Cracking open the Sinclair ZX81
In 1981, Sinclair Research released the ZX81 as a follow-up to their earlier ZX80. The ZX81 was manufactured by the Timex Corporation and sold as a kit (£49.95) or fully assembled (£69.95). In 1982, Timex started selling the ZX81 in the US for $99.95 as the Timex Sinclair 1000.
Follow along as we take a peak at the hardware inside one of the first low-cost home computers.
Photo by: Bill Detwiler / TechRepublic
Caption by: Bill Detwiler
Bill Detwiler has nothing to disclose. He doesn't hold investments in the technology companies he covers.
Bill Detwiler is Managing Editor of TechRepublic and Tech Pro Research and the host of Cracking Open, CNET and TechRepublic's popular online show. Prior to joining TechRepublic in 2000, Bill was an IT manager, database administrator, and desktop support specialist in the social research and energy industries. He has bachelor's and master's degrees from the University of Louisville, where he has also lectured on computer crime and crime prevention.
geeeez i'm feeling quite old if the author of this can't mention these are TTL chips or mention its function.Other marking is the date code 8222 = Year week. btw Touch sensitive? if it was one thing NOT it is sensitive, this is just an old fashioned membrane "El Cheapo" keyboard.
I had used an ICL mainframe at university, then had access to a PDP11 - a glorious improvement. When I left uni I was forced to use a Compuserve/IBM mainframe then I moved job and found I had access to a PDP11 again - a savious. After a few months I was thrown off to make rrom for the accounts people (technology always took seconf place!) and was given a teleprinter connected toanother offsite mainframe. I could not use it as the first typed character only appeared after 6 or so more had been entered due to the geometry of the type head. I argued for a PC but was turned down on cost grounds, so I went out and bought, with my own money, the newly released ZX81 and a memory module and printer, being the cheapest available solution. For the next 3 years I developed all sorts of scientific programs on that little machine and it performed really well. It was a joy not to have to rely on someone else's mini or mainframe. It is true to say that the ZX81 revolutionised my life and changed my career. Looking back it might seem primitive but it was truly liberating. Clive Sinclair's ZX81 was a work of consumer electronics genius. Needless to say, I did not buy a C5! I did, however, subsequently buy a Jupiter Ace which ran Forth (for you youngsters out there who have never heard of it, Forth is a reverse Polish notation stack language) and got even more intellectual pleasure out of that. Happy and simpler days!
Wow!. That was my very first one. I have leaned Basic and Assembler on the membrane keyboard. The memory extension needed some extra tape to avoid that I lost my input. Those were the days.
The keyboard is a 'touch sensitive membrane', a flexible plastic surface with the actual switches under the surface. While easy to clean and water resistant, it is very difficult and slow to type on, because you have to press hard and very deliberately to use the tiny, closely-spaced keys. hire a programmers
My first computer:- I was one of those who financed the whole thing. I mean I paid in advance for a kit that I received 3 -6 months after payment. So I like many did a build up instead of a break down, I had the extra memory and printer, did my shop accounts on it , spent hours Basic programming. What has happened to peek and poke?
