I am working on a project that uses the Eurotherm 3504 controller to control valve position based on input pressure.

I had completed the exact project a couple of years ago but I used the ABB C505 controller. The C505 is now obsolete and so I have had to use the Eurotherm 3504.

The challenge I face is this - The ABB controller had a processor watchdog functionality and could output a controller fault if there was a loss of watchdog or a break in the output loop (I tested this by disconnecting one of the output legs and I got a controller fault as expected).

The Eurotherm controller does not seem to have this functionality. I have thoroughly researched the datasheets and have come up with 2 solutions.

1. Watchdog - I plan to use the Instrument Diagnostic functionality of the eurotherm controller as a substitute to the watchdog of the ABB.

2. Output Loop Failure - I plan to use the 'Loop Break Time' function of the eurotherm controller to represent an output loop break.

My problem with the second point above is according to the eurotherm datasheet, loop is considered to be broken if the PV does not respond to a change in the output. I do not know if that is the same as breaking one leg of the 4-20mA output signal.

Any thoughts?

Thanks

 

Those are fairly easy to code around with most processors. An external time delay relay that you reset with an output can work as a watchdog, but you have to make the reset edge triggered. (e.g. Reset through a capacitor) And you can make your lowest process value say 5ma. so 4 is a broken wire. I'm not familiar with the Eurotherm, but there is often a particular value for 0 ma.

Regards
cww

 

Thanks for your reply. I am currently working on the design.