Inside the Black Box 6: Timex Sinclair 1000 / Sinclair ZX81

This little back box was almost unknown in the US, but kickstarted the home computing in the UK (and Brazil!) Built by the team of mad genius Sir Clive Sinclair in 1981, it was the first computer to sell for less than $100. Join us as we learn the basic architecture of a computer, and see how just four chips and a little plastic in the ZX81 introduced a whole generation to computing.

[@borzak101]

They were incredibly popular in the US. Every town had a Timex/Sinclair user group that met often. Here in my relatively small town they met once a month and had about 100 members show up each week. This was before anyone even heard of Commodore or Apple on the home front. These were everywhere for a while.

[@JoulesperCoulomb]

A few notes: The EAR and MIC sockets are for audio input and output to/from the cassette recorder. They are labelled to match the labels on the recorder's sockets, hence EAR is the input to the computer and MIC the output. They are connected (via some filtering) to the video output pin of the ULA. Of course, when this pin is used for audio I/O, video output is suspended. The audio signal gets fed to the TV and is the cause of the bizarre patterns during loading or saving. Modern TVs often simply show black at these times because they refuse to attempt to constitute a picture from the audio. The ZX81 wasn't intended to have any audio output other than for saving to cassette. In SLOW mode the CPU is dedicated to video generation at all times other than during frame fly-back, which is the only time user code is executed. This makes the computer very slow, but it was a good way to produce a very simple and low cost system. The later ZX Spectrum has a more advanced ULA that handles all aspects of generating the video signal and so the CPU can execute the user's program all of the time. However, this arrangement means access to the lower 16K of RAM (and the ULA's I/O port) is contended because this is where the picture data is held.

[@brianhackit7900]

Fantastic videos. All of them. Thank you so much. Really appreciate that. OK that's enough now

[@vinaduro]

Another amazing, and super informative video. I just can't wait for the next instalment. Thank you for sharing your time and expertise with us Dave.

[@bitset3741]

Great video. The TS-1000 was one of my early computer memories, and I have had 3 of them including the one I have now. In the US they were pretty popular and actually available for checkout in many of our public libraries. My elementary school had TRS-80 computers, the high school had Apple II but the local library had TS-1000s. I got a paper route and earned money for my first computer, which ended up being an Atari 800XL.

[@Graype07]

Your videos are amazing! I can't wait for the next one! Thanks

[@TheAka58]

The ear socket in the ZX81 was connected to the ear socket on your tape drive for loading stuff. The mic socket in the ZX81 was connected to the mic socket on your tape drive for saving stuff. The ZX81 was my first computer back in 1981.

[@ronmaxwell5394]

This Timex Sinclair 1000 was my first ever home computer. :-)

[@gyozopajter535]

great video! thank you! gently with the keyboard wires, they can easily break away!

[@doncasterrunner]

Really enjoyed the video. An Amiga one would be lovely please!

[@Ivanskis]

My baby, how I miss you. One time I spent hours and hours programming a board game and when I tried to save to my cassette it would fail.....

[@RabbitEngineering]

Typing in from magazines, oh the frustrations.

[@BertGrink]

+Dave Nunez (Rabbit Engineering) I remember it well... a friend of mine got a ZX81 in around '81/'82 and it got me hooked as well, so we would spend long sessions typing in programs from a UK magazine called YOUR COMPUTER where we took turns at typing; one of us would sit with the magazine and read out the listing to the other, who then typed it on the ZX, and after roughly an hour we would trade jobs.Of course, we would often type the programs wrong, and then spend hours hunting down the mistake, or the listing in the magazine would have printing errors, which we would then have to sort out on our own. Of course this could be VERY tedious, but we also learned a lot in the process, so yeah it was a great time which i remember with fondness :) On a side note, i like to think of the ULA as the forerunner of what later became known in the era of IBM-compatible computers as the chipset - i.e. North- and South-bridge chips, since what it did was orchestrating the flow of data between the various other parts of the computer. Finally i'd like to commend you on a great presentation; good job, and especially a kudos for pronouncing the name right: Zed-Eks. Thanks for making this video, you now have one more subscriber :D Cheers from Denmark.