Back around September 1980 I was entering my final year of an agricultural science degree when I noticed the ZX80 was on sale (if I recall correctly presented as full-page adverts in Sunday paper magazines or similar). I was therefore are far too busy to get one at that time but I fully intended to get one the following year when I graduated. Meanwhile the ZX80 had evolved into the ZX81, which was smaller, and I felt it was actually better looking. So I purchased a ZX81 in the summer of 1981. (Sorry Bill Gates, you were not the first to name your computer related products after the year of origin, e.g. Microsoft Windows 95 or Office 2003, etc). In 1981 my recollections were that pocket calculators were the only "device" that people carried around with them and only the more advanced had scientific functions or the ability to run simple programs or even show more than a handful of lines on the display. If you wanted to communicate on the move - well CB radio was in its early days in the UK. Meanwhile, back at university I had been introduced to computer terminals with keyboards connected to a distant mainframe that frequently took a while to respond when you pressed the return key and only let you log in between certain hours. Nevertheless, this had allowed me to be introduced to writing simple programs in BASIC on a screen that at least allowed you to run them in real time and correct any mistakes quickly. This was a vast improvement on typing out cards and sending batch jobs full of statistical analyses away to be run only to find they either crashed, jammed, looped or else just didn't do what you wanted or expected them to do or maybe presented the output in a layout that you had not desired. Getting the jobs back often happened two or three days later, thus it could take weeks to get anywhere! Thus, at last the ZX81 offered me the chance to "do proper computing" at home and for me at least it was a "must have". Thank you Sir Clive! Plugging something into a TV was a pretty novel experience too. Yes VCRs were beginning to become popular in the early 1980s later to be followed by video cameras but I do recall just a few years previously having a an early videogame. Therefore, the ZX81 wasn't quite the first thing other than a TV programme that was currently being broadcast to appear on your TV screen but it was the first to give you anything like control of what you had there! TVs only had aerial sockets at that time so you had to manually tune them to a particular channel that hopefully didn't interfere with anything that was being broadcast. My experiences of the ZX81 concur with some of the other comments in this column, especially the frequent crashes due to poor connections with the memory pack. I believe it added 15k to the existing 1k memory to give you 16K in total. It was a struggle getting things to run with only 1K to use but it was a more stable experience. I did get a proper keyboard for mine and soldered it in manually as my big fingers were too cumbersome to work well with the standard keyboard. I do remember trying to get things to run in "machine code". I'm not familiar with the precise technical explanation but I gathered the motivation to use machine code was that you were attempting to communicate with the computer in its language rather than having it communicating with you in your ???language??? and thereby asking the ZX 81 in effect to translate between the two every time. It was therefore a much more efficient set-up and it did indeed run programs a lot faster but any error in the program tended to completely crash the system. Attempting to debug binary listings was a step beyond me, therefore any nascent ideas I had of becoming a computer programmer were sadly extinguished at that point. Nevertheless, there were some games around to "enjoy". I remember a chess game that replied with moves that frequently put pieces in positions off the board! There was also an adventure 3d maze game that I had not seen the like of before so it was fun playing it back then. After the ZX81 I had an Acorn Electron at home then use of an MS DOS PC at work in 1987 followed by a DOS based Amstrad at home in 1990 before a string of Windows PCs came along right up to the present.
I still have two of these.. including the gigantic 256K memory expansion. I have a Sinclair ZX81 and the Timex/Sinclair 1000 .. if I remember the nomenclature correctly. One of them is still in the original box. I wrote primitive spreadsheet like software to keep track of customers on a paper route.... an adult-job of 400 customers on a motor route in a rural-ish area. I was a programming student in college and was trained on 80 column punch cards and a mainframe.. and did an internship at Hasbro Toys where I found out you can count Mr. Potato Head parts with a mainframe based inventory system. It was during my internship I saw the ads for them.. it was an exciting time. I could actually afford the thing!!! A real computer of my own!!! The programming I did on these is where I REALLY learned programming.. and learned it well. Even got into some assembler programming on it for little subroutines. Still have maybe as many as a dozen books on how to do all kinds of cool stuff with them. What a treat to see this "teardown" article.!!!!
I had a Spectrum (next one ofter the ZX81) and it had huge problems with heat. There was a heatsink half the size of the PCB which was attached to the voltage regulator (this appears to be the major source of the heat). I'm pretty sure the square plate in your teardown is an improvised heatsink, and the metal strip appears to be in spring-loaded contact with the silvered metal on the inside of the case, thereby turning the interior into yet another improvised heatsink.
My mum passed away last year and while it was a very sad time for the whole family, this article reminded me of the time we spent with this silly little computer. It was the start of an electronic journey that she and I shared for my entire childhood. I was 6 years old, when my dad came home with a secondhand ZX81 that he bought from a work colleague who couldn't make sense of it. Because she was at home, mum spent a whole day typing in programs from computer magazines, with me watching and helping. Thousands of lines of code just to make a few ASCII characters jump. But it was wonderful and I hate to think how my life might have turned out had I never got the ZX81. I briefly met Sir Clive Sinclair a few years ago on a London bus and relayed this story to him. He seemed pleased. I will always be grateful for what this little box of tricks did for me, my mum and the future I'm living today.
Had lots of fun learning on that thing. Still have it. Needed to save an old TV to use as monitor and Just for fun I light it up to use my old star tracker program.
