function code-83 ) 2 C0 F1
ERROR CLASSICLADDER- MODBUS-EXCEPTION RECEIVED:83 (Excep=2)
ERROR CLASSICLADDER- MODBUS-INCORRECT RESPONSE RECEIVED FROM SLAVE!!!
Serial flush all!
INFO CLASSICLADDER- Modbus I/O module to send: Lgt=8 <- Slave address-1 Function code-3 Data-8 34 0 1 C7 A4
INFO CLASSICLADDER- MODBUS Length we should receive=4 (+3)
Serial config...
Serial writing...
Writing done!
select() for serial reading...
Serial reading...
5 chars found
INFO CLASSICLADDER- Modbus I/O module received: Lgt=5 -> (Slave address-1 - Function code-83 ) 2 C0 F1
ERROR CLASSICLADDER- MODBUS-EXCEPTION RECEIVED:83 (Excep=2)
ERROR CLASSICLADDER- MODBUS-INCORRECT RESPONSE RECEIVED FROM SLAVE!!!
Serial flush all!

It did make a difference - the exception code is now 2 - illegal data address

So the function (3) is corrected but seems the address is wrong
is address 2101 legal? in your test you used 2100

ahh I bet you did set 2100 in classicladder - try setting 'modbus element offset' to 0

Chris

Sorry I meant set offset to 1

Hello

2100 and 2101 are correct. i tried them both, in hex value and in decimal.
I also tried different offsets 0-9. none made difference.