pH loop in error mode

[Mzukulu]

HI guys,

I have a strange one here, we have a FF pH transmitter, if we change the mode to Aut, the faceplate reads ok!

the moment we connect it onto a control loop, the control loop goes into error mode, and remains in manual, however, changing the pH transmitter mode to manual, the control loop goes into normal mode, and can be changed to auto.

this is a yokogawa pH on a CS3000, we have other pH transmitters and they work just fine.

[rezabejd]

Which "mode" are you changing? AI block? Transducer block?

Is the PID a host block or a standard FF block in a device?

Was the AI ever assigned to a different device, even an earlier revision of the same device?

I have very little experience w/ CS3000, but on other systems, I have seen a curious problem which seemed to be a problem with the LAS (Link Active Scheduler). As I recall "Downloading" the whole segment made the problem go away.

[jcpilman]

We have seen this same situation. I'm not sure of all the circumstances, but I believe it was usually related to poor communication due to segment installation problems. Have you done any diagnostic testing on this segment?

I think the LAS is having difficulty with the scheduled communication. You can test this by connecting the transmitter for unscheduled communication. Normally, with CS3000, you connect the OUT of the FF-AI block to the IN of the PID block. This forces the measurement to be communicated during scheduled communication. As a test, you can connect the PV of the FF-AI block to the IN of the PID block. This will allow the measurement to be communicated during the unscheduled portion of the cycle. We have gotten some transmitters to report their live values by trying this.

I recommend you look at the segment schedule in both cases to see the differences. (Fieldbus Builder -> Tools -> Display Schedule). Also, I do not suggest this as a permanent solution.

...John