I can barely remember but its like you all were thre with me learning everything the HARD way. Thanks for the memories, Im a real computer tech now some 29 years later and can thank the timex corp for the countless hours of frustration & education. This was the only thing like a computer I could afford at the time. Now this seems like a millinum ago, oh yea it was! I love the teardown series, because its how I became a repair tech. Just have to see whats inside and how it ticks. Sometimes you get lucky and and fix it. anyway its always fun. Thanks
Half of these comments are about how it was our first computer. I didn't even notice when I first posted. I love it. :)
Man, I loved playing with that thing... the screen going nuts when you'd load/save from the cassette... and how damned hot the RAM pack would get. Oof! Thanks for this. :)
I still remember the joy when I recieved this wonderful little miracle. I was sitting infront of it not really knowing what to do, but wanting to code the whole universe in it. Sweet memories.
My first computer, and as a first year C.S. major, it was too much to resist. Early on, Sinclair offered either the kit form, for (I think) $59.95, or built for $99.95. As a starving student, I ordered the kit. However, when the box arrived, it contained the already built ZX81, with an apologetic note from the company, asking if this substitute was ok, since they were out of stock on the kits, and they wanted to expedite the order... pre-built for the kit price. YAY!! Instead of spending a weekend soldering wires and praying I didn't wreck something, I was able to plug it in and start programming.
Drove all the way from Montgomery AL to Birmingham after scrapping up $100 to buy this PC. Developed my first program which was based on the board game Stratego.
I don't remember the model number, but mine did not have the 16k 'brick' memory module. It had to be ordered separately. I also remember if you messed up on syntax error, it would reject it and place a line of reversed video at the bottom of the screen showing where the error was. Let, nor, and - basic lang...oh my...!
Esta fue la maquina que ayudo a cristalizar mi primer contacto con la programacion y el sueno de poder hacer cosas increibles.... todavia la tengo, guardada como un gran tesoro. como esa pequen#a lupa que abrio la ventana a otro universo. Saludos a todos y vivamos tambien de estos bellos recuerdos...No olviden que tiene como accesorio un teclado externo que permite introducir codigo en forma mas agradable.. Saludos a todos. ler 2011
I discarded my Model 15 teletype in favor of the TS1000. What an advance! The BASIC interpreter was pretty slow. Running code written in assembler was probbly thousands of times faster! But it was a fun and cheap way to learn programming. I even had a 64K module for it. You had to run a RAND USR command first in order to use it. I have a couple in my basement yet - I wonder if they still work?
I used to go into our local Sears store every chance I got to play with that thing I thought was a miracle. Soon after that, my Dad bought us a Vic-20 with a tape drive and Lunar Lander. He had convinced me that membrane keyboards would not last.
Back in '62 I got a Digi-Comp 1 mechanical computer. It had a spring based action to cycle through 3 bits of data. Very advanced for its time. In '82 I bought a kit version of the ZX-81 for the fun of building it. Imagine my surprise when I hooked it up to an old 13" B/W TV set and saw the cursor displayed! I waited for the price to come down before I bought the 16k memory expansion. A friend of mine bought a 64k aftermarket expansion module but the ZX81 could only address 16k at a time. He had to throw switches to access the additional memory banks. We had a club of about a dozen users who traded published software and subscribed to ZX-81 pubs including InSync and Sync. The selection of published software was incredible: games, utilities, I even have compilers for LOGO and FORTH 2 languages that are now out of favor but were popular with publishers and hobbyists back in the day. The cassette interface was atrocious and it often took several tries to save or load a program from tape. I also had a thermal printer with paper calculator sized roll paper. I wired a keyboard to it for comfort as the membrane keys were tiny. The kit instructions were very emphatic about grounding yourself during assembly and being careful not to damage the IC chips. If you look closely at the pictures above you will notice that the chips on the motherboard were not directly soldered to the pcb but were instead inserted into connectors that you first soldered to the board. The metal strip along with the silvered cases were to prevent RF interference. I never had a problem with that in my home from this unit. The square metal part was a heat sink for the voltage regulator that took the voltage down from 12v to 5v. I still have mine but haven't touched it in years. Perhaps it will go to the Smithsonian! You have to give Clive Sinclair a lot of credit. He wanted to make a 1 board PC at an affordable price and he succeeded.
It actually did have some sound capability. Sort of. If you turned the sound up on the tv, you could hear the buzz of the clock as noise. It was possible to write loops that changed the frequency of this buzz to make rudimentary music.
