modbus error ... some times

Hello,
We have 4 modbus rtu card.
Some times we have modbus error.
I started the application in terminal mode and here is the log.

INFO CLASSICLADDER- MODBUS Length we should receive=5 (+3)
Serial config...
Serial writing...
Writing done!
INFO CLASSICLADDER- after transmit delay now...25 milliseconds
select() for serial reading...
Serial reading...
5 chars found
INFO CLASSICLADDER- Modbus I/O received (Hex):
Length=5 bytes <- Slave: 3 Function: 90 Data: 3 CRC: AD C1
ERROR CLASSICLADDER- MODBUS-EXCEPTION RECEIVED:90 (Excep=3)
ERROR CLASSICLADDER- MODBUS-INCORRECT RESPONSE RECEIVED FROM SLAVE!!!
Serial flush all!
INFO CLASSICLADDER- Modbus I/O to send (Hex):
Length=41 bytes -> Slave: 3 Function: 10 Data: 0 0 0 10 20 2 0 2 0 2 0 2 0 1 0 2 0 2 0 2 0 2 0 2 0 2 0 2 0 1 0 2 0 2 0 2 0 CRC: 59 AD
INFO CLASSICLADDER- MODBUS Length we should receive=5 (+3)
Serial config...
Serial writing...
Writing done!
INFO CLASSICLADDER- after transmit delay now...25 milliseconds
select() for serial reading...
Serial reading...
8 chars found
INFO CLASSICLADDER- Modbus I/O received (Hex)

Can anybody tell me how can I interpret this error ?

Best regards
Thierry Vorms

It seems the device is sending back an error message as a response to a request.
As to what 'Function code 90, Data 3' exactly means may be described in the manual of that particular modbus device.

Have you tried increasing the debug level to see what the request was?

linuxcnc.org/docs/html/ladder/classic-ladder.html
DEBUG LEVEL - Set this to 0-3 (0 to stop printing debug info besides no-response errors).