What's inside the ZX81 RAM Expansion?
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- Опубликовано: 11 сен 2024
- Today I'm gonna rip this thing open and see what's going on! Come take a look.
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ah, those memories flooding back when in autumn of '81 my father came back from a large European computer fair and produced two very small boxes from his suitcase: a ZX81 and the 16KB RAM pack, surely an issue 1. This sent me into a whole new universe.
Fantastic, computers seemed more wonderous from that Era, oddly more complex and more compelling all at once than modern machines. Now it just seems like hardware gets faster but not much new comes around. Thanks for watching!
I was 28. I had no computer experience at all other than USING the computers at work. I got one, and here I am, fully self-taught, safely retired after spending my last 5 working years doing computer-related stuff that no-one else could do...all because of the ZX81...and the RAM pack...and the thermal printer...and Basic and Machine Code and Assembler.
I used to love these plug-in modules, I used to have a repair business back in the early 80’s and the number of spectrums we took in could almost pay the rent. Any dislodging of a plug-in module (which was easy to do as there was no retaining mechanism) could and often did blow some chips
I came into the hobby just after the chip/board level repair stopped. I am happy to have learned the skill and I bet it was a lot of fun just moving hardware and paying the bills!
The design of that pack is what you could politely refer to as "highly cost optimized" one of the reasons (apart from the iffy mechanical connection) that it's sometimes unstable is that it generates some of the RAM timing using RC networks driving regular 74LS series TTL chips. This makes it highly sensitive to a bunch of things like ambient temperature, the precise voltage of the 5V rail, which vendor supplied the chips and you looking at it in a funny way.
That old school DC/DC converter didn't help either - it was needed because the 4116 needs +12V and -5V in addition to the +5V, but generated significant RFI - the metalized coating on the housing kept the radiated interference below the FCC limits, but still induced significant noise onto the internal signals.
I looked at a bunch of these things back in the day, and although they did work most of the the ones I checked were running the RAM outside of it's timing specs - typically by violating the tRAH(min) spec. The use of disc ceramics in an RC network for delays is pretty sketchy, too.
I can't really tell if the cost savings was pure genius or an accident of some idiot pushing a deadline.
I don't remember where I read/heard this, but my recollection is that the module's engineer originally included a capacitor as he knew that power interupts would happen, but Clive nixed it on cost grounds... My first exposure to computers and programming was with the ZX81 followed shortly by a Spectrum - happy times (except when the 16k came loose!). Thanks for showing the innards
I had that RAM expansion and the connector was so bad that I did solder the expansion to the bus connector permanently in order to achieve stability.
The bus connector weren't gold plated but tin plated and it didn't help a bit.
Could you imagine if this thing was gold plated? It would double the costs!
Crazy to think that this system sold with so little memory 2Kb (what's even more crazy is that it had 16 times more memory than the Atari 2600 and 8 times more than the TI99/4a if you count just the scratch ram). Cool little expansion. To stop it wobbling around and crashing the system "a piece of Blue tack about the size of a runner bean" should do the trick, well that's what I learned from watching Micro men, nice teardown ;)
the Sinclair came with 1k, the Timex came with 2k. Did you see Hjalfi's vid of programming the machine, that was impressive!
The one extra circuit generates a negative voltage. The RAM chips used (4116) require a dual voltage supply (+/- 5 volts). ( see the ZX spectrum )
In addition, the ZX81 outputs the addresses and data in parallel, but the 4116 requires the address signals multiplexed (RAS / CAS). This means that the chip uses the address pins 2x, which means the chip needs fewer pins and is cheaper to produce.
Internally, the ZX81 has a static RAM that does not require this logic.
Nice, thanks for sharing!
If static RAM was used, would there be no need for anything but the RAM itself?
@@carminone Unfortunately not completely, but only 2 components. The ram and a 4xNAND (74LS00).
In order to use the ram externally, the internal ram must be deactivated, this is done with RAM CS, which is permanently set to 5V. Then the states MEMRQ and A14 must be evaluated. The ram may only be active when Memrq is low and A14 is high. Since the RAM works with negative logic, it must
Memrq = 0 + A14 = 1 >>> CS = 0.
If you then take the 74LS00, the Memrq is inverted and logically linked to the A14 NAND.
The extra circuitry was for power conversion for the ram chips from the edge connector of the zx81.
Cool, thanks for the info!
That was built at the Timex factory in Portugal. Lot's of Spectrum and Timex stuff came from there.
Sweet, thanks for sharing!
I recall getting a Sinclair Spectrum and opening it up to add an extra 48k of memory. The upgrade kit even came with a dummy practice chip.
That's pretty cool. I think there was a way to add RAM internally to the ZX81 (aftermarket) as well.
I saw the video title, “what’s inside the ZX-81 RAM expansion” and thought, “RAM!”
And hey - I was right! 😜
there was some cardboard too... probably the most important part!
Many years ago, I made my own 16K expansion. I think my circuit had fewer ICs. I made the multiple voltage part with a 555 timer chip. The voltages other than 5V drew little current.
That's awesome! Its one of the things I still want to learn how to do.
@@TemporarilyOfflineRetroTech The multivoltage RAMs are a thing of the psst I think so there is less to learn there,. I used RC delays to time the RAS-flip-CAS timing. It is a case where minimum times have to be worried about but the maximums are a long way out of the range of concern.
My grandfather put epoxy between the computer and the expansion pack to stop the pack from wobbling and resetting the system bus.
That is a really good idea!
bringing it all back
This is all fresh for me, didn't have one as a kid and missed out!
The original UK versions didn't come with any of the shielding - just a plastic case
Those were simpler times!
I really enjoy the ZX81 content ... keep it up. :)
Thanks - I have to buy some more stuff for the next installment. I think I'm all outta gear
well the two bits of installing cardboard explain the overheating
True, that's the problem for sure!
It's just ram and and 74ls xxx chips for the address decoder. You had to dind a way to support the ram pack so it didn't hang down on the edge connector. There were after makes 32 48 and 64k packs and some rs232, parallel , voice synthisizer , joystick interfaces and even a disk interface. I had a ts 1000 with 64k rs232 interface and disks running cp/m
I have been looking for one of those ZXpand units for sure. Thanks for stoppin by.
Impressively more complicated than I would have thought. Love hand drawn circuit boards. Any idea what it cost back in the day?
I did love the hand drawn circuits as well - I have a series here on building a ZX-81 kit - a really fun build. I love how they did "so much with so little" on that. I don't know what that expansion costs, but I'm sure it was as "cheap as possible" knowing Sir Clive Sinclair's mission. Thanks for watching!
No flight simulator? I had that!
I can imagine a flight sim on the ZX81, that would be neat to see!
there are more ICs there than in the ZX81 itself ... LOL
I found that very strange too
WOW!!!! There are RAMs inside the ZX81 RAM Expansion!!!! 🤣🤣🤣🤣
And you still clicked!
You can build a modern version of this out of just 2 parts.
But what two parts?
@@TemporarilyOfflineRetroTech A new Hitachi 16K HM62256ALP-10 32,768-word × 8-bit High Speed CMOS Static RAM, and a SN74LS00N multiplexer (about $1 buck). And 2 sockets. And of course the edge connector which is really hard to find. No other parts needed. Check out the HM62256A Series datasheet. The entire thing condensed down into 1 RAM chip. And it uses way less power. You don't need the GND tab because the edge connector provides GND. You could also build a 32K version, which Timex had planned but never produced.
@@TemporarilyOfflineRetroTech PS: You can also replace all the 473Z ceramics on the original ram pack boards with tiny modern ones (473A) which are much smaller. Just be careful when resoldering.