bad wiring or bad modules

I'm working on a project on a SLC 500 PLC. I have order 2 DC modules and they are 1746-IV8 and 1746-OV8. I have tested the +24VDC on the power supply on the PLC and reads 22.9V and after wiring the new modules the Voltage reads 1.5 volts. I'm wondering if the wiring is wrong or the new modules were bad.

Welcome to the Forum !

The "V" indicates that they work opposite the more popular "B" modules.

1746-IV8 is a Sourcing input module It provides +24V DC and expects the input devices to connect to DC Common.

1746-OV8 is a Sinking output module. It provides DC Common and expects the output devices to be connected to +24V DC.

Whether or not your wiring is "bad" depends on how you hooked it up.

Is the "DC supply" the little auxiliary supply on the 1746-Px Power Supply module, or are you talking about a larger DC Power Supply mounted externally.

According to the manual:
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1746-in027_-en-p.pdf
the switches (or whatever you have inputs wired to) are to be connected to -VDC and the (2) VDC connections at the bottom are to be connected to +24VDC -- this is on the IV8 module.

On the OV8 unit, similarly, +24VDC is the common for your output devices as well as connected to the VDC (top) terminal, while -VDC is connected to the lower (DC COM) terminal.

To bring the 22.9 VDC down to 1.5 VDC you need a pretty heavy draw if not a dead short. I would think this would fault the processor (speaking from experience ). Sounds like the 24VDC supply is bad but I’m not really sure. One way to find out is put a multi meter in line and check the current draw. I don’t remember what the current capability is on the power supplies but I do remember it’s not very much. If the attached modules are drawing to much current (via the connection to the 24VDC not the backplane) it will draw down the voltage but again if it takes it down that far I would expect the processor to fault.

I'm wondering if the wiring is wrong...

Click to expand...

That is a good guess for pulling 24 volts down to 1.5 volts.