The Curious Case of Jupiter Ace

The Jupiter Ace was before my time, but I found the user manual to be a very well written Forth tutorial: https://archive.org/details/Jupiter_Ace_Users_Manual_1982_Ju...

I had one of the early Jupiter Aces - the circuit board said something like v1.00 - and it had some added soldered wires on the board, correcting some errors, but it worked well enough. Forth was definitely what attracted me to it - I was frustrated with Basic on my Sinclair XZ81, and Forth seemed like a big step forward. And I think it was.

I was 14 or 15 at the time. The first summer I wrote a whole load of games, mostly in Forth, but sometimes Forth just wasn't fast enough. I got a copy of "Mastering Machine Code on Your ZX81", and learned Z80 machine code. If there was an assembler available, I didn't have it, so this was all hand-assembled. Getting jumps right was a total pain, as was debugging. Generally, it either worked first time, or you started from scratch again. Usually I got there in the end.

I sold those games through a ad in "Your Computer" magazine, and earned back the price of the computer several times. But recording and shipping tapes one at a time got tedious really fast, so I didn't take that any further.

My Ace got modified quite a bit over the next couple of years. An extra 16KB RAM pack was an early addition, taking it to a whopping 19KB. Because Forth was so compact, I never filled that up.

My mother worked for Chubb cash dispensers (ATMs for Americans) at the time, and at some point they scrapped a lot of equipment. She tipped me off, and we went round there one evening and liberated lots of cash dispenser pieces from a skip. Amongst these were several numeric keypads with lovely mechanical keys. I dismantled several keypads to take out all the individual keys and made a proper keyboard to replace the original dead rubber monstrosity.

At school, I was taking O-level Technology, and you had to do a final year project. I'd seen articles in "Your Computer" about competitions where a micro-mouse robot would have to find its way through a maze. That seemed like a cool idea to me, so I decided to make one, despite not really having much idea how. I hadn't done any digital electronics at that time, but my friend Johnny Tombs (who later went on to be a Professor of Electronic Engineering) had decided to make a parallel I/O port for his ZX Spectrum using TTL logic for his Technology project. I'm not sure where Johnny learned about digital electronics, but he knew what he was doing, and designed a nice elegant circuit board and etched it at home. The Ace had the same Z80A CPU as the Spectrum, so I figured his design could be adapted. After a lot of pleading, he gave my his design and spent some time explaining to me how it worked and loaned me his copy of Watford Electronics catalog. A couple of weeks later all the components arrived, and I spent a busy few days soldering up my modified I/O port on veroboard. Compared to Johnny's professional looking version, mine looked like a mess of wires and chips, but it worked!

So then when it wasn't playing games, my Ace rode about on the top of a fairly flaky micromouse, made in the school metalwork shop out of aluminium sheet, some lego motors, and some light sensors to detect maze walls, exploring mazes. It didn't win any prizes but it did get me an A in O-level Technology.

My school was throwing out an old teletype, so I scavenged that. Then I modified the parallel I/O port to output +/- 12V on one pin, and wrote software to bit-bang RS232 at 110 baud. Back before the Internet, just finding the specs for RS232 was not so simple - our local library was a bit limited in that way - but I got there in the end. A lot of guesswork and trial and error. I don't think anyone made a printer for the Ace, so I may have had the only one. Being able to print code listings really helped, even though the teleprinter only printed capital letters.

Somewhere over the years, with my parents moving house multiple times, the Ace disappeared. Many years later, I found one on Ebay, and still have it. But somehow I never fell back in love with it - it just wasn't as good as I remembered my rather non-stock one being at that formative time in my life.

Awesome story. Which year did you create the micromouse?

Any change you have pictures of it?

I agree completely about that sense of discovery. No-one around me knew anything about computers and the information available was limited, but somehow figuring things out for myself was a key part of the attraction for me. And it served me very well as the foundation for a lifetime of research too. If you're passing through London and fancy a beer, look me up, and we can reminisce!

The earliest home computers had tiny amounts of memory: my TRS-80 Color Computer started out with 4K of RAM and BASIC in ROM although within a few years I had it filled out with 64K of RAM. There weren't a lot of languages that would fit in 4K, but they included BASIC, FORTH and assembler (like the actual assembler.) [1]

My FORTH experience with that was writing a subroutine-threaded FORTH for the 6809 under the OS-9 operating system in about 3000 lines of assembler. I wrote to the Forth Interest Group and they sent me a card which had a list of standard words in FIG FORTH, mine complied with that except that I used the Unix-like system calls from OS-9 for file I/O instead of the block scheme most FORTHs used.

[1] The popular 6502 (Apple ][, Atari 400/800, C-64, ...) was particularly weak in support for compiled languages because it had few registers and fewer addressing modes but it was easy to write a FORTH for too.