Bad Quality on Modbus RTU over Serial link

Hi ignition

I have been doing a trial on your Software. But I just couldn’t get a simple modbus rtu connection going.
If you can explain how it works, I believe I can convince my company to invest in your softwares.

Step1: Base on your instruction, I have set OPC Item Path to “[Modbus]1.HR1
The Value doesn’t appears. Result :
Value = “BAD”
Quality = BAD


Well just to get it out of the way: if you're evaluating for commercial use then you should be using the standard version of Ignition. You can reset the 2 hour trial as many times as you want.

Have you looked for errors in the Ignition logs? Are you sure the serial settings are correct? That there's data at this address? A device at the other end of the serial line turned on and configured to respond?

Hi Kevin

Below are the Ignition Logs

Below are the serial settings

And i have confirmed using Modscan that there's data at this address.
And the device at the other end of the serial line turned on and Modscan is able to retrieve data from it presently.

Are you sure parity is "None" ? That is not normal for Modbus RTU.

In order to educate myself, I did some googling and found this document from Modbus that says (For both RTU and ASCII modes):

Even parity is required, other modes ( odd parity, no parity ) may also be used. In order to ensure a maximum compatibility with other products, it is recommended to support also No parity mode. The default parity mode must be even parity.

Remark : the use of no parity requires 2 stop bits.

So No Parity is acceptable (depending on what you are talking to), but the number of stop bits shown above doesn't match the Modbus requirements.

Personally I'd throw some sort of serial capture device on the test rig and see what Modscan is actually sending down the wire.

1 Like

Hi Pturmel

I have tried a couple of settings from Parity: Even/Mark/None/Odd/Space
the result is still the same.
Quality : Bad

I think you need a serial port sniffer to see what traffic you are getting, both for Modscan, and for Ignition, so you can compare.