having a problem with main door sensor on this 044 slant

[Badbaud]

We put jumpers across all of the mechanical door switches so only the door optic was being used.
Closing the door still gives door open M.
We plugged in a optic bypass circuit that allows the door optic to transmit to the cabinet optic via a long cable with some electronics inside it to pass on the signal over the wire.
This way the door and cabinet optics are lined up when the door is open.
We still did not get a door closed.
We plugged in our optic transmit voltage detector. It is simply a visible LED in series with a 390 Ohm resistor that is plugged into the connector the door optic would be plugged into.
The visible LED did not light, well it did but so faintly you had to cup your hand around it to see a flicker.
If the visible LED does not light then the door optic transmitter is not getting enough electrical power to emit it's invisible light.
This narrowed the search down to the optic signal being defective.
Next we got the scope and looked at the transmitting optic signal on it.
What I saw was a full swing of voltage as expected but the pulses were extremely narrow.
Those pulses were so narrow the LED and the optic transmitter did not have enough time to turn fully on.
Normally the optic would be pulsing on and off at a visible rate, as verified by the LED test.
I am assuming some electronic component in the system is putting out weak pulses.
During our venture into diagnosing we had swapped out the MPU and the door open error did not occur.
We also swapped the backplane board, same result.
So that narrows it down to system wiring or a component failing inside the system.
Wiring Ohm'ed out good.
My question is, does the signal originate  from one of the I/O cards and it it the one on the left or the one on the right?
What are their individual functions?

[Badbaud]

I never found the cause but I did solve the problem.
I used the scope and found the wire that carried the signal that drives the door optic.I then determined which wire on the cabinet side optic passed on the received signal to a I/O section of the game.
I then tied the transmitter wire to the common of the main door switch and tied the received signal wire to the normally open of the switch.
When the door is closed the door switch connects the optic transmit signal wire to the optic receive signal wire the two signal paths are connected and the game sees a door closed.