Sinclair ZX81 : spécifications techniques

Quelques accessoires : la célèbre extension RAM Memotech et l'imprimante thermique ZXPrinter

memopak.jpg (5106 octets) ZXPrinter.jpg (5551 octets)

Des accessoiristes ont proposé des boîtiers décents pour mettre son ZX81 et les extensions. En France, il y avait même un modèle portable dans un Minitel. Ci dessous, le célèbre DKTronics :

zx81custom.jpg (11370 octets)

L'intérieur est beaucoup plus bricolo, avec l'extension RAM dépliée :

zx81custom_inside.jpg (13802 octets)

Avec une extension RAM 16 Ko et la ZX Printer :

Les coffrets sont parfois pathétiques, genre "L'Initiateur", fait pour initier au dégoût de l'informatique.

Le lecteur aux yeux de lynx découvrira trois variantes de ZX81. De base (à gauche), avec le clavier pipeau-mécanique (au centre) et enfin une version pipeau-mécanique toute grise, pour faire plus professionnel (on imagine)

Sinclair ZX81 FAQ

Version 1.3.Révision 13/6/95

Une FAQ en trois sections:

  1. Information sur le ZX81 pour ceux qui ne le connaissent pas.
  2. ZX81 Emulators.  De l'info sur les émulateurs.
  3. ZX81 Hacking et Trivia.  Des questions et réponses en tous genres.

1 - Information for people new to the ZX81.

Q: What the hell's a ZX81?

A: Good question. The ZX81 is a small home computer that was quite popular in the early eighties. It was built by Sinclair Research (who also made the Mk14 kit, ZX Spectrum, ZX80 and QL computers.)

It was a highly revolutionary design, containing only four integrated circuits. Most machines at the time needed twenty or so. This allowed the machine to be produced very cheaply, making it the first computer in the UK available for less than a hundred pounds.

The ZX81 was released in North America as the Timex-Sinclair 1000, and marketed by Timex. A TS-1500 was a TS-1000 with 16K of RAM built in.

Over a million ZX81's and TS-1000s were sold worldwide.

Q: Where can I get the latest information about the ZX81?

A: Read the newsgroup comp.sys.sinclair. It's mainly about the ZX Spectrum, but quite a lot of ZX80, ZX81 and QL stuff appears in it. It's also available as a mailing list. Send the message:

subscribe sincnews

e.g.: subscribe sincnews joe@somemachine.someplace.somecountry

to You can send messages to the newsgroup/mailing list by sending email to A word of warning though, there's about 250 messages a week, so be prepared to be swamped if you get it through the mailing list.

You could also try looking for the latest version of this FAQ. It should be available on the following URL:

There's a few useful WWW pages too:

Q: What are the specifications of the ZX81?

A: Integrated Circuits


40 key touchpad keyboard.

Screen Resolution

Memory Map:

Q: What's a RAM pack?

A: A device that could be plugged in to the bus connector to increase the memory size of the machine. 16K was the most common size. 32K and 64K packs were also avaiable. The original 1K was displaced when a RAM pack was added. The 64K RAM pack only gave you 56K as you still need to have the ROM in the Z80's memory map.

Q: What was the ZX Printer?

A: It was a printer (shock!) that you attached to your ZX81 or ZX Spectrum via the bus connector. It ran off the power lines in the bus, so you needed a souped-up power supply for it. It used a special sort of grey, metallic, thermal paper that looked like a rather evil sort of toilet roll. It could be quite noisy (and occasionally smelly) whilst in operation and the print quality was less than desirable.

Q: Where can I get a ZX81, and what's the going rate?

A: Since the ZX81 is no longer in production, you'll have to find someone willing to sell you one. They're not exactly antiques yet so you should still have a chance of getting one quite cheaply.

You can find (at least in the UK) lots of second hand ones for sale in magazines like Micro Mart, at car-boot sales, school fairs etc. The going rate is about 10 pounds, maybe 10 pounds more for a 16K RAM pack. If you're good at haggling you might get it for 10 pounds complete. Other hardware is a bit more tricky to find, and consequently can by quite expensive.

In the United States, there is a company in New York City called Zebra Systems currently selling ZX81's in kit form for $30.00 U.S., 16K Rams for $15 U.S., Alphacom Printers, ZX81 books, and other items. The ZX81's they have use NTSC video modulators and their current policy is only to ship within the U.S. and Canada. The link to their web site is listed below.

Q: What are the best ZX81 games?

A: Have a look at the all-time ZX81 charts. There's also a voting template so you can vote for your favoutite games.

2 - ZX81 Emulators

Q: Are there any ZX81 emulators?

A: Yes, I'm aware of four at the moment. I'd rank them in the following order:

In summary:

The Atari emulator is just about perfect for playing most games on, but due to the slow speed of the original, typing in long programs is a bit of a chore.

Xtender is good for typing in programs, because of it's high speed. However, due to the poor keyboard response, it's fairly useless for playing games that require simulaneous keypresses. Carlo Delhez (the programmer) is currently working on a new version of Xtender that will fix this problem.

