A customer has three Krohne Optiwave 7500 with 4-20 out that are tied into a Siemens panel. They're for level on three cooling towers that are tied together with a bypass chamber.

We're getting intermittent but frequent communication errors. Krohne and Siemens reps have ruled out installation, wiring, configurations, and power in and out on both ends. Radars weren't grounded initially but are now. All three have been acting the same way for weeks now.

We had two of the three working fine for a month at one point and the third for a week. The three towers were in bypass mode so they could keep the towers full while we troubleshot.

Is anyone aware of general or specific compatibility issues between Krohne and Siemens PLCs?

When we closed the bypass valve and put the system back in auto, the radars ran for a couple hours and then, one-by-one, dropped out again.

Here's the kicker: there are twenty other radars of the same model installed elsewhere in the plant that are tied into a different manufacturer's panels that work just fine. The other radars are wired the same and use the same power supply.

 

How are they "communicating"? What does a communications failure look like?

 

The COMM message is on the screen on the radar itself. It's a simple 4-20mA out to the Siemens panel.

The message goes away when connected to the other PLC.

 

What does the COMM message say?

What does the Siemens report when the Comm message is there, and when the COMM message disappears?

Does the 4-20mA continue to operate? If not, what happens? Does it go full scale in one direction or another? Does it lock up when it should change?

Remember, no one but you can actually see what's happening, so you have to describe what is happening.