ZX Spectrum build, No ULA.

[@alexloktionoff6833]

This is what I was looking for - all discrete no rare ULA and DRAM and VGA output! Please continue

[@DrMattRegan]

Enjoy!

[@StratsRUs]

Quality viewing.Warm nostalgia for me.

[@DrMattRegan]

Glad you got something from it!

[@andrewkieran8942]

I came to watch this to understand your approach, and how you'd solve the timing issues. Then, I was impressed with your construction technique. Those things are all impressive, but then I fell in love with the pic of your puppy!!

[@DrMattRegan]

Yeah she's great. Normally I'm very allergic to dogs (and cats), but she's a cavoodle and no allergic reaction at all!

[@jorgeferreira6727]

Congrats! That is a very clever way of sharing the bus with your personal video hardware. In this days of fast hardware, FPGAs, extra powerfull microcontrolers and massive frameworks, its an almost lost art. This kind of videos are quite a refresh so I don't totally forget some old tricks. Thank you for a really interesting and refreshing video.

[@fastlandnet]

I love these words

[@DrMattRegan]

Thanks for the feedback. It's a fun project. You might like the upcoming bits as well.

[@mheermance]

Neat. Your point to point wiring looks so much better than mine. I find that when I am halfway through I realize I have painted myself into a corner, and the latter half of the build is trying to avoid melting wires from the first half.

[@DrMattRegan]

Exactly! I do have that problem later as well, it happened a bit on the SAP6502 build. Some pins were several wires deep.

[@vanhetgoor]

Marvellous! This is how computers should be built. Away with the ULA, I never liked that kind of electronic parts, all that mistry, that hidden science, all those little secrets squeezed into one chip. It is a lot of work but the results are so beautiful to look at, it is craftsmanship. When the computer is not in use, you can frame it on the wall to look at.

[@DrMattRegan]

Thanks for that. Back to basics. I think we should throw out the Z80 too - kzbin.info/www/bejne/oZewfp2hZ51rmdE

[@TheTurnipKing]

The thing is, the reason behind the ULA makes perfect sense when you see the Harlequin board, which implements the ULA in 74 series logic. Indeed, you just have to look at the complexity difference between the ZX80 and 81. It's just unfortunate that these chips had to be reverse-engineered instead of documented, they could have been reimplemented in all sorts of ways

[@DrMattRegan]

Yep, the harlequin uses the same technique as the Apple 2 (and TRS 80). I used it too here kzbin.info/www/bejne/p5Pdq62hg7V8eas Any guesses as to why i chose an EPROM?

[@schmitzbeats6102]

@@DrMattRegan I would say, to make different screen modes, via "Firmware". Like Timex 2068, Sam Coupé or ULA+

[@TheTurnipKing]

@@DrMattRegan I've seen some cases where they've been used as a substitute for relatively complex logical devices, but I'd be lying if I said I understand the process here. WHY is relatively easy. PROM is cheap. How, i don't get. Yet.

[@csbluechip]

Can you make a short video showing how you are doing the wiring please. Especially one (bus) wire to multiple points. It's not obvious where the insulation is coming from. Loving this series. Thanks

[@DrMattRegan]

Thanks, will be working on a build techniques video soon.

[@AndyGraceMedia]

This is absolutely gold. Loving it. Really clever solution to the problem. On construction technique I'm also impressed. I think I'd lose the plot with that sort of point-to-point IC socket wiring. I've prototyped using soldered break-off SIP machine-pin sockets designed for wirewrap and that was hard enough.

[@DrMattRegan]

Thanks Andy, it's a fun little project. It should end up at < 30 chips.

[@AjinkyaMahajan]

Impressive work !!!

[@DrMattRegan]

Thank you! Cheers!

[@kirknelson156]

thats some genius level problem solving, enjoying the series 🙂

[@DrMattRegan]

Thanks for that, glad you like it.

[@schrodingerscat1863]

Personally I have always used wire wrap for projects like this but your point to point wiring technique looks great and as you point out much cheaper than wire wrap which is insanely expensive for the sockets. Using an EPROM as a replacement for the ULA is also a great idea.

