The multiplexer consists of a transistorized
device selector and a relay calling card for each
16 lines of controllable devices. The device selector
connects the 16 read and write lines, one
at a time, to two 16 lines buses as commanded by
eight control and sense lines from the computer
I/O and changes the signal level for relay control.
The device selector signals the calling
cards to connect one or more devices to the two
16 line buses (but not more than one to each bus)
and tests the 16 line bus for low impedance. If
the remote-local switch on the console is in remote,
the calling card can connect the device to
the bus. If the R/L switch is in local, the
calling card is locked out, and the device selector
will see an open circuit and signal the computer
that the device is not controllable.