S4C missing Input signal

I have an S4C controller here for an IRB6400 robot, I have some of the I/O connected to another robot and other equipment, recently I have added another 4 Inputs from a PLC.

The problem I am having is the robot keeps missing the signal even though it has a constant 24v going to it. It reads the signal most of the time while the robot is running through the program however it seems to not be able to read the signal during some parts of the program.

I have tested this by watching the indicator leds on the I/O board and it lights up and then it will start to flicker and turn off. I have tested this with a multimeter held to the specific Input I am testing and it has a constant 24v the whole time even when the led turns off.

The power for the bank of inputs I am having problems with is coming from the robot itself.

Does anyone have any idea what is going on, I have the first bank of inputs wired up and they work perfectly so I am unsure what is going wrong.

I have attached photos of the I/O

Quote from hermann

- You should measure against Pin 9 of the same connector where the signal is (X4). May be a weak 0V connection to this bank.

- try a direct connection to 24v to the desired input
- otherwise hardware defect of the DSQC

I have measured against pin 9 of the same connector, it was just easier to take a photo as shown.

I should mention, when the robot is stationary the I/O remains on constantly and never seems to cut out.

I will try connect constant 24v to the board the next time it is running and check it.

I found the problem, it was a partial internal breakage in the wire for pin 9. Replaced it and all is working fine now.

Thanks for all of your suggestions.