Read back Snap output modules


#1

I’m a new user to Snap I/O.
When I write to an Analog output module to set an analog value, can I then read the Output value back to make sure the the output is set to the value I requested ? I am checking for a fault in the output circuit… Perhaps the module failed or something attached to the output is preventing the output from reaching its goal.

If we can’t read back from the output module, then I can add an additional analog input to the output to verify the output compliance. I would rather not have to add the hardware of course but can if necessary.

Thanks very much !


#2

Hi dfbonneau,

Welcome to the forums!

The quick answer is yes, you can read it back in by using an analog input module…

The long answer follows…

We used to do this on nominated important valves at the hospital I used to work at.
The Opto modules are rock solid. I have never had one go bad, so I have never needed to check the module output, but control valves, now they are a whole different story!
These things, because they are mechanical, are exposed to heat and vibration a lot of time, and because what they are controlling is a mechanical device that can jam, stick, or suddenly require more force to move, these require feedback for sure.

Not all valves have this feature, we had to spec it for those we needed it, but you can get valves that offer 0-10v or 4-20ma signals that reflect their actual position.
The cool thing about this signal is that it (usually) has very little in common with the input signal (other than power).
What this means is that if the actuator strips some gears, or the magnetic clutch kicks in, you can see it if your Opto code tracks the output signal against the input signal.
From there you can send an email/text or sound an alarm, what ever is appropriate for your application.

(Some digital valves have similar feedback switches etc, so you can monitor those as well).

In my mind, this is the best and only sure fire way of knowing that what you sent really happen.
Just picking up the voltage or current on the output module does not really give you what you need. It is barely half the story.

Cheers.