[GreenKeys] Model 33 Reader Control Mod

[William Wuttke]

Over the years there have been a few questions regarding the Model 33 Reader Control function (usually for DEC PDP-8 computers). The original mod used a relay to control the tape reader. I came upon a problem when trying to use my Model 33 reader with my HAL ST-8000 modem at 45 baud. The ST-8000 has only a 256 byte buffer, so a long tape would overrun the buffer. I used a heavy-duty solid state relay (really overkill) instead of a mechanical relay and modified my “automatic reader" Model 33 ASR to use it. (The original mod was for a “manual reader” Model 33 ASR.) It’s a very simple mod. The solid state relay is controlled by a 3-32 VDC signal and works well straight from an Arduino output pin.

I also made an Arduino “chameleon” device to allow the M33 to operate at either 110 baud ASCII or 45 baud Baudot. At 110 baud ASCII, the reader control is controlled by CTS from the ST-8000. At 45 baud, the reader is controlled by the Arduino’s TX buffer. I also built-in RTS control for keying the ST-8000. I also did some testing of pulsing the reader control relay every 165 ms to allow sending at 45 baud (although characters are still sent at 110 baud) - it worked well in pulsing mode.

Here’s a link to the mod instructions and modified schematics:

https://www.dropbox.com/s/0mcqfd4m980zaqp/2018%20Reader%20Control%20Mod.pdf?dl=0

Have a Happy New Year - looking forward to GreenKeys Night,
Bill Wuttke
KE3BK