[@RabbitEngineering]

Actually, the ULA idea is making a big comeback in FPGA cards used in massive datacenter racks, for applications like cryptography where you will need to revise the "hardware" often. Stick in an FPGA, and then just reprogram it when you need.

[@stargazer7644]

You didn't hook headphones into the ear jack. The computer had no sound. You plugged ear and mic into the ear and mic port on your tape deck. That's how tape decks are labeled. Ear is actually an input.

[@craiggilchrist4223]

My first computer was a ZX81 and they used to come in kit form to build yourself in true Sinclair fashion. Im from the UK and Sir Clive Sinclair was a pioneer here. I remember the old Ram Pack wobble due to the poor connection on the card edge connector. Sinclairs official fix solution was to use Blue Tac to keep it from moving. Classic. Anyone interested in the Story of Sinclair and Acorn i suggest you watch the program called Micro Men on You Tube.

[@BertGrink]

+craig gilchrist Hi Craig, as far as i can remember, the main cause of the dreaded RAM Pack Wobble was that in order to save a penny, Sinclair Ltd. omitted a small bit of plastic, known as the Key, from the connector on the RAM pack, the absence of which could lead to the RAM pack sliding slightly sideways and thereby shorting out the connections on the PCB. Since you're from the UK, you're probably aware that a large number of third-party vendors soon started making RAM packs (and other add-ons), many of which were both cheaper than Sinclair's own offering AND had the aforementioned Key fitted. Probably the best known of these third parties was a company called Memotech, which sold RAM expansions in several different sizes: 16KB, 32KB and even 64KB; they also made printer interfaces, and even a High Resolution graphics adapter, which sadly was horrendously slow. All the different modules from Memotech came with a small square of adhesive velcro which you would attach to the ZX81 and whatever module you had acquired. I mention this for the benefit of others who may not have heard about them, so please forgive me for stating what may be obvious to you. Greetings from Denmark :)

[@craiggilchrist4223]

Its all good my friend, i do remember the third party ones. The good old days.

[@BertGrink]

craig gilchrist Good days indeed :)

[@alexabadi7458]

I modify them, adding a new composite video output and a 5v switching power regulator, I do sell quite a lot of them on Ebay (USA) giving them a new life!

[@ZXSpectrumvideos]

Timex from Portugal had a lot of success back in the day. I'm not sure, I think Timex closed around 1984, but the Portuguese division went on until 1989, 1990

[@CraigMaloney]

These were quite popular in the USA. But the competition between Atari, Commodore and Radio Shack caused them to not be quite as popular over here. Plus they were a bit underpowered compared to the competition. That said it's still quite an engineering marvel and one of my favorite machines.

[@TheTurnipKing]

+Craig Maloney They threatened to be quite popular, causing Commodore to cut prices aggressively, effectively driving Timex out of the market.

[@MrJ0mmy]

i like your videos

[@ilexgarodan]

As a Canadian, we NEVER got the Timex Sinclair computers. You had to import them from America. You couldn't find Sinclair hardware or software here in Canada. Importing them from the U.K. was counterintuitive, due to the differences in voltage and using NTSC as opposed to PAL.

[@RabbitEngineering]

+Tedmeister that's too bad, they are really cool little machines - hey, never too late to pick one up off ebay!

[@ilexgarodan]

True. Though, I'm more interested in the ZX Spectrum.

