Dear experts,

I have a PI-controller in OB35. The setpoint is read from a DB. Every 5 to 20 seconds OB35 reads 0.0 from the DB, despite the value is not zero and stable. I have searched through my code and I can't find any point in it where the setpoint is changed (when this happens).

What could be the cause?

When I transferred, in OB1, the value from the DB to a real-tag, and used this real-tag in OB35, it became stable.

I wonder if I have missed some basic knowledge here?

CPU is an old 6ES7 315-2EH14-0AB0, V3.2.

Thank you for answering,

I only needed a simple PI controller so I entered a discretized model in OB35, i.e. I didn't use any ready-made functions.