Modbus communications - problems when using Data Exchange

[PeteLam]

Thanks, I'll try the CPU system tag and see if that's the issue. I've used this basic setup on another machine, but didn't have any issues. Difference was that machine only had 1 temperature loop to monitor versus 12 on my current project.

[PeteLam]

Well, it turns out that it wasn't the CPU, which peaks at about 60% and idles at around 25%. However, I think I have found the root cause of the problems.

It doesn't seem to be programming related at all. Just a noise issue on the cabling. Because COM3 and COM4 share a common connector, I used a single multiconductor cable to connect the HMI to a demarcation point in my control enclosure. The length of cable was about 6-8 ft.

When I replaced this cable with another cable I jury-rigged, the communications faults basically disappeared. The new cable just used single-conductor 20awg wires. I twisted three wires into one bundle for the RS-232 link to the Smartmotor, and I twisted two wires into a separate bundle for the RS-485 modbus connection. There is no shielding, and I did not even bother to put a shell around the DB-9 connectors.

I'm going to build a new, more professional cable using two 20awg, three-conductor jacketed cables, mostly because that's what I have on the shelf.

Is there any compelling reason to use shielded cable? FWIW, the RS-232 is running at 38,400 baud and the RS-485 is at 9600 baud. I don't recall that shielded wire was required for either of these connections.

[mark.monroe]

Whether to use shielded cable or not has a lot to do with where you are running your cables. If you are running it right next to your power lines than you may want to use shielded cables. Every application is different. You may want to consult an Electrical Engineer to see what they recommend.