How to make a hardware setup for monitoring BER of modbus

[Nikhil N]

I have to devices in communication with modbus protocol. Master is PC and slave is compressor(100ft line,RS485 standard, modbus RTU protocol). I have to build a setup which will show how much the error in communication. I am coupling some white noise to the line and some burst signals too. I could see error has happened in communication from communication log file which is getting from the Omnimbt software that I am using in master. But instead of this I need to get the BER(bit error rate) and show persons who will be monitoring the performance of modbus communication. Can anybody suggest some possible ways for doing this?

 

[anorlunda]

If you know exactly what was transmitted and what was received, and if there was no error-correction, you can compare transmitted/received bit by bit.

 

[berkeman]

modbus RTU protocol

Jeeze, this is turning into a long project!

Does the RTU protocol include a checksum or CRC with each packet transmitted? How long of a checksum or CRC?

 

[Nikhil N]

If you know exactly what was transmitted and what was received, and if there was no error-correction, you can compare transmitted/received bit by bit.

My doubt is how to determine the error, whether by comparing the signal with noise and without noise coupled as we can see in the noise analyzer(ref:figure below) or by determine in bit level comparison(BER)?

 

[Nikhil N]

Jeeze, this is turning into a long project!

Does the RTU protocol include a checksum or CRC with each packet transmitted? How long of a checksum or CRC?

It has CRC with modbus signal

 

[berkeman]

It has CRC with modbus signal

How many bit CRC? If each packet has the CRC checked, look at the packet error rate and multiply by the number of bits in the average packet length.

Your test should also send numbered packets, so that any loss of a packet without a CRC error logged is seen also.