Troubleshooting AB Analog Output Module

I have a system with a 1769-OF4CI that sends signals to I to P devices that drive control valves. Recently I ran into an issue with one of the outputs where the operator would command the valve to be open X% and the actual output at the I to P would be lower than X by more than 10%. The highest output I could get at the I to P was like 16.8mA. The OK light is on for the module and I have hooked the wires up to spares on the card and get the same result.

When I remove the wires from the Analog Output module and drive the signal with a process meter the I to P works as expected and matches the % I am driving. It's only when I hook the circuit back up to the module that I experience the current drops.

What are ways I can troubleshoot the module to see if it is the problem? Can I measure the current coming out of the module while disconnected from the end device?

You need a load to complete the circuit, otherwise no current will flow. Is your process meter reliably accurate? If so, you've very likely got a failed output, it's not impossible. I have seen analog inputs fail similarly, registering maximum 18mA despite 20mA being on the wire. If all your terminals act the same, I would look for a configuration issue on the card channels.

This is not split-ranging with two I/P's in series is it? Many A/O's don't have enough voltage to drive two I/P's or two positioners in series.

Use your process meter or calibrator set for measuring milliamps in place of the load and see if the card can supply the proper current. That will divide the trouble between a) the output card b) wiring and load

First I offer this to help
Ohm’s law applies here as with all electronics
Measure the voltage across the Input it is important to know so we can figure this out
Most MA inputs have a 250 ohm load resister that the input current passes through the internal interment actually measures the voltage across that resister using an differential op-amp to control the device. Some devices have a 500 ohm load resister and there may be other values out there
Then you have the output most driver out at 24 volts to drive the device but there at some only output 12 volts and I know of a few that only use 5 volts
So from what you posted it sounds like your source power may be only 12 v you could also see the same thing is you tried to series the devices 24v source through 2 250 ohm resisters in series = 500 ohms the actual signal read into each device would be 50%. The output drive has no way of knowing what the actual load is it will try an deliver the current you ask for but there is not enough source voltage to do that than the current output will not be there.
This is why it’s important that a ma output only goes to one device if you need to go to multiple devices you must uses a splitter

Thank you all for the suggestions. When it comes to analog modules I can second guess myself when troubleshooting because of how expensive these cards are these days so I get in my head and worry I might do damage to the card.

Process meter has always been reliable when using it, and configuration of the card is the same as it has always been. The card has been in service for a while, and just started experiencing these issues recently so I was able to rule out card config pretty quickly.

Only one I to P in the circuit - I have dealt with split ranging before and ran into that issue though.

With the circuit connected I measure 0.737V at the output terminals on the card. At the terminals of the I to P measurement is 0.686V.

One other thing to note, it was suspected that the I to P was the issue so that was replaced and there was no change to the issues. The new I to P was left in place and matches what was in place previously. The I to P has 180 Ohm impedance.

That would mean that you have .051 volt drop in the connection wire that would seem a little high to me. What is the open circuit voltage on the output module? Check your wiring connections you may have a loose connection someplace
It would also mean that you only have less than 3 ma going to the I to P device
Do have access to a current simulator if so connect that to the I to P device and see how it responds
Your output module could be bad can you actually measure the ma single from the output module
A good multi meter should be able to measure the current

Does the I to P device have a power supply connected to it? Like 24 VDC or 120 VAC. Or does the I to P device just have the 2 wires from the 4-20 mA output card connected to it?
It sounds like there is a power drain somewhere. If this worked before and is not now and you have changed the I to P device and changed the output on the card then I am guessing wiring or a 24 VDC power supply somewhere.
Do all of the other current outputs on the card work correctly?

doctord said:

Does the I to P device have a power supply connected to it? Like 24 VDC or 120 VAC. Or does the I to P device just have the 2 wires from the 4-20 mA output card connected to it?
It sounds like there is a power drain somewhere. If this worked before and is not now and you have changed the I to P device and changed the output on the card then I am guessing wiring or a 24 VDC power supply somewhere.
Do all of the other current outputs on the card work correctly?

Click to expand...

The I to P just has the two wires from the 4-20mA output card connected to it. The power in the loop is coming from the bus interface of the card and I wired to different outputs on the card and got the same results. There are no other active devices getting signals from this card right now because they are down for maintenance.


Something I did not check very thoroughly was wiring. I visually inspected, but did not tug test them to verify they were all tightened down. Something very well could've been left loose.

A few things on follow up
You stated that the system was in shutdown mode
The I to P device must have a separate power supply to it, is it even powered at all ?
What is the data value you are sending to the module ?
How do you have the module configured ?
That module is capable of multiple output configurations
0-20ma and 4- 20ma are both possible
In 4-20ma the module can produce and output of 3.2 to 21 ma
In 0-20ma the module can produce an output of 0 to 21 ma
It can also be as -10 to 10 output
So check the module configuration and the data sent to it carefully
I could see the output dropping down to that level If the configuration was set wrong or even if the data word sent to it were a neg value
If it were commanded to a negative value ( Possible if the value is the output from a PID control ) or in shutdown the system commands it to a negative value to ensure that final output pressure is zero
Here again we don’t have the full picture so we can only give you some ideas where to look

I wrote a nice detailed reply, and the computer ate it. So here it is in much shorter a form.

1) Check the program and make sure nobody put a high limit on the output. This is often a quick fix for a valve that is mis-behaving.

2) Remove the wires at the card, and connect your meter directly to the card. Run the signal through its full range. If you don't have full range at the card, and there is nothing in the program capping the output, then the output point on the card is bad.

3) If the signal is good at the card, check the signal at the device. Is it correct? If not, check every junction, terminal, and splice in the wire. You probably have one that is loose or going bad.

4) Still not working? Run a temporary wire from the card to the device and see if that fixes the problem. If it does, then repeat Step 3 until you find what you missed, or just replace the wire.

Hope this helps,
Bubba.