[GreenKeys] Model 15 to a computer

Wed Nov 3 12:15:49 EDT 2021

 I have a model 15 that I'm just now getting the time to work on its restoration. As part of procuring this TTY, my goal has always been to connect it to my vintage computers in the same manner I use my Model 33 (see http://deramp.com). To this end, I designed an interface board to make this happen. Until I get my model 15 fully restored, I can't fully prove out the interface board, but both are progressing nicely at the moment.

Here's a picture of the interface board: https://deramp.com/downloads/teletype/Model%2015/pictures/60ma%20Interface.jpg

On the right side, the interface board provides 1/4" phone jacks for connecting to the printer loop and the keyboard loop of the Model 15. On left side is a barrel jack for a 15v AC adapter, an RJ-14 jack for RS-232 connection using modular DB-9 or DB-25 connectors, and an audio jack for connecting AFSK feeds (e.g. ITTY).

The RS-232 connection uses ASCII at 9600 baud and includes a handshake line to throttle the sending computer. With a terminal emulator, it's just a matter of turning on the "hardware handshake" checkbox. For vintage computers, virtually all serial boards bring out the CTS pin of the UART to the DB-25 and that will throttle the transmitter without any software changes. This allows the source computer to use ASCII at a standard baud rate instead of having to worry about a five bit code or 45.5 baud support.

Presently, I have tables installed for ITA2-TTY and USA-TTY for the ASCII-Baudot conversion. I've also added the ability to generate ASCII keycodes that can't be generated by the Model 15 keyboard by defining an "escape" sequence starting with either FIGS-FIGS or the blank key.

The audio interface demodulates AFSK (presently 2125 Mark - 2295 Space as used with ITTY) and send the demodulated output directly to the printer.

Another goal of the interface is to make it "regenerative" in that is does not require an external high voltage loop supply and does not use a switcher on-board to generate the high voltage required. Instead, a regenerative approach is used in which the snubber circuit for the selector magnet coil captures the energy from the mark to space transition and then dumps that energy back into the selector magnet during a space to mark transition.

For the following paragraph, reference this scope screenshot https://deramp.com/downloads/teletype/Model%2015/pictures/60ma%20Interface%20Scope.jpg

Consider an idle marking state in which a low voltage supply (e.g., 15vdc) is supplying 60ma holding current through the selector magnet coil. A subsequent transition to space charges a snubber capacitor to high voltage in about 2ms as the selector magnet’s field collapses. At the next transition to mark, the capacitor is dumped back into the selector magnet. Coil current reaches 50ma within about 2ms as the capacitor discharges. The low voltage supply then builds up to, and holds, 60ma until the cycle repeats at the next mark to space transition.

Mike

     On Wednesday, November 3, 2021, 12:24:25 AM CST, Joe Duszyński <joeduszynski1 at gmail.com> wrote:  

My decoder recently died so have been toying with replacements ideas that would also incorporate some sort of serial interface to the computer

The least soldering and building or thinking too hard.....
An old computer with built in serial interface is probably the easiest thing using optocouplers and say heavy metal as your (go between) 
Its pretty easy to find an old computer with built in serial. To tell if its suitable run heavy metal on it with a loopback to pins 2&3 on the serial port at 45.5 baud
If that computer sends what it receives then "Nagle's" board at https://www.aetherltd.com/connecting.html would be your magic device interface.
All the parts for that board are available.

Arduino...
Lots of solder, cerebral usage, wires laying all over the place and programing......................
I started playing with arduino's recently because as I stated my decoder died and I wanted something a tad more sophisticated to do the decoding and to have some sort of serial interface too would be nice to send to the teletype from computer all in one box instead of two boxes I had. My prev decoder ran the teletype and would put the text on a video monitor and it was about 30 years old. I also had on the loop a serial interface and would sometimes get a mess if both talked at the same time.
On the arduino front I have the Sending from the arduino down pat. Decoding from the loop I just got working. Now I need to add a second "loop decoder" and feed demodulated RTTY input into the second loop decoder is the project for this weekend...
For that project I heavily borrowed source code and circuits from two sources and modified it for my needs.
https://k183.bake-neko.net/ji3bnb/page13_d.html.. (PLL CIRCUIT and some code)
and 
https://heepy.net/index.php/USB-teletype  (Some of the code) 
The volpe board kinda would do what I want BUT I'm also not into soldering SMD's (Surface mounted devices) so since it was designed for a "32u4 AVR"I decided to adapt the code to run on a "Leonardo/Arduino board" and add a second decode input for the RTTY.
Nice thing about having two decode loops (one for the keyboard/teletype loop) and one for the RTTY the arduino can monitor the traffic and either block the keyboard or block the RTTY when one or the other is sending at least thats the plan. And its a hell of a lot easier than all that SMD nonsense.

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