Insufficient data received error in modbus

[paddy_p]

View attachment commdata.txtHi,

I have a monitor which is programmed as modbus slave (RTU ). When I am connecting single device and checking the RS485 communication using Modbus poll and RS232 to RS 485 converter, it works fine. But when I connect another similar device in loop at that time it gives insufficient data byte error and CRC error.

Please advice what could be the problem.

Thanks,
Paddy

 

[FvM]

But when I connect another similar device in loop

I won't describe a RS485 bus as a "loop". Are you talking about something different?

What do you exactly mean with "a monitor which is programmed as modbus slave"? A monitor should connect passively to the bus, a slave is an active device answering requests to its device address.

Possible reasons for communication failure:
- two slaves using the same device address
- a RS485 bus related problem, e.g. wrong termination or missing bias resistors

 

[paddy_p]

Hi,

I am sorry for using wrong term. I mean to say multidrop connection. The problem was occurring because of improper voltage (reverse voltage i.e. VCC is connected to GND and vice versa) to bias resistors.

Thanks for your support!!

Regards,
Paddy