Ricoh D131 Service Manual page 1518

2.4 TRAY FULL
A pair sensors are used to detect the tray full condition. At the start of every job:
Tray full sensor (Emitter) [A] emits a signal to tray full sensor (Receptor) [B].
As long as the signal remains unbroken the multi-folder will continue to operate and
feed folded paper to the top of the unit [C].
When the top of the stack grows high enough to interrupt the signal between the tray full
sensors [D], this will signal the machine to shut down the line temporarily.
After the operator removes the stack from the top tray, folding and paper exit will
resume.
SM 125
Tray Full
D454/D615