This 16kB expansion module would easily be bumped often causing the entire ZX81 to lock up, thus losing the last 2 screens of code one entered ! I eventually solved the problem by building a wooden frame to keep it locked together. Saving one's program could be done to a standard cassette tape and took quite a while so who felt like doing that every 30mins to mitigate against those bumps ?!! Also my first PC :)
So I bought a surplus keyboard, painstakingly mapped the membrane keyboard connections to the fullsize keyboard, and hardwired it to the TS-1000 with individual wires and solder. Guess I had too much time on my hands.
It was my first computer. First with only 1Kb of RAM, and then with 16Kb Expanded RAM and Centronics Parallell extension (for printing).Also had thermal printer. It were expandable up to 56 Kb RAM . It can also change character set via software or hardware It also has sound via PC. An strange thing for a computer without a buzzer An "comecocos" like game with less than 1 KB of RAM A chess game with less than 1 KB of RAM. A 3D Maze game with the 16Kb expansion SET. Support for modem, scanner, ....
My father bought me this computer along with 16K RAM, thinking I might be interested in computers. Well, here I am today, a computer professional. I learned Basic programming as well as assembler on it, and I wrote some small games for myself. I remember saving the programs to a tape recorder - once the cable wasn't attached well and the screeching blasted my ears! I also subscribed to a magazine or newsletter (remember printed newsletters?).
I have one of the biggest collections of Sinclair Spectrum memorabilia in the world and i have to say i would love to have got my hands on a fully boxed ZX81. I almost picked up a ZX80 a few years back but i decided to concentrate on the Spectrum 48K and 128K series. I thought about Sam Coupes but they didn't interest me either! nice work on the destruction of a beautiful piece of machinery! :D You can buy a 128K from me and do the same to that if you want! ;)
Mine was the ZX Spectrum 48, but just the mention of the words Sinclair and ZX jumpstarts my memory - all 48K of it! Buying a game for it was always a treat, and the impatience of having to wait until I arriving home was unbearable. I used to look down on users of the Commodore 64, asking myself if they knew what they were missing out on! I designed a book-lending library system for it using Basic and loading from and saving data to cassette tape. I had the volume control worked out to perfection by listening to music with earphones and setting the volume to as loud as possible up to the point of just being uncomfortable to listen to. Tir Na Nog and the Hobbit were two of my favourite games. Those were great times before the internet and IBM-compatibles distracted us from real computing. I introduced my young son to computers then and he decided that computing was going to be his way of making a living - he now works for Fujitsu. No need to say that I still have my favourite computer although it is rarely turned on. Perhaps I should put it on display in my lounge for every visitor to see. Thank you Techrepublic - perhaps a Tear Down for the ZX Spectrum soon ?? (wishful thinking).
All hail thee, noble Sinclair! Thy 1K RAM Thy baby BASIC Thy Zilog Z80A The clever chip The dancing chip Lord of the 8-bit bus The chip from God Never shall we forget
I never used or owned a ZX81 myself, but what I heard from those who did was: they were do desperate in their shoehorning, they not only used self-modifying code (hard enough to do with ROM code already), but they even jumped into the middle of multi-byte instructions!
