[GreenKeys] TTY loop interface

[Jim Haynes]

Many years ago I tried (unsuccessfully) to sell a concept of how to
hook up Teletype gear in a station.  The idea was that every selector
magnet would have an electronic driver, and every signal generator would
produce a signal, and the standard signal would be a voltage of + or - 6V.
This was before RS-232 came out (and was intended for a different purpose)
and probably before MIL-STD-188.  At the time I had in mind low voltage
selector circuits that could be keyed with power transistors rather than
120V circuits, but today even the latter are easily handled with solid
state relay ICs.

The problems this solves, compared to current loops, are that you don't
have to keep current in the loop to keep something from running open,
and you don't have to keep the loop closed when you disconnect something.
(Also you don't have 120V loops running around your jack panels or 
whatever that can give you nasty shocks.)  The selector magnet drivers
can be biased so that the selector circuit is closed when there is nothing
connected to the driver input.

Think of the signal circuit as a hub rather than a loop.  You can connect
as many printers, reperfs, etc. to the hub as you want, so long as their
drivers are designed not to draw too much current from the hub.  You can
connect as many keyboards and other signal sources to the hub as you
want by putting in diode or transistor gates so that any one of them can
drive the hub spacing when the rest are marking.  You can, if you wish,
have separate receive and send hubs with a regenerative repeater between
them.

Jim W6JVE

 	---

 	"Ya can argue all ya wanna, but it's dif'rent than it was."
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 		Meredith Willson, The Music Man