| 摘要 |
PROBLEM TO BE SOLVED: To allow continuation of operation in spite of the occurrence of abnormality in one of a plurality of oscillators.SOLUTION: Detection means 24 is provided for monitoring whether oscillation means 20 normally generates an operation clock or not. Detection means 24 of a second light emitting unit U1-2 monitors a second operation clock generated by oscillation means 20 of the second light emitting unit U1-2. When the oscillation means 20 is normally operating, selection means 22 of the second light emitting unit U1-2 is set to a first selection position P1, and the second operation clock generated by the oscillation means 20 is outputted from a transmission section 28 of the second light emitting unit U1-2. If abnormality of the second operation clock is detected, the selection means 22 is switched to a second selection position P2, and a first operation clock generated by oscillation means 20 of a first light emitting unit U1-1 and outputted from a transmission section 28 is inputted from a reception section 26 of the second light emitting unit U1-2 and can be used as an operation clock of the second light emitting unit U2-1. |