[@BertGrink]

+Tedmeister Hey there... just in case you're still interested in the ZX Spectrum, it was marketed in the North American region as the Timex-Sinclair TS2048, and in an enhanced version called the TS2068 which additionally had a Programmable Sound Generator (AKA music chip) and a ROM Cartridge port on the right hand side of the (slightly improved) keyboard. Sadly, however, these enhancements meant that a small percentage of the games for the ZX Spectrum were incomptabile with the TS2068 caused by differences in the ROM code, so they had to make a ROM cartridge with the original ZX Spectrum ROM to solve this problem. Both models will occasionally pop up on ebay, but they tend to be somewhat expensive, being a bit of a collector's item nowadays. Cheers from Denmark. :)

[@desiv1170]

The TS1000 wasn't popular in the US to the public. But it was fairly well known in the nerd circles. ;-) I REALLY REALLY wanted one back then, but my parents said no. I think the keyboard really made them not take it seriously. I bugged them for a few years, but in April 1983, Commodore dropped the price of the Vic-20 to $99. It had a much better keyboard, so they got that for me. I did eventually get my TS1000 (and a US model ZX81 too) about 4 years ago... It also had a sales burst when Commodore had a deal where you got $100 off your C64 if you "traded in a computer" and the TS1000 was around $50 by then. So people bought them to get $50 off of a C64. :-)

[@TheTurnipKing]

29:95 Another important feature of the Z80 is that it was capable of refreshing dynamic ram automatically with no support components... another cost saving.

[@VauxhallViva1975]

I had one of these. 16K RAM pack was notoriously unreliable with it's connections. Breath near it, and you could lose your program. I soon gave up on trying to use the RAM-pack at all. The main machine's 1K was practically useless - even in 1981. My one ended up like many others - as a novelty door-wedge keeping a door open. ;) It WAS my very first computer though, before I moved over to the Atari 800XL.

[@ian_b]

The problem with the display wasn't that the CPU was underpowered (for its day). It was that there was no dedicated display chip unlike in more expensive computers of the time, or PCs. The Z80 was thus forced for most of the time to do nothing but drive the display, since when it is doing that it cannot go away and do anything else.

[@HidekiAdam]

Unless you were in FAST mode, in which case the picture was only generated when the machine was idle, executing PAUSE or INPUT

[@ian_b]

True. :)

[@dowekeller]

Ear and mic were for cassette loading only, the ZX81/TS1000 had no sound output.

[@MrStevetmq]

ULA Uncommitted Logic Array, forget the resistors and capacitors, this has the logic gates (AND,OR,NOR,NOT) that are needed to glue the system together. If you look inside a system like the TRS-80 model 1 level 11 that is very simler(sorry for the spelling I am dyslex....) in design(apart from having 16K not 1), it used MANY! more logic chips than the ZX-81. The ULA was the main brake though that made the ZX-81 cheep but so hard to repear because the ULA was not a standard part unlike the TRS-80 that used all standard parts. The ULA was a custem design chip.

[@TheTurnipKing]

+MrStevetmq semi-custom. I understand that Ferranti produced the ULA chips in relative bulk and customised them to fit client tasks. The ZX81 is an evolved version of the ZX80 design using the ULA to replace a large number of expensive glue logic chips allowing Sinclair to drive down the cost of the ZX81. It's quite similar but the software is slightly simpler and the ZX80 doesn't have FAST and SLOW mode.

[@MrStevetmq]

TheTurnipKing That is what I said. Thank you for adding detaile

[@markevans2294]

@@TheTurnipKing There's some additional logic to allow for FAST and SLOW mode. Mostly generating an NMI signal. This could be retrofitted to the ZX80. With modern ZX80 clones having this included. There was an upgrade option to fit the ZX80 with a ZX81 ROM and keyboard overlay

[@TheTurnipKing]

@@markevans2294 yep. The ULA was absolutely the right call from Sinclair's point of view. It reduced the entire computer to just four ICs and simplified assembly considerably, but it does unfortunately mean most modern clones are more accurately described as zx80 descentants due to the relatively unobtanium nature of ULAs.

