[GreenKeys] Regenerative Loop Driver for Model 15 Teletype

[Mike Douglas]

The loops are separate coming into the board so that full duplex operation with a computer is simple, but the board also has a half-duplex mode in which anything typed on the Model 15 keyboard is sent to the printer as well as to the RS-232 output.

Mike

> On Dec 8, 2021, at 3:47 PM, Harold Hallikainen <harold at w6iwi.org> wrote:
> 
> I also thought about the selector magnet and keyboard in series. However,
> the selector magnet flyback voltage comes from the top of the keying
> transistor, and it would not be there if the loop is interrupted by a
> series keyboard, instead.
> 
> Here, I'm using the typical high voltage DC supply with series resistor to
> drive the loop. I use TRS connectors so the shell can be grounded. The
> jacks also have shorting switches so the loop remains closed when
> equipment is unplugged.
> 
> The boost circuit is clever!
> 
> Harold
> 
> 
>> On Wed, December 8, 2021 2:30 pm, Mike Douglas wrote:
>> Jim,
>> 
>> One of my main goals with this project is to use the Model 15 as an
>> operator console with my vintage computers (e.g., Altair 8800, IMSAI 8080,
>> etc.) in the same manner as I use my Model 33. This requires full duplex
>> operation, and therefore, separate send and receive loops. But I'm a child
>> of the computer age, so Irv Hoff and I have different goals for the
>> equipment :)
>> 
>> Mike
>> 
>>     On Wednesday, December 8, 2021, 03:10:48 PM CST, Jim Haynes
>> <jhhaynes at earthlink.net> wrote:
>> 
>> That's very interesting - using such modern technology to drive an
>> ancient
>> Teletype.
>> 
>> "Back in the day" Irv Hoff was always insistent that the Teletype keyboard
>> and printer should always be simply in series, and his "Mainline" stuff
>> always worked that way, so there was a single 2-conductor cord between the
>> machine and the electronics.  I was not particularly taken to that point
>> of view, and in fact my designs at the time worked around converting the
>> Teletype external interface to something related to RS-232.
>> 
>> But I did fiddle around with, never built, a "leg combiner" circuit that
>> allowed the keyboard and printer to be simply in series in a loop but
>> provided separate receive and transmit legs.  The trick is to detect
>> whether a spacing signal came from the keyboard or from the receive leg,
>> which involves setting a latch.  If anybody wants to look at this circuit
>> just ask.
>> 
>> 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
>> 
> 
> 
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