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 Post subject: ZX81
PostPosted: Jun Mon 07, 2021 4:30 am 
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Location: Wilderville, Oregon 97543
Not sure if this falls into the "Equipment" category. Anyone fiddle with the ZX81?


Last edited by jsweinrich on Jun Mon 07, 2021 2:01 pm, edited 1 time in total.

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 Post subject: Re: ZX81
PostPosted: Jun Mon 07, 2021 5:44 am 
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jsweinrich wrote:
Not sure if this falls into the "Equipment" category Anyone fiddle with the ZX81?

Are you referring to the Sinclair computer from the 80's? If so, yes, but I started with the ZX80 built from a kit.


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 Post subject: Re: ZX81
PostPosted: Jun Mon 07, 2021 2:34 pm 
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Same here, I started with the ZX80 and then sold it to my supervisor and then bought the ZX81. I repackaged the 81 in a bud box with a perf board to expand RAM and any future circuitry. I added I/O ports. At that time I wrote a program for my son of 2 years old called "Mr. Dot". It was just an etch-a-sketch program. I used a Magnavox Odyssey game joystick for him to easily control it.

Later, I repackaged it into another bud box in a more rugged configuration to control a camera and a rotating mirror to track comet Halley. The program was part Basic and part Z80 machine code. The machine code was for the timing portion so it would be repeatable.

My son and grandson are coming to visit us in August so I decided to resurrect Mr. Dot. Since I have used the 81 TVs have changed a lot. The 81 would work with all TVs back then when they were all analog. Now the digital TV are much more picky but I did find one that will work. Just waiting for a tape recorder I bought off ebay to get Mr. Dot up and running. I also bought a Magnavox Odyssey joystick off ebay.

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 Post subject: Re: ZX81
PostPosted: Jun Mon 07, 2021 3:38 pm 
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I have one in the attic. There was an article in one of the electronics magazines about adapting a real keyboard to it, and I found one at Radio Shack while it still sold surplus parts, and adapted that successfully to the ZX80.


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 Post subject: Re: ZX81
PostPosted: Jun Mon 07, 2021 4:19 pm 
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Macrohenry wrote:
I have one in the attic. There was an article in one of the electronics magazines about adapting a real keyboard to it, and I found one at Radio Shack while it still sold surplus parts, and adapted that successfully to the ZX80.


My brother did the same thing with is ZX81. Played with it a bit but I had a better computer. They were fun to play with, not much you could do without RAM expansion.

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 Post subject: Re: ZX81
PostPosted: Jun Tue 08, 2021 2:37 am 
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Here are the schematic I made in the day to expand memory and ad 8 bit I/O ports.

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 Post subject: Re: ZX81
PostPosted: Jun Tue 08, 2021 3:18 am 
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Very cool! I remember the day I finally received the kit. IIRC, it was $99 - a 'real' zx80 computer with 1K of memory for $99 in kit form. From order to delivery I think it was a month, or maybe months. Also, if I remember correctly, the kit was delayed so long that someone else had come out with a floating point math chip and I bought that and had it before the kit was delivered. I stayed up all night assembling it and then typing in programs. I too added a real external keyboard, also from Radio Shack. I think the keyboards were originally in a TI product. I bet it's all still up in the attic in a box, along with ZX81's and 64K memory packs I later bought from thrift stores.

Ahh the memories :)


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 Post subject: Re: ZX81
PostPosted: Jun Tue 15, 2021 1:52 am 
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I started computing by taking a course from the university on BASIC, "phoning" in my lessons with a yellow-paper terminal and a phone cup. Later that year I got a ZX-81 kit and built it, and added on the 16k RAM pack. I moved to the East Midlands in England for 6 months and bought a UHF converter at an electronics shop in Nottingham to replace the US VHF.

I spent a lot of my spare time programming a baseball game on the ZX-81 during the winter and early spring. I went to a Sinclair meet/fest in Westminster, London in December (man, that was jam-packed) and bought a plastic keyboard with actual keys, cheap, but usable. The ZX-81 board and RAM fit in the keyboard. That made the programming easier.

The baseball program display was a scoreboard with team names, innings, runs in the inning, and how many outs, and ultimately the final score. I created a diamond in the center of the screen with 0 for the bases, which turned to a 1 when a guy was on base. The lineups were fixed, it required a restart to change.

I assigned realistic batting averages for each player with a power rating of 1-4 to each player. The lineups were the Braves and Dodgers of the era. The display would display the batter's name and whether he got a hit, what kind of hit or an out. A guy on first got moved along to second if the batter got a single, and so forth. A triple with 2 guys on base would increment the score for the inning by 2 runs on the scoreboard and a guy on third.

It worked well, and I had a lot of fun with it. After a while I decided to use peeks and pokes and develop a graphic splash screen. Unfortunately, that was too much for the 16K RAM, the system would crash.

I lost track of what I did with the ZX-81 and the cassette taps with the programs. I think I traded it with a friend for an Atari 400, for which I replace the pressure sensitive keyboard with a teletype keyboard. Fun times!

Ted


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 Post subject: Re: ZX81
PostPosted: Jun Sun 27, 2021 3:19 pm 
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Joined: Jan Thu 16, 2020 12:29 am
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jsweinrich wrote:
Not sure if this falls into the "Equipment" category. Anyone fiddle with the ZX81?


The first thing I did when I got my ZX-81 was to add an XOR gate and switch into the video circuit. Then switching one input of the XOR high , or low, either inverts or non inverts the video (leaving the H & V syncs alone) so as to get the usual illuminated characters on a black background. I found the white background with dark characters less agreeable.

The other thing I did was to make a proper keyboard. At the time there was a mechanical button overlay option that went over the original keyboard, but that was not tactile enough, so I made a panel with a microswitch under each button so it felt "clicky" and connected that in with a ribbon cable and D connector.


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 Post subject: Re: ZX81
PostPosted: Jul Wed 07, 2021 2:45 am 
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Ah, we were hardware hackers of the first order, made so by the limitations of the tiny ZX machines. Mine was the Timex equivalent, seems most of us used the TI99 keyboards that were so common and cheap in surplus after TI left the market post-haste to end its bleeding of millions of dollars in the price war with Tramiel and Commodore.

My hackery was not so elegant as others shown and described in this thread. Still, it got the job done and was much more stable in operation. It had a fairly accurate internal counter that was easy to repurpose as a clock. I used it with a program I wrote to handle multi-step darkroom developing processes.

The bare-board Timex in the second image sports 48k of RAM, 16k on-board with two additonal RAMpacks.

The ZXSP emulator for Mac works very well, even the "cassette" interface is functional. https://worldofspectrum.net/emulators/
-Ed


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