Insufficient data received error in modbus

[paddy]

Hi,

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.

I am using CCS compiler of version 4.119 and modbus_slave example.
For single device there is no problem but it gives error when I add another device. Please find attached text file.

Please advice what could be the problem.

Thanks,
Paddy

[RF_Developer]

I am not sure what you mean by "loop". RS485 is multidrop, there is no "loop".

There is unlikely to be a software problem, as it works in one hardware configuration and not in another. The problem must be related to the hardware.

The problem may be incorrect termination or incorrect bias, or possibly incorrect bus topology.

The bus must only have terminations at the ends, not somewhere in the middle.

The bus should be biased at one point only. Additional bias resistors may work, but performance may be poor and unreliable. Mistakenly biasing the lines the wrong way round is also a hard problem to diagnose and may well work in one configuration but not others.

There should not be any branches of the bus. It should go from one end (terminated, and possibly biased) to each intermediate device in turn and on to the far end, which should be terminated but not biased.

So, check all your RS485 bus wiring, terminations and biasing.

[paddy]

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