[GreenKeys] Model 28 etiquette

[Jim Haynes]

The most basic LESU has the power fuse and the power switch, mounted
so the switch rod that comes out the front of the cabinet can operate
it.  And also, I believe, a terminal block for the selector magnets.
You can leave out the LESU but then you have to do something about
a power switch and fuse.

Typical options are
    Line shunt relay - shorts the signal line when power is turned off,
       so that if your machine is in a series loop with some others
       you can disconnect the keyboard and typing unit without opening
       the line and putting all the others out of service.
    Line relay (polar) and power supply.  If you have a line relay
       you need the power supply to power the selector magnets circuit
       and the bias winding of the polar relay.  And the selector
       magnets have a terminal block in the LESU so you can connect
       the line relay.
    Motor control relay
       There are several kinds.  One is a latching relay, such that
       some stunt box sequence, typically FIGS-BLANK-H switches the
       motor off and a momentary open on the line switches it on.
       Others can be operated by line polarity: reverse polarity turns
       the motor off, normal polarity turns it on, and there is enough
       time delay that the motor stays on when receiving signals.
       Still another kind turns off the motor after some duration of
       idle line, turns it on when there is a momentary interruption.
    Line-Test key - seems to be usually a feature on Bell System machines.
       allows putting the keyboard and printer into a local loop for
       testing.  Requires power supply.