Allen-Bradley 1734-AENTR B energizing outputs when disconnected

Good Morning! I am new here and have limited experience in working with PLCS, and desperation has brought me here with a problem, hoping some of you might be of assistance, although this might be more on a wiring side of things than a programming side.

I am working with a 1734-AENTR B Point I/O block, with five 1734-IB8 24VDC sinking input modules, two 1734-OB8E 24VDC sourcing output modules, and two 1734-IE2V analog voltage input modules. I have a variety of 24v sensors connected to the input modules and analog modules. Both the Point I/O and the field sensors / devices are powered from the same 24V machine circuit.

I have noticed that there is what seems to be a substantial amount of voltage present on my output terminals when the whole 1734-AENTR is powered on, about 9v. I suspect that this is leakage from the solid state output modules, but it is worth noting. When I disconnect the main supply terminals providing the 24v supply to the 1734-AENTR, all terminals on the chassis become energized with about 10v, which turns on a siren and a few lights connected to the outputs. This does not seem to be correct, any ideas on what might be causing this? It is worth noting that I have 7 more 1734-AENTRs on the same machine set up in a similar way that have the same problem when I disconnect the supply terminals on the 1734-AENTR. If I had to take a very uneducated guess, I suspect that some of the field wires connected to the input modules that are stil high with 24V are somehow energizing the whole chassis?

I appreciate any input or assistance. I can provide more details if needed.

Hi good morning and welcome to the forum. I have attached a very good description of sink/source regarding I/O modules and devices that I copied off of this forum that was created by Ron Beaufort. He is retired now and ran a great PLC training "boot camp". His wisdom and advice are priceless, as it is with many others here.
I would verify devices, wiring, grounds and connections. Hope this helps.

Have a look at the output configuration
you can set how they should react when communication is lost
Each output is set separately
Turn ON
Turn OFF
Remain in last state
There may be a reason the origenal programmer wanted then to turn ON

saultgeorge said:

Hi good morning and welcome to the forum. I have attached a very good description of sink/source regarding I/O modules and devices that I copied off of this forum that was created by Ron Beaufort. He is retired now and ran a great PLC training "boot camp". His wisdom and advice are priceless, as it is with many others here.
I would verify devices, wiring, grounds and connections. Hope this helps.

Click to expand...

Thank you for that document! While it is a helpful resource, I have verified that my sensors are the correct type and are wired for a sinking input.

GaryS said:

Have a look at the output configuration
you can set how they should react when communication is lost
Each output is set separately
Turn ON
Turn OFF
Remain in last state
There may be a reason the origenal programmer wanted then to turn ON

Click to expand...

That you for the idea, I was able to verify that all my outputs are configured to turn OFF in the state of communication loss. The problem still remains, unfortunately.

I've been retired for years and hope I'm not just confusing the issue. I recall my first experience with the solid state output modules - I was using them to control motorized valves. The valve actuators, as it turns out, also used solid-state inputs. So, a solid-state output device controlling a solid-state input - There was no place for the voltage to "bleed-off" when the output module switched to the "off" state.

I compensated by adding NE-2 neon lamps (with appropriate input resistors in series) across the terminals of the output modules. Hence, when the module output went "off", the neon lamp instantly bled off the voltage and corrected the issue (the problem I was overcoming was that the valves would not shut on command).

This is a bit off the wall but are you sure that you don't have another PLC accessing the output on Inputs Rockwell allows up to 3 PLC's to read the Inputs they say that only 1 can write to the outputs but maybe the other is not Rockwell
see what happens when you Ethernet wire from the Output module
Do they work correctly when connected to the PLC ? if not the may be stuck on

MasterBlaster said:

I've been retired for years and hope I'm not just confusing the issue. I recall my first experience with the solid state output modules - I was using them to control motorized valves. The valve actuators, as it turns out, also used solid-state inputs. So, a solid-state output device controlling a solid-state input - There was no place for the voltage to "bleed-off" when the output module switched to the "off" state.

I compensated by adding NE-2 neon lamps (with appropriate input resistors in series) across the terminals of the output modules. Hence, when the module output went "off", the neon lamp instantly bled off the voltage and corrected the issue (the problem I was overcoming was that the valves would not shut on command).

Click to expand...

I am unfortunately unable to add such devices to the circuit, as this is a brand new machine I am installing according to schematics. I would have to convince someone in engineering that they made a poor design choice, and you know how that goes. I did swap some additional loads (another siren and some more lights) to the same output modules on one chassis and found that my voltage remained consistently about 10v no matter the additional loads. I am able to dissipate this if I connect a neutral back to the chassis without the 24v wire, but if I disconnect both the problem remains.

GaryS said:

This is a bit off the wall but are you sure that you don't have another PLC accessing the output on Inputs Rockwell allows up to 3 PLC's to read the Inputs they say that only 1 can write to the outputs but maybe the other is not Rockwell
see what happens when you Ethernet wire from the Output module
Do they work correctly when connected to the PLC ? if not the may be stuck on

Click to expand...

I have verified that the outputs and inputs function correctly in normal conditions, and the problem only manifests with the 1734-AENTR completely disconnected from supply power, thus disconnecting it from the Ethernet circuit due to lack of being powered.

It also sounds like you have a floating ground. This maybe by design or a loose wire on you 24V power supply. Check to see if your shop is using floating power as a source supply, or a fixed ground, most are grounded. This could be the source of mystery power if your testing to ground. Just food for thought.