[@DrMattRegan]

Yeah, wire wrap sockets are hard to get in 32 pin packages.

[@schrodingerscat1863]

@@DrMattRegan Can't say I have ever had problems getting any size socket but they are bloody expensive, Mouser have 23 pin DIP in stock.

[@flomojo2u]

Wow, what a blast from the past! I built SO many little embedded boards using those Radio Shack prototyping boards and blue Kynar wire. I stuck with 68000-series CPUs, including a 8MHz 68000, 16MHz 68020, and 33MHz 68030. They all worked great, though I used PEEL programmable logic for decoding the addressing signals for the 68020/030 boards. Very simple boards, mostly EPROM/flash, SRAM, logic, and random stuff, including an IDE hard drive using my own file system. It was lots of fun reading the ATAPI specification for all the registers and commands!

[@DrMattRegan]

Excellent. I'm thinking of doing a series like the kzbin.info/aero/PLjQDRjQfW-85S5QkX8wZbkqichM6TLYYt but for the 68000, a TTL-68000 CPU. Would that be of interest?

[@R.Daneel]

I used to do full wire-wrap, but would occasionally use this technique for parts of a circuit. One trick I used if I needed more than 2 wraps on a pin (or it was getting too busy) was to add an empty socket, wire the croweded pin to the empty socket and short socket pins together on the top of the board by sticking a wire in the socket.. You then had multiple pins to use for that one signal. Once a few pins were used this way, you could even cut off the rest of the socket with edge cutters if it wasn't needed. Sub and thumb.

[@DrMattRegan]

Sounds good. I'll try that if necessary. You can sometimes get, 4 connections if you use 2 wires

[@MeeBilt]

Great stuff! I used the exact same point-to-point-wire technique on my first DIY computer back in 81-82. It was based on the 6502... when I got my first job I suddenly got access to a wire-wrap gun so I used that for my next build.

[@DrMattRegan]

Yep, I think they are both becoming lost arts.

[@MeeBilt]

@@DrMattRegan Indeed, thanks for a great video and an interesting project! I never owned a ZX Spectrum but I borrowed one from a friend for a month to try it out - I remember those cool little micro-drives.

[@DrMattRegan]

Thanks for the feedback, enjoy.

[@nexpro6985]

Thanks for the vid. It popped up in my yt feed. I swear KZbin is listening to my thoughts. The last thing I did on my laptop before watching this (on my phone) was to add a map of the Zx Spectrum memory to the appendices of the Machine Lightning manual I am reproducing. The memory map indicates contended ram vs un-contended.

[@DrMattRegan]

Excellent. Glad you enjoyed it. Interesting little machine the Spectrum.

[@ESPArduinoIDE]

I've always wondered why the authors of the clones didn't do this right away? Great job!

[@DrMattRegan]

Thanks. Yes, Superfo uses a second SRAM for storing the video. I still haven't to get it to work at speed yet (still working on the VGA output), but for the moment, the 74HC374 seems to do the trick.

[@nickmoore385]

That makes sense now. I was wondering what the second RAM chip was for on his ZX-128- Max design.

[@melkiorwiseman5234]

Very interesting, and it occurred to me after watching this part, that you could add another latch or two to enable you to bank switch part of the RAM (the part which doesn't hold the video memory) to allow the computer to be expanded to run either multiple programs or programs with large amounts of data. Edit: Maybe even simulate a RAM drive for fast loading of programs.

[@DrMattRegan]

Yeah, good thoughts. It may make sense to put video RAM in I/O space.

[@melkiorwiseman5234]

@@DrMattRegan You could address video RAM using the OUT command. One method would be to have an output port to select which 256 byte section of the RAM (or which non-RAM device) you're addressing, and another to address the actual byte in video RAM (or device) which you want to access. But my original thought was that, if the video RAM is located somewhere in (for example) the lower 32K in RAM, then you could bank-switch some of the unused RAM in the chip using the upper 32K of memory space. (I'm assuming you're aware of the concept of Bank Switching)

[@sarkasaa]

Well, that subscription was a no-brainer!

