Modbus Poll Checksum errors.

I have a PLC connected to a modem. I can tap into the modem via ethernet and read the Modbus messages coming in. So I use Modbus Poll.

I'm getting a bunch of Checksum Errors and Insufficient Byte Received messages. How do i resolve this? Is this a cabling issue?

If I let this go long enough, sometimes the errors will go away on its own.

Any ideas on how to resolve this?

What kind of modem?

What's it converting?

From what to what?

You mentioned that you do have good communication sometimes. I would check to make sure that the cable shield is only connected to one end.
What baud rate are you using? After checking the cable I would lower the baud rate to see if the errors are less.

Regards,

If you’re trying to use a modem to pass Modbus RTU you need one that is compatible with it. Modbus RTU uses gaps between words to identify the “end of message” and most modems (especially radio modems) will generate gaps of their own. When the modem’s gap reaches the receiving device, the data is usually clipped which will produce checksum errors and packets that are short (I.E. insufficient bytes received). Most dial up modems can handle Modbus RTU but the communications lines need to be pristine and the modem should be at 9600 baud or lower (19.2Kbaud requires a higher level of compression which will produce gaps). Radio modems must be specifically designed to handle Modbus RTU. The few that do (that I’m aware of) add a buffer on the receiving end that is longer than the time it takes to send the word so all of the data is recombined before it is released to the Modbus RTU device.
If you’re using Modbus/TCP then try slowing down the poll rate and see if that fixes the problem.

I also forgot to mention that when using Modbus with a PLC you need to have it set to PLC addressing.
http://www.modbustools.com/poll_base.html

I was also getting a Checksum Error using Modbus Poll to communicate via RS-485 to a MicroLogix ML1100. By examining the serial data, I observed that the PLC was echoing back the complete Modbus message followed by the data requested. This does not seem to follow the Modbus standard. I found an obscure setting in Modbus Poll that ignores the reflected data and makes the error go way:
Connection Setup --> Advanced --> Remove Echo (if applicable)

DRSapach said:

I was also getting a Checksum Error using Modbus Poll to communicate via RS-485 to a MicroLogix ML1100. By examining the serial data, I observed that the PLC was echoing back the complete Modbus message followed by the data requested. This does not seem to follow the Modbus standard. I found an obscure setting in Modbus Poll that ignores the reflected data and makes the error go way:
Connection Setup --> Advanced --> Remove Echo (if applicable)

Click to expand...

Your problem may be in the RS485 port of the PC or in the RS485 converter that you are using.

A RS485 comm transmits and receives over a single pair, when the port transmits it should automatically disable reception and then enable it waiting for a response but it seems that your port or converter does not do this switch correctly and keeps the reception enabled while transmitting and then receives the same bytes that it is sending.

What port or converter are you using?

lfe said:

Your problem may be in the RS485 port of the PC or in the RS485 converter that you are using.

A RS485 comm transmits and receives over a single pair, when the port transmits it should automatically disable reception and then enable it waiting for a response but it seems that your port or converter does not do this switch correctly and keeps the reception enabled while transmitting and then receives the same bytes that it is sending.

What port or converter are you using?

Click to expand...

Good observation. It likely is the USB to RS-485 converter. It is a converter from Keller that I use with their level sensors. Thanks.

It could also be a timing issue. In the Modbus Poll connection screen, try increasing the "Delay between polls" setting. I believe it defaults to 75ms and that may not be fast enough when going through modems or when using a USB to RS485 converter.

Modbus ascii is also possible.
http://www.simplymodbus.ca/ASCII.htm

Post some Wire Shark logs and we can have a look at the packets.