May be this appeared in US, but in Europe the Sinclair did not have it, but suffered from excessive heat on par of the keyboard, and the plastic could even be deformed at one hot point (both on the upper keyboard part, and under the box. This made the device unconfortable to use for long, and it could even damage the keyboard surface. So my fealing is that this is for driving the heat to a larger surface (this explains the addition of the metal cover coating here : the strip interconnects these surface to make heat dissipated more conveniently (notably the heat from the very inefficient clock generator with a basic quartz and capacitor driving too much current, and of the CPU plus the TV analog modulator module. Note: the ZX dd not have any graphic board. In fact, when running in "slow" mode, it was the CPU that was interrupted and that computed the scan lines from a bitmap font in rom and the very small "text" buffer in memory. IT was just able to generate 24 lines of 40 characters, and the program was only running when the video signal was in blank mode at the top and bottom of the screen. In "fast" mode, the CPU was no longer intrrupted, and the program was running with the video turned off (the "fast" mode was also used to compute the "audio" wave form for recording and reading programs in the cassette, from a very basic 1 bit output, the same that was also used to generate the video frames, this explains why you were seeing black and white stripes on the TV when reading/writing from/to the cassette. The computed digital signal was then directly entered into the TV modulator that filtered with passive analog filters the very imprecise signal. Yes there was no sound, but you could still emulate it (programming in assembly, entered in memory via "PEEK/POKE" instruction in BASIC) using the same device used for ourputing to the cassette or to generate the display. IF you were very good, it was still possible to count all CPU cycles to make some processing, and still being able to run a useful programs. Don't speak about Hifi sound ! Later, hobbyists have used the 8-bit port interface for the RAM extension or printer to produce a more decent sound using a D/A convertor built with an array of callibrated resistors, connected to an external amplifier, or experimented ith it with basic electronic, transforming the "computer" into a device controler. There was so littel memory that, at that time, many just learned the assembly language to program it, and symbolic assemblers were developed to help this conversion into a BASIC program consisting in DATA/PEEK/POKE instructions that could be loaded from the cassette. Some games could be developed. At that time, I could program the Z80 processor directly because I could remember the bytecode of most instructions (but a tabular conversion table was a great help): to program it, in fact you were first writing programs on paper. I also remembered the number of CPU cycle for each insttruction, to make best use of the available CPU time between video frames and between frame lines. But its interest was it was damn cheap for the child I was at that time. Comparatively, the price it was sold at that time would be about $150 today, much less than most PCs that are sold today (but there was no real choice, because other computers were really much more expensive, comparatively).
The thing that fascinated me was that the Sinclair used the video refresh signal as the refresh for the dynamic RAM, and to walk out rows of dots from ROM for the characters. This was part of the reason that the screen went blank if you were running any complex programs ("complex" for a Sinclair, that is) A fun trick to play on another ZX owner (who didn't know about it) was to enter a quick program on their machine that used PEEKs and POKEs to copy the table in ROM that was used to generate characters to RAM, and set the pointer for the screen subroutine to point to the RAM version - which would be backward, rotated ninety degrees or upside down. (I accidentally realised that this was possible as a joke while working out how to print 80-column text on the 32-character Sinclair thermal printer. (Of course, it could only print 32 *lines* at a time, necessitating paste-up for a full page, but i was impressed with how clever i was.)
I bought the Timex Sinclair +16 while attending an off-campus Computer Science class while unemployed (the class had C-64's - too rich for my wallet at the time). In any case, I was able to do my assignments on the Timex and also wrote programs for a Surveying course I was taking simultaneously. What a great way to learn! Changed my career choice. My opinion on the block of aluminum up front; It may be heat-sink but I also suspect that it is doing double duty as a counter-weight to the memory module. If I recall, the entire assembly was a little light in the front with the module attached since the bottom of the module did not touch the surface that the computer was resting on. At least, that is how I fondly remember it...
The metal strip that touches the metallic inside of the case is to reduce electromagnetic interference that the computer will emit when it is running. My C-64 was a pretty effective VHF jammer. This would probably be to meet FCC guidelines, and may not have been required in other countries. The metal tab connected to the voltage regulator is a heat sink, or a radiator to dissapate the heat produced by the voltage regulator so that it doesn't burn out. FYI - the C-64, my first computer, if you don't count the BASIC programming cartridge for the Atari 2600, also had the BASIC command shortcuts, and the graphics characters mapped to the keys.
The metal piece attached to the +5V voltage regulator looks like a heat sink. The metal strip with it's connection to ground and the silver lining on the case looks like an attempt to reduce RF emissions. Otherwise, the 74LS157s are quad two input multiplexers which would be used to multiplex the column and row address bits into the 4116 dynamic RAM chips, the 74LS393 is a counter and I would suspect is being used for refreshing the RAM using the Z-80's RFSH signal and it's outputs as the refresh address. The inductor, transistors, diodes, Zener diodes and other components on the 4116 board are most likely to be a converter from +5V to +12V and -5V converter as the 4116 used +12V, +5V and -5V power supplies. Why the striped covering for the capacitors -- that's the interesting question. Brings back memory of the days when solder was a major part of computer ownership.
This was my first computer that I learned to interface IO and control via assembly. I Also got English made "Memotech" high-end version that started out as memory modules for the Sinclair, then evolved into a cool aluminum cased computer. The orig movie "weird science" used this cpu as the 'girl' creation PC. They had the full floppy drive cabinet and I was jealous.