[@DrMattRegan]

Welcome. Enjoy!

[@borayurt66]

Exactly. I don't understand more than half of it, but the parts that understand were enough to be hooked.

[@rappzz]

Awesome! I'm sticking to Logisim for now. Seeing a physical thing run must be thrilling though 😁

[@DrMattRegan]

Hope you enjoy it!

[@ITGuyinaction]

👍👍👍 Excellent! Very interesting project! Even if it's not an exact "copy" of ZX Spectrum. Concerning these technical details, it's probably necessary to watch the video 2-3 times to understand everything even if the general idea is easy to catch. P.S. A bit pity that you didn't design printed circuit board.

[@DrMattRegan]

I'll likely do a schematic at the end.

[@ITGuyinaction]

@@DrMattRegan You mean schematic diagram or board design?

[@schmitzbeats6102]

Marvelous! For quite some time I wondered how the ULA could be replaced by a giant lookup-table. You are makeing that work. Also I would appreciate a more in-depth video or article about your construction technique. Specifically which materials you use.

[@DrMattRegan]

Many thanks! Yep, am thinking of putting forward a video or two on the construction technique. Stay tuned.

[@billcarson9565]

Excellent, subscribed!

[@DrMattRegan]

Welcome aboard!

[@saszab]

Same here!

[@Xsiondu]

That was awesome

[@DrMattRegan]

Thanks, glad you liked it.

[@Sinclair-uq7jq]

Can you make the video ram double the processor speed so there's no contention

[@DrMattRegan]

I’m not sure that saves any hardware (except maybe the 374). You still need to be able to tri-state the Z80 address bus.

[@Sinclair-uq7jq]

@@DrMattRegan is that expensive?

[@Sinclair-uq7jq]

Dual ported or double speed are those the only options?

[@DrMattRegan]

@ you can stop the CPU with a BusREQ, or slow down the clock. Have a look at the my Spectrum DRAM video.

[@Sinclair-uq7jq]

@DrMattRegan but the processor isn't even 4mhz the enterprise can manage 8mhz now see stefandrissen video mod player enterprise

[@garry5280]

Theres an original prototype spectrum in the cambridge computer history museum built in much the same manner.

[@DrMattRegan]

Interesting. I've seen the wire-wrapped version of the first PC. I think i remember seeing one of the 68000.

[@alexloktionoff6833]

Could it be simpler to have two SRAM chips, probably even 3: 0 - ZX RAM 1 - pixels buffer 2 - attributes buffer? Clearly separate XZ ROM+RAM from Video and just use WAIT when Z80 writes into video memory. Even adding 1 or 2 SRAM chips could reduce total chips count, assembly and debugging?

[@DrMattRegan]

At some point, the memory storing the video data to displayed needs two paths into it, one from the CPU and one from the raster generator. Other than dual ported memory or VRAM, we are going to need some sort of multiplexing on the address lines, and this is where many of the chips are.

[@DavidHembrow]

Excellent Indeed. Better than the original in terms of CPU speed. Full speed memory access in the contended memory will of course make some timing dependent software incompatible, but I'm sure you've thought of that already and I expect it to be addressed in a layer video Btw, have you tried loading the I register with 01xxxxxx ? You'll then see snow on the screen with the original hardware.

[@DrMattRegan]

Thanks David. Yeah, i'll use the wait signal to slow the CPU down using the circuit in the last video to emulate the DRAM memory access. VGA will mess things up badly too, so i may do a PAL version also. I've heard about the snow issue, but i haven't looked at it in detail, I suspect the 01XXXXXX XXXXXXXX as the refresh address probably triggers the clock halt circuitry in the ULA. It triggers it during port accesses. The raster scan sweep should keep the contents of the DRAM refreshed,. Thanks for that, i'll look into it.

[@alexloktionoff6833]

Actually I'd like to assemble without slow-downs, but more fast Z80 like 14Mhz @@DrMattRegan

[@bkahlerventer]

It might be overkill, but have you thought of using dual-ported RAM for the video RAM to avoid the bus contention?