[@Clancydaenlightened]

did Sinclair release the the zx81 under the Sinclair name then timex? because i have a US NTSC zx81 (inside the mobo is red with usa issue one silkscreened near expansion connector with a single 1k x 8bit ram instead of 2 1k x 4bit ram found it at a swap meet for $5)

[@BertGrink]

Yes, it was released in the United Kingdom in March 1981 under the name 'Sinclair ZX81', and in the United States in July 1982 as the 'Timex-Sinclair TS1000' with twice as much RAM (2KB versus 1KB in the UK version). Also, as mentioned in the video, the US machine had a TV channel selector on the bottom, but in every other respect the two models were identical.

[@Clancydaenlightened]

Gert Brink Nielsen doesn't answer my question i have a ntsc zx81 usa that is sinclair branded not timex?

[@BertGrink]

aadz93 I'm sorry, i misunderstood your question. To be honest, i didn't know that there were ZX81-branded machines on the US market, but my guess is that they may have been parallel imports. Given that both the UK and the US models utilised the same PCB, with just a minor difference in the components around the TV modulator, it wouldn't have been difficult to change these components to make the computer NTSC-compatible. Thus it is very probable that some entrepid person would buy DYI kits from England, make the necessary component changes, then solder the machines together and sell it as a NTSC version. Hmmm... I just googled your question, and found this page atariage.com/forums/topic/201836-us-released-zx81/ which states that there WERE actually ZX81-branded machines n the US market. I must confess that i'm quite confused now. I'm sorry for not being able to give a definitive answer.

[@Clancydaenlightened]

it's fine, i was looking for an answer to that question for awhile, i figured it was an older release model, appreciate it ! quite interesting

[@BertGrink]

aadz93 You're very welcome, it was interesting for myself too!

[@emmettturner9452]

Mic is short for microphone, not Mickey. "Mike," not "Mick." ;) The keyboard reminds me of a Texas Instruments Speak 'n' Spell. Any plans to tackle that one?

[@RabbitEngineering]

+Emmett Turner I'm going to call language difference on that one - in South Africa we used to all say "Mick" not "Mike" :-) Speak and Spell would be a nice one to tackle actually - speech synthesis is something most people consider mysterious so it would make a nice episode.

[@emmettturner9452]

+Dave Nunez (Rabbit Engineering) Awesome! Wish I still had mine. BTW, any reason why we don't get to see this thing work? I guess that's not the point of these videos (it's not like we saw the Famicom or NES controllers work), but I'm sure most Americans like me are so unfamiliar that they would have loved to have seen it boot/operate.

[@RabbitEngineering]

+Emmett Turner I don't have a TV with an analog antenna input into which to plug these things any more!

[@kumbah2006]

+Dave Nunez (Rabbit Engineering) - you can get a special replacement circuit board from RWAP that will give you a composite video output to your TV's yellow socket. You should be able to find them on sellmyretro dot com. Good luck !

[@RabbitEngineering]

+kumbah2006 thanks, I'll look into that!

[@stargazer7644]

Actually the other way around. The ts1000 would be known in the us, the zx81 was known in the uk.

[@pablopicaro7649]

dec 2020 its almost 40 years old

[@SianaGearz]

Mhm, i wonder how much did the processors really cost? 1983: MOS 6502: $5 MOS 6502A: $7 Zilog Z80: $4 Zilog Z80A: $5 Zilog Z80B: $12 I think this computer needs Z80A. However, i expect the prices would be perhaps up to 2 times as much in 1981 when the system was designed. By all reason, CPU and RAM are the only parts in the box that actually cost anything, but not for very long. And oh, ULA is a masked logic chip; like mask ROM, but with logic gates instead of data. So you don't need to make millions of them for them to be profitable. And they don't contain passive components.

[@mUbase]

the ear and mic were for tape loading and savging. NOT for sound... hmmmm... of course you could generate sound through the RF if you were good with assembly...

[@robertwoodliff2536]

Why manufacturer in Portugal........at that point, cheapest place in Western Europe to manufacture at that time.

[@andrewbevan4662]

ZX81 had no sound....