I couldn't part with my first computer, that was purchased in the 60's from the local drug store with my paper route income. I became a computer expert, after I realized there was nothing more fun than computers. I am currently functioning as a University Database Administrator, after hardware, device drivers, diagnostics, software, programming, my U.S. Patent and an early retirement. I can never learn enough and I missed computers, so I went back to work doing what I love. That small investment changed my life for the better (never watched much TV after that - busy developing new computer technologies, and playing the latest games!)
I loved my ZX-81 (assembled from kit). Features that made it unique for its day: 1. The incredibly efficient operating system that made it possible to operate with only 1K RAM, including the display memory. It worked, and helped to make me a very efficient assembly and BASIC programmer. 2. The single ULA chip, that made it possible to build a computer with a chip count of only four. 3. The advanced BASIC interpreter with the auto line numbering and the single-touch functions, as mentioned above by mmoran. 4. The membrane keyboard, which was a lot easier to use than one would think. 5. More negative "features" were the crash-on-jiggle feature of the 16K module and the cassette tape storage system. We adore our pets in spite of their annoying quirks, right?
I had one of these back in the stone age (about 1982), and really had a great time with it. I got a third-party keyboard, which was set up by taking the entire motherboard out of the original case and putting into the keyboard. I put in 64K (!!!) of RAM, and attached a small high-speed tape drive -- also known at the time as a "stringy-floppy," and a four-inch thermal printer. I was a true power user! The fun part was writing a bubble-sort program that would sort a text list. I used that to sort the catalog for the local video rental shop (another new high-tech innovation) and print out a new listing every week. Got free video rentals for a year or so.
The 16K RAM pack had to be held tight to the unit with a rubberband, since the connector was very insecure. I think that the cardboard insulated strap is a ground strap, since it touches the metallic lining of the case. Maybe they were initially worried about static electricity. that the cardboard insulated strap is a ground strap, since it touches the metallic lining of the case. Maybe they were initially worried about static electricity.
Wow ... This was my first computer, too. I used it to learn Z80 machine code (with the help of Rodnay Zacks "Mastering the Z80"), spent a year or so writing and publishing utilities and selling type-in programs to computer magazines, then parlayed that shallow experience into a founding Technical Editorship at Family Computing Magazine. An actual, whole, long-term career in technology publishing and software development followed. Got to love the little wedge. I fixed the jiggle issue with my 16K RAM module by building a rigid base that clamped the keyboard and module units in place. It had concealed wiring for the power supply too, and (memory fades slightly here) an on/off switch, because otherwise you had to turn off the device by unplugging from the mains.
This little baby is what got me started with computers. My dad bought the kit, put it together, played with it for a couple of weeks and then lent it to me - he never got it back. Absolutely amazing what you could do with 1K RAM. I eventually added a third party keyboard and 32K memory board. My dad next bought a BBC Micro but he kept hold of that - I had to buy my own and stuck with it until I got my first PC way back in 1988. Thanks for the article, it's brought back some very happy memories.
And an amazing little thing it was, for the time. The BASIC interpreter shoehorned into that 2K monitor ROM mapped keywords to keys, so if you were at the beginning of a line and hit "L" it would give the command LET. At that point, it switched to normal text entry mode. And if you attempted to enter a line with a syntax error, it would reject it and reproduce it at the bottom of the screen, inverse video, with the cursor positioned at the point of error. It was my first programming "tutor." Had to be careful when using it, though. If you bumped or jiggled the 16K "brick," like as not you'd crash it. And loading programs from cassette was always a treat. Volume control setting on the cassette player was crucial. Seemed to work best if you adjusted it so the white and black bars on-screen while loading were of equal width, but even so you were lucky if it loaded successfully more than half the time. I spent countless (and ultimately fruitless) hours trying to design an external automatic gain control circuit that would make it more reliable. Thanks for the memories, Bill!
Going back a lot of years now, I remember one friend programming an S-100 bus computer to play "music" through an AM radio by using the RF noise generated by various instruction sequences. Like they say about the dancing bear, it isn't how well he dances, it is that he dances at all. He never did succeed in making the music and the display from a Cromenco Dazzler video work together which is what he was attempting to do.