[@DrMattRegan]

Yeah, i'm not a huge fan of dual ported memory, hard to get, expensive etc. I'd rather stick with more vanilla parts, but that's just my preference. I would use them for crossing clock domains though.

[@akkudakkupl]

This might end up being a better ZX Spectrum than the original ;-)

[@DrMattRegan]

At least it won’t use annoying ULA

[@alanclarke4646]

VGA output and no "dot crawl". That alone makes it better😊

[@ostrov11]

спасибо, хорошая работа.

[@DrMattRegan]

Glad you like it.

[@ancipital]

Rather than soldering it all on - did you think about doing it all with wirewrapping techniques? I've not tried this myself (yet!) for anything though could be entertaining to give it a go. Also makes it easier to fix mistakes. I remember the first minicomputer I worked on (DEC VAX 8600), if you looked in the back, the whole backplane was wirewrapped!

[@DrMattRegan]

Yeah, I've tried wire-wrap in the past, but i actually prefer point to point soldering. I find wire-wrap (for me) to be slow. I learned to do this back in the day, when wire wrap sockets were expensive. Also i want to put it in a spectrum case.

[@ancipital]

@@DrMattRegan Understandable going into a spectrum case, no room for wirewrapped. Good video though so thanks for the interesting start to finish of getting that working.

[@zxspectrum16KB]

If u can get video ram without contention running on zx spectrum try some freescape titles they were very slow can u overclok the cpu to 16mhz and 32mhz? Try adding ula+ support or spectra + ulax?vdac2?

[@DrMattRegan]

Interesting, will look into it when i get a chence.

[@zxspectrum16KB]

@@DrMattRegan please do

[@frankowalker4662]

Cool. And no ULA. 👍

[@DrMattRegan]

Thanks, it's a fun project.

[@frankowalker4662]

@@DrMattRegan I built the Harlequin 128 a couple of years ago. That was a lot of fun.

[@DAVIDGREGORYKERR]

Has anyone tried building a TI-99/4A with 512K of RAM and ROM so it will work as intended and not throw the out of memory error

[@DrMattRegan]

TI-99/4A is a bit outside my experience unfortunately.

[@fredimachadonet]

This is awesome! Would it be possible to share the Arduino and C++ code to generate the video via USB? I wanted to do something similar with the TRS-80 Model 3 clone that I'm building, specially because I don't know how to build the video circuit.

[@DrMattRegan]

Yep, i'll make mention of it in the next video.

[@ZXSpectrum128K]

Ula+ ulax vdac2 spectra have you heard of these?

[@DrMattRegan]

Yeah, I’m probably not going to go down the path of improving the ULA, kinda more interested in replacing the Z80 with TTL logic.

[@ZXSpectrum128K]

@@DrMattRegan what about z80 at 8mhz and video ram at 16mhz?

[@Mr.1.i]

Do u think people would of paid an extra £5 on the price of a zx spectrum so it could have vram

[@DrMattRegan]

Hmmm. Adding the upper 32k of Ram (to make it 48k) avoids the contention for those memory accesses. Probably better to spend the money there

[@yanparasuacurtisao]

Witch Wires are this? Im building a z80 handwired computer and im using much silicon wires that i need to decap all te sides before soldering then

[@DrMattRegan]

These are wire wrap wires, but I’d recommend using the ones with Kynar plastic around the wire. Kynar is quite heat tolerant and easiest to work with.

[@GregCoonrod]

Just out of curiosity, why do point to point soldering instead of wire wrap?

[@DrMattRegan]

I always found wire-wrap a bit cumbersome and slow. I find this technique easier myself.

[@_wave64_]

Can you share what wire and insulating you're using?

[@DrMattRegan]

So this is the exact wire i use www.jaycar.com.au/blue-wire-wrap-wire-on-spool/p/WW4346?pos=1&queryId=5bf58702e3ae70ed5bcbb145f0e4f3ef It's from a local retailer, but it is kynar brand, you should be able to find it on ebay.

[@alexloktionoff6833]

Is it possible to isolate Z80 A & D buses with just 10k resistors like in original ZX?

