I wasn’t particularly vested in this machine but, i wanted to see if there was life and if so how much life.
I turned on the machine using the circuit breakers at the rear of the machine. These felt a bit clunky but gave a satisfying click and the fans came to life with quite a lot of noise.
The drive lights flashed regularly, as if beckoning me to pop in the boot disk. I answered that call, but there was no attempt to read the disk.
I still had the unit open, and I could see an LED on the power supply flashing with the drive LEDs, so I figured it was probably a power fault. The CRT wouldn’t switch on.
I pulled all the cards from the rack (except the CRT supply which was a bit tricky) and the supply came up fine – solid LED. This prompted me to measure all the power supply lines and work out which supplies go where, including the computer bus.
The CRT still wouldn’t switch on, but I thought that it might need one of the cards that I’d removed. (The front panel switch is wired via the keyboard, as it turns out.)
Replacing the cards and connectors progressively went well until I connected the keyboard.
I checked which “pins” on the processor board edge connector provided power, –5V and 15V, and found that the keyboard did indeed have a short from 15V to ground. I cut a track to narrow it down. There was a tantalum on the remaining piece so, I took a wild guess that it would the cause. It was. I replaced it and made the cut track good.
After that, the unit fired up. The screen turned on, and the unit requested a boot disk. I gave it the only one that I had, and it booted.
Unfortunately, it wasn’t picking up anything that I typed. That led me back to the keyboard.
The keyboard is a Cortron unit. The company still exists and still makes keyboards, but I can’t see them lending much support.
The unit is almost certainly a custom unit made for lexitron. I was able to find some documentation for another computer that described how the cortron keyboards worked. I also had a good poke around the board. The working description for the other keyboard seemed to accord with what I could see on this unit.
The keys use contactless magnetic cores. When they are pressed, a pulse can be transmitted from one winding to another.
It’s all controlled from a microcontroller. The switches seem to be arranged in a 10 x 10 grid. The microcontroller addresses the array via two decimal decoders. I expect there must be a sense line back – I need to find that. The micro controller presents a byte to the host processor reflecting the key that’s been pressed or released. I imagine there’s some debouncing, and they must have done something about “shift”.
The keyboard also presents the state of the front panel switches. A mux routes either the keystroke or the switch state to the 8085 board depending on a select (address line) from the 8085 board. I suspect that the processor sends a “come and get it” strobe (maybe an interrupt) when a key press is detected.
The keyboard also has status LEDs (eg caps lock) and those are written by the 8085 board to a register on the keyboard. Ie the keyboard seems to just be a peripheral device on the 8085 bus rather than be accessed via, say, a serial port.
The bad news is that I could see no strobes. The micro controller looks dead to the world, although it is happily clocking. It is almost as if it has lost its program. That’s a real problem because I doubt that I’m going to be able to find that ROM binary. It is a UV erasable part rather than a mask program – there must be some kind of finite life.
This is going to be a bit of a problem to remediate. I don’t fancy writing that code using the original processor – it was probably in assembler.
At this point the sensible thing to do was to say, “well i tried” and then grab the drives and the RAM and chuck the rest into recycling. I put it down for a while before deciding that i could make a replacement based on a teensy board using the Arduino development environment. This board is vastly more powerful than the microcontroller, so i’d be able to work in c.
There are two pieces to be solved:
- the protocol and character coding (maybe ASCII) to the 8085 board
- how to detect keystrokes
This was a genuine case of suck it and see, and i knew it would be time-consuming. I decided to park the project until i could find a way to duplicate disks. In 2019, i don’t think greaseweazle had entered my conciousness.
