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README.md
lqp02-teletype.ino

README.md

This is a project motivated by a desire to create a daisywheel teletype, to give a similar experience to that of a Model 33ASR teletype, but with upper and lowercase characters. Thanks to Hugh Pyle and his fantastic Twitch channel for the inspiration. Thanks also to Chris Gregg and his project: https://github.com/tofergregg/IBM-Wheelwriter-Hack and to this very similar project by Stephen Casner: https://github.com/IBM-1620/Junior.

The goal is to intercept user input from the keyboard and send it to a remote computer and receive the echoed characters from the remote computer and send the corresponding commands to the printing mechanism for hardcopy output.

I obtained an IBM Wheelwriter 5 from the Tektronix Surplus Store (open two days a month, first and third Thursdays, from 2pm to 4pm) on the Beaverton Oregon campus of Tektronix. This typewriter came with the PC Printer option, which hangs on the back of the case and interfaces by a 6 (or 7?) pin bus. The cost of the typewriter was $25. I purchased two new-in-box ribbon cartridges from the same location for $0.50 each.

In the process of working on the IBM Wheelwriter, I was given a couple of DEC LQP02 serial daisywheel printers. This repository reflects the work on the LQP02. These have some advantages, including the availability of a native RS-232 interface and full ASCII character set wheels.

This project uses a Teensy 3.1 with a PS/2 keyboard, and two serial ports. One serial port talks to the printer and the other talks to a remote computer. It decodes keyboard events using the Ps2KeyboardHost library from Steve Benz: https://github.com/SteveBenz/PS2KeyboardHost, sends them to the remote computer, reads data coming from the remote computer and sends them to the printer's serial port.

The printer is constrained in the rate at which it can turn bytes into print, so flow control is important. It turns out that the printer does XON/XOFF flow control, and the remote computer does RTS/CTS hardware flow control, so the Teensy program looks for XON/XOFF from the printer and stops reading from the computer serial port when it last saw XOFF, which lets the hardware UART's flow control just work.

One of the disadvantages of the LQP02 is that it was intended as a strictly output device, and as a result being able to see what you are typing is not prioritized. I saw a video on youtube (https://www.youtube.com/watch?v=TRxz4x45i54) which used the vertical motion control to roll the platen to lift the active printing line up above the printing mechanism at appropriate moments so that you can see what you have typed. On the LQP02, 12 lines seems about right.
When rolling the paper back down to continue printing, I am rolling an extra line down and then back up one to hopefully improve vertical registration. Thanks Blake Thomas for the idea.

There are a bunch of TODO's:

  • Add a toggle switch to switch between Line and Local mode;
  • Support the mini-DIN8 connected Sun Keyboard;
  • Support output to an IBM Wheelwriter;
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