On the Schematic page W6PM1SI1A on lines 14, 15, 16, 17, and 18 note the photoeye emitter and receiver PEGCS and PEGCR. The emitter sends a beam and the receiver closes a contact upon blocking of the beam. The receiver is a dark operate receiver, meaning the contact closes when the beam is no longer being received or is blocked. The contact PEGCR energizes relay coil CRGC. When the relay energizes it closes a contact that makes an input at the 1st I/O card at MTA4 pin 6 (1MTA4-6). If you measured the DC voltage at 1MTA4-6 to TB7 on a din rail connector or 1MTA31-10 on the processor card, you should have 0vdc. If you 12vdc the input is not being made.

The receiver has a red LED indicator that will glow steadily whenver the receiver is seeing the modulated light from the emitter and will flash to indicate marginal receipt. This LED is visible through the lens or a window on the side of the barrel of the receiver unit.

Emitters have a LED that lights steadily to indicate AC power availability. This LED is visible through the lens or through a window on the side of the barrel near the lens.