[GreenKeys] Polar Relay

[Jim Haynes]

I think the first UARTs were made by Western Digital - I remember Teletype
was aware of them, probably under a non-disclosure agreement, but that was
before they became production items.  And I had heard that DEC was the
original impetus for the development.

Teletype first used ring counters to direct signal pulses into flipflops,
or from flipflops to the signal line.  In fact the first of these used
Western Electric point-contact transistors that were expensive and rather
unreliable.  Teletype switched to a circuit that used an NPN and a PNP
transistor together to get performance similar to an SCR.  This was all
in time-division multiplex equipment for the Navy.

Then someone at Bell Labs showed how to use shift registers as 
distributors and Teletype adopted that.  For receiving, set the shift 
register to all marks, shift in the bits, and when the spacing start pulse 
arrives at the end of the register you have the whole character in the 
register.  For transmitting load the character into the register, shift in 
marks, and when it contains all marks stop the clock because the character 
has been shifted out.  (You don't worry about the character finishing too
soon because it has trailing marks; you aren't going to get another
character until the tape reader or whatever has one ready to send.)
We used unijunction transistors as bit clocks.

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man