Look at this Cool Find (Memorex 2051 Terminal Keyboard)

[SgtPeppers]

I was given this Memorex 2051 Terminal Keyboard around 2 years ago by some old man we knew. I didn't think much about it at the time, but it came to me to post it to r/Mechanicalkeyboards which led to people being quite curious and it ended up on KBD.news (https://kbd.news/Memorex-2051-1173.html). Since I have a desire to document this on the Wiki, here are some images of the board and it's PCB. Also, it is now in possession of my as we have plans to try getting it working via USB or PS/2. The keyboard seems to be using Hall Effect switches, specifically the Micro Switch SD series, just like the Space Cadet keyboard.

Here is my friend's typing demonstration: [youtube]https://www.youtube.com/watch?v=KwPSjyFhrnw&t=10s[/youtube]

Also, here are some images (Please allow higher resolution images):
There is also an Imgur Post Available: https://imgur.com/gallery/Y0mAFLo

The Memorex 2051 Terminal Keyboard in a high resolution, for once, outside the advertisement.

2051 Keyboard.png (3.99 MiB) Viewed 6924 times

IMG_20220102_174534.jpg (27.14 KiB) Viewed 6924 times

IMG_20220102_174537.jpg (23.79 KiB) Viewed 6924 times

IMG_20220102_174541.jpg (26.67 KiB) Viewed 6924 times

The Keyboard's PCB (Again, sorry the the low resolution)

IMG_20220102_174557.jpg (28.62 KiB) Viewed 6924 times

IMG_20220102_174602.jpg (30.6 KiB) Viewed 6924 times

IMG_20220102_174605.jpg (30.44 KiB) Viewed 6924 times

IMG_20220102_174608.jpg (29.96 KiB) Viewed 6924 times

[Bjerrk]

Very cool - Any pictures of the switches (i.e. with a few keycaps lifted)?

[SgtPeppers]

Very cool - Any pictures of the switches (i.e. with a few keycaps lifted)?

I believe the YouTube video shows it, but lemme get some photos up. You might have to wait a bit, though, since I need to do some maintenance to my home internet.

[MMcM]

From the photos (and a few more clearer ones), it is possible to mostly work out how this operates. It also helps to understand how the 5251 keyboard communicates, because some of those ideas were borrowed, too.

• Program
  • 8048 microcontroller (4)
  • 74373 latch (2) and PROM (3) implement extended program memory in a standard way; that is, part / all of the program is on that chip and not the microcontroller itself (8048 is not reprogrammable).
  • Both the microcontroller and PROM programs can be read off, but they would need to be desoldered to do that.
• Matrix scanning
  • Switches are 3-pin (4B2E), which are meant to be intermittently powered.
  • 74154 demux (5) outputs to a single row so that the Hall Effect sensors in those switches run.
  • MCU port 1:0-5 reads column inputs for pressed switches.
• Communication
  • Substantially the same as IBM 5251. Compare https://archive.org/details/bitsavers_i ... 5/mode/2up.
  • 74374 register (1) holds parallel output for terminal; it is latched by MCU writing (*WR) to the bus (DB0-DB7).
  • This is odd connector pins 11-25, with bits 0 and 2 swapped for some reason.
  • MCU port 1:6 clocks the 7474 flip-flop (7), which loads low into it. This is the signal that data is ready.
  • Connector pin 24 (DATA strobe, open collector, no pull-up) is driven by the state of the flip-flop.
  • The MCU only sends when connector pin 26 (16 on DB25, DELAY strobe) is held low, which it senses through T1.
  • On receipt of the DATA signal, terminal must bring DELAY high again, to signal processing, and then low again when ready, which will preset the flip-flop back and signal MCU.
  • If connector pin 20 (POR) goes low, MCU is reset and flip-flop is also preset high again. This is power-on reset.
  • If connector pin 22 (CLICKER) goes low, 75451 high-current driver (12) powers the solenoid; there is also a diode to protect against backcurrent.

What cannot be gathered from looking because it's determined by the firmware:

• The parallel output is scan codes and not ASCII.
• Every key sends a unique scan code.
• These codes are not in any obvious way related to the IBM ones; they seem to come straight from the matrix coordinates.
• Only the three shift keys (open arrows) send break codes, which are one different from their make code. Because of this the key to the left of A is more useful as CTRL than CAPS.

[basicbass]

Some better photos

[basicbass]

The pcb and traces

[SgtPeppers]

Some better photos

Thank you so much for that. Take good care of it, my friend.