TS1000 has no real advantages over either of the others, but might appeal to original Timex Sinclair owners.

Q: Where can I get software for the emulators?

A: There's some software on the site in the directory pub/sinclair/snaps/zx81. These are mostly BASIC programs, since there's no widely available method for loading in old tapes.

While you're there, check out my game: 'Deathrider.' :-)

You can also get a huge bundle of software if you register Xtender.

Finally, try picking up copies of all the emulators. Most of them come with a few examples.

Q: How can I get .81 files to work on Xtender?

A: Usually just rename them to have a .P file extension. The keyboard controls in games might not work properly though.

Q: How can I get .P files to work on the Atari Emulator?

A: Try renaming them to .81 files. They might be ones that were 'converted' to Xtender files by the above process. If that doesn't work, you can try the program PTO81 (from which chops a byte off the end and fiddles a few system variables.

This is only available as a PC/DOS program at the moment. Contact me if you desperately need a ST executable version. Since the only compiled language I've got for my ST is STOS, I'm none too keen on writing one though.

Hi-Res graphics, programs that POKE the ROM (e.g. to change the speed in Xtender) or use Xtender's file handling system won't work, though.

Q: How can I load my old games off tape?

A: There's a program by John Elliot called ZX81TAP that is available on my ZX81 page (see above) that can, in theory, read ZX81 programs from tape, using a Spectrum Emulator and save them as P files to disk.

Unfortunately, I can't get it to work on my system, but this might be down to dodgy audio equipment, leads and a crap sound card. If anyone can get this program to work, I'd be very interested to hear from you.

3 - ZX81 Hacking and Trivia.

Q: Could you get sound on a ZX81?

A: Yes. Just about, anyway. You could drive the cassette port rather like the ZX Spectrum's speaker. So if it was wired up to a cassette recorder or an Amplifier, you could get sound from it. There was a program, that was printed in a magazine, that could sample and replay sounds using the cassette port.

You could also get sound through the television, by rapidly alternating between FAST and SLOW modes. There was a 'Sinclair Organ' program in Sinclair Programs that did this quite nicely.

There was also the Zon-X add-on, that contained the AY-8192 sound chip (as used in the Atari ST, Spectrum 128 and Amstrad CPC machines) to create rather better sound effects.

At the moment, no emulators support ZX81 sound using any of these method.

Q: Could you get hi-res graphics on a ZX81?

A: Yes. There were a number of ways.

1. The Z80's i register pointed to a font table. You could move this around in the ROM to create swirly graphics effects. The game Astral Convoy did this. Unfortunately, the design of the ZX81 made it impossible for the character set to be anywhere in the 16K range, so you couldn't have a user defined set without extra hardware. With a 64K RAM pack you could disable the ROM shadow between 8K and 16K and design your own UDGs.

2. CRL released a Hi-Res graphics toolkit. This contained an extremely hairy interrupt routine that replaced the ZX81's usual display update routine. Basically, it pointed the i register to a strategic ROM location, and tried to 'best fit' the HRG display to what it could find in those locations. It also used a feature of the ZX81's hardware that allowed you to have 1 pixel height characters to improve it's chances.

You still could, at best, only have 128 different combinations out of the 256 possible ones. These were the 64 'characters' and their inverses. So this is really pseudo-HRG.

A number of games by Software Farm, including Hi-Res Invaders and Rocket Man, used this technique.

With a 64K RAM pack you could also, alter the font, allowing an even better chance of getting the characters you need. Some people made an alteration to their ZX81's internal wiring, so that the i register can point to the 16K RAM area, allowing them to do the same.

3. There were a number of hardware add-ons that worked with software similar to the above routines to allow UDG's and true-HRG. DK'Tronics and Quicksilva produced devices that worked with some of their games.

Q: Could you get colour graphics on a ZX81?

A: Yes, but not without extra hardware. Prism released an adaptor that required some soldering inside the machine. It got little attention, because the ZX Spectrum had been out for a while, and the ZX81 was on the way out.

Q: Could you really run a nuclear powerstation on a ZX81?

A: Well, Clive Sinclair said you could. Then again, he made some pretty outrageous claims about a lot of his products. I wouldn't give it much chance, since mine crashes about once every 2 hours without a RAM pack on, and once every thirty minutes with one. :-)

Q: Could a 1K ZX81 really play chess?

A: Yes, but not very well. 1K chess was released in 1982 by Artic Computing. It had no graphics and could only play white. Nonetheless, a marvel of programming skill to get a chess game into 1K.


Final note

This FAQ is in its very early days, so don't be surprised if some of the information is rather vague, and occasionally incorrect. This is due to it being largely composed of my memories from fourteen years ago, gossip from comp.sys.sinclair, rumours and speculations. Please feel free to Email any corrections to me if you know better.

See you in comp.sys.sinclair, Matt Barber, University Of Greenwich, Email:

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