[@DrMattRegan]

Yep, should be possible to use resistors, but you still need the 74HC374.

[@leechatt9709]

Superb work! I did a Zx81 like that and it never worked. Interesting idea cant wait to see this clocked at 3.5Mhz. I wounder how much faster it will go especially when youve put the design on a PCB. I suppose it does work as your making videos about it? Cool! Im currently working on a Nes Emulator. the PPU runs every tick and the C6502 runs at a Modulo of 3. Are you going to release the schematic and build parts when your done.

[@DrMattRegan]

It’s a work in progress. Did you use a CMOS z80? I did point-to-point wiring for a ZX81 in this series kzbin.info/aero/PLjQDRjQfW-84WG47-5UjPz1BrXxc1acvd which did run at 3.25 MHz.

[@leechatt9709]

I used a Zilog Z8400A PS. Have you ever thought of using a Teensy or a stm32 for the ULA?

[@DrMattRegan]

I'd rather not use modern parts. The EPROM and SRAM were available in the 1990s. Will do a schematic once finalized.

[@leechatt9709]

Cool old skool

[@ZXSpectrum128K]

Can send u links if u need them....

[@Mi-583]

You could try to put the links in your ch description and tell people to go there within the week before you take them down?

[@ZXSpectrum128K]

Cant reply to your comment keep getting error

[@DrMattRegan]

Huh... I see this one. Not sure what is going on with youtube.

[@ZXSpectrum128K]

@@DrMattRegan I'm not complaining they reinstated my channel after small claims tribunal

[@peddersoldchap]

Would it be possible to use this EPROM to replace the ULA on a real Spectrum? PS - I'm not that experienced with electronics, but I know how to solder and use a multimeter. And do basic circuit calculations.

[@DrMattRegan]

No not really. This design uses SRAM, while the spectrum uses DRAM

[@peddersoldchap]

@@DrMattRegan Would it be possible to adapt the design?

[@melkiorwiseman5234]

@@peddersoldchap I note that there was never a reply, but the reply would probably have been "not really, because I'd have no use for such an adaptation since I'm not using a ULA, but feel free to look into that yourself if you think you'd have a use for it." My reply would be that as long as the number of inputs required doesn't exceed the number of address lines on the EPROM, and the number of outputs doesn't exceed the number of data lines on the EPROM, it should be possible to do it, but you'd have to figure out for yourself how to program the EPROM to provide the correct outputs from all possible inputs. Logically, it wouldn't be all that hard. It's just a bit of a slog, having to take account of every possible input and then ensuring that every possible input provides a sensible output, and compiling a bit-pattern for every memory location so that it can be programmed with the correct pattern later. The only remaining issue is ensuring that the EPROM which you use is of a type which can respond quickly enough to keep up with the input signals.

[@peddersoldchap]

@@melkiorwiseman5234 thank you for your thorough reply. I would love there was an open source solution for implementing a ULA.

[@melkiorwiseman5234]

@@peddersoldchap The only part that a EPROM could not do would be to generate signals, such as from a clock circuit. If you needed more than 8 output lines, you could parallel the address and access lines for two EPROMs to get 16 outputs. Maybe someone will come up with a solution, but I could probably do it myself if (and only if) I knew all of the possible inputs to a ULA and all of the corresponding required outputs. That's on the proviso that, as previously stated, the ULA didn't have any clock or timing role.

[@dogwalker666]

I built my first zx81 from a kit.

[@DrMattRegan]

How did you go getting it working? Any problems?

[@dogwalker666]

@@DrMattRegan Actually it worked first time I was so impressed, Modified it to give composite video output to a Sony monitor, much clearer.

[@DrMattRegan]

Probably the biggest problem was the keyboard interrupt. I tried using the interupt acknowledgement sequence to cancel the INT signal. I got frustrated and just used the raster generator to limit it to 30 t states and that worked.

[@dogwalker666]

@@DrMattRegan oh right.

[@68HC060]

A latch could probably do it...

[@DrMattRegan]

@@68HC060yeah, not sure. The data would become unstable during the second read.