[GreenKeys] To SMD or not to SMD?

[Jim Haynes]

I've been trying to get around to writing an essay on ASR set design.
The idea goes back to 1959 when I was working on the low-voltage selector
with transistor selector magnet driver.  The concept I wanted to sell
was that every selector magnet should have a driver, and it should work
on something like RS-232 voltages.  (RS-232 didn't exist back then,
but the concept was around)  And keyboards and tape readers should
likewise generate low voltage signals rather than be in current loops.
Do all the switching inside the set with low-voltage signals and have
just one circuit to interface the internal connections with the outside
world, which at the time was usually current loop.

Either I was too junior a person to be taken seriously, or I didn't
try hard enough to sell the concept, but it didn't happen and as we
know the 101 Data Sets and the related Teletype equipment all had
silly low voltage 20ma current loops.  My point was that it's much
easier to switch things when you are switching voltages rather than
currents, and at low voltage being compatible with transistor circuits
you could have accessories like a regenerative repeater that didn't
have to cope with high voltage current loops.

Later on I built my own 35 ASR set with no K-KT-T switch and with
controls as I felt they should be.
    Line-Local toggle switch for the printer
    Line-Local toggle switch for the keyboard
    Line-Local-Off pushbuttons for the punch
    Line-local-Off pushbuttons for the tape reader

The 35 is easier to do than the 28 because it doesn't need a purely
mechanical punch to allow the operator to punch fast.  100 wpm should
be plenty fast for the punch operator.  The reason for the K-KT-T
switch in Model 19 and Model 28ASR is to allow the punch to run faster
than the signal generator because the operator can punch tape faster
than 60wpm.

I tried again about 1966.  RS-232 existed by then.  With a little bit
of creative interpretation of the various RS-232 signals I figured
we could make TTY sets with nothing but RS-232.  This was in contrast
to the 33/35 with its 100-wire interface to the 101 Data Set and
Model 37 was starting to have a 50 wire interface.  But I couldn't sell
this to the old geezers at Bell Labs, although some managers at Teletype
could see the advantages.

The inspiration for that one was that some of the Bell operating
companies were not buying the 101 Data Sets and the expensive TWX
model 33 and 35.  Instead they were buying the private-line 33/35
machines, adding the RS-232 converter box, and using them with 103
series Data Sets for TWX as well as DataPhone and other services.

There was also MIL-STD-188 which used low voltage polar signals,
with polarity opposite to RS-232 for some reason.  I'm sure the military
would have liked to have TTY sets that didn't require those signals to
be converted to high voltage current loop.

Jim W6JVE
 	---

 	"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