PLC5 RIO chassis interfaced with a Contrologix 5571 processor

I am working for a company that does chrome plating and the line is VERY old lol
So they did a controls upgrade(before my time here) and replaced the PLC5 controller with a Contrologix 5571 along with a few racks for inputs and outputs. But one of the PLC 5 I/O chassis and a SLC chassis still remain communicating over DHRIO. The PLC5 chassis has a RIO module and about 14 Input/output cards.
I have an issue on multiple cards where the outputs are OFF but they have 120v on them. I have removed the wire and tested the terminal...still 120v. I have put a latch bit after the OTE for that output to make sure it is not coming on without me seeing, and it is not. Also I have swapped the output card as well as the whole chassis. The SAME outputs are STILL on! I am not sure what is causing this, as this is the first time I'm working with a PLC5 chassis.

Any insight or troubleshooting tips would be greatly apprecieated!

Thank you! Cheers

You have triac output cards installed (1771-OAD). See this thread for more information. Pay extra attention to Ron Beaufort's post, warning about what happens when you remove a load from one of these outputs. We use a PLC 5 for our plating operation still, and see this as well. Our electricians are aware of the potential hazard, and know to turn all disconnects off prior to performing any maintenance.

Welcome to the PLCTalk Forum community !

Measuring 120V AC on the output terminal with no conductor connected despite the output point being switched off is totally normal for AC solid-state "triac" outputs. It's not specific to the A-B circuitry or the PLC-5, but of course you see more with those products because they were popular when 120V AC was still the dominant industrial automation voltage.

As noted, yours are probably 1771-OAD modules if they're 16-point modules manufactured 20 to 30 years ago.

A triac in the OFF state will "bleed" a few milliamperes, which you can easily sense with a multimeter, and will actually drive a low-current device like an LED or a neon lamp, or sometimes is enough to keep a small relay from completely shutting off.

Triacs can also "fail ON", which is what it sounds like; their gate inputs get damaged and they turn on uncommanded. If you work with solid-state AC power controls, you will find that they have supplemental circuits to disconnect and prevent exactly that sort of behavior. An AC solid state heater control, for example, will always have an overtemperature protection device.

The easy way to tell if your triac has failed ON, versus simply bleeding current, is to connect a simple load like an incandescent lamp or a relay. Once there's a path to AC Common and a few milliamperes can flow, the measured voltage on the output point should be close to zero.

Thank you for the info I apprecieate it!

I had a similar issue had some new device that just needed 120vac input from a plc IO card. It works on previous project but on contrologix/CompactLogix IO ac output. But it did not work on the PLC5 output AC card. I had to use an isolation relay dry contact to get over the issue.
Maybe isolation relay is necessary or a dry contact output card is necessary?