Those are internal communication lines used between the 900-4RL(s) and 900-2Q(s) boards in whatever combination they are arranged in. They go straight through the power supply board. It is only used as a connection point.
Their function is:
-sequence any EL device pulls so that only one retracts at one time due to the 16A surge. Example, if two 4RL’s are installed, each loaded with 4 EL99 devices and all get an input at the same time, board one will sequence all its 4 devices then allow board 2 to do the same.
-if used as interlock function, they allow large interlocks to exist using more than one 4RL board. These lines tell the other boards that a door is open.
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