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I wrote a ZX Spectrum emulator some years ago when I was trying to learn more about how CPUs work, and I gained a big appreciation for these older types of hardware that can actually be understood by mortal beings. Love your content, very inspiring and insightful!

Hey James, if you make any more PCBs (like with a longer piece for the cartridge connector), you may like to try a gold over nickel finish. Not only does that look drop-dead gorgeous (especially on a green, black or blue board) but it will improve the life and reliability of the finger connector over a tinned finish.

Look forward to seeing v2 of this board.

It's always cool to see capability expansion on the old retros. Looking forward to see what you come up with.

Really nice video, I find you content very relaxing, thanks for sharing! It's cool to see you working on a spectrum, one of my favourite machines, and I love the idea of your project idea, good luck!

This is the exact Spectrum setup I had as a child. I always wondered what the flap next to the joystick ports was for. Now i know! Great stuff.

Hi James! I’ve only recently discovered your channel and am working through the CPU videos. I found you through the VGA peripheral series. It’s been hard not commenting on those older videos, I wanted to wait for a newer one that was active. I love this personal journey you are taking. It resonates with me in so many ways. I share a similar childhood computing background and am also a software engineer that took a liking to electronics and recently CPU and advanced chip design. It’s like being a hobby twin! I really like this trip into the ZX spectrum world. It’s easy to take for granted the powerful tools and electronics we have today and “dismiss” retro tech but the truth is the engineering that went into these is phenomenal and done by intellectual giants. Getting to the stage of understanding fully the design and implementation of these as a hobbyist rather than an EE graduate is no small feat and very rewarding. This was state of the art thinking using the tools of the day. I love what you are doing and can’t thank you enough for sharing.

Even though I lived in New Zealand, I was fortunate enough as a kid for My first computer to be a 48K ZX spectrum with Interface 2 joystick port - and the only ROM I ever had for it was Jetpac (Came in a very nice glossy box too with a custom plastic inlay that held the ROM cartridge)

Great fun ! that looked like Joust with a bit of Defender thrown in !....cheers.

Well done getting it working

Nice video very interesting, in Holland we had the MSX system also everything from tape. Your video gives me a idea to create something like this for the MSX

That was fun! Jetpack brings back lots of good memories

If I remember correctly, Spectrum expansion was quite clever: expansion device had a rom that could be "swapped" instead of the normal ROM. When you type or run a command that ROM does not support, it triggers an interrupt. At that point expansion port device switches different ROM to be used. Now execution continues from the other ROM at same address. That can handle all the new commands weather they are typed or run. Of course, after running the extra command for the device, it either returned from the interrupt or triggered another interrupt that triggered switching back the normal internal ROM and after returning from the interrupt, the code continued from the next point as if nothing had happened. I used to have a book with full ROM disassembly (explained) and full schematics. Some tricks like interrupt triggering on invalid command (to switch ROM) were quite clever I think.

YAY! I Love your videos! I noticed that you didn't show the full contents of the box!!!! I bet there's more in there!!! ... if so, pretty sneaky of you!

Nice to see you with a speccy :D I have something that may be useful for you, it's the ZXShield, it allows you to connect an Arduino to the speccy and virtualize any kind of peripheral or memory. It is published in my github (you can find the link in my profile) and there are examples on creating a wifi card with telnet software, a remote program loader, a poke search system, virtual roms, midi sound card, etc etc. Even if you don't use it the content of the repo may give you some ideas about things to add to the speccy ;)

Great work 👌

Is it doable? Memory request would be to tell the system to put data onto bus right? With the ROMCS asserted I assume the system addresses the rom, otherwise it may be reading from it's memory or even writing to it. Since there is no more available signals to discern a write to the expansion/coprocessor you'd have to assume writes to some address in memory/system is writes to the coprocessor then?

I had the Interface 2 plus one game cartridge with my Spectrum. Yep, JetPac! I never did find any other cartridges in shops. I got mine with the Spectrum that I bought second hand with saved up paper round money. I never knew where the previous owner got the game cartridge.

it's because the interface 2 slot was specifically for connecting rom cartridges: mainly, for swapping out the base 16k of memory which is where the sinclair ROM is usually mapped. The full expansion expansion port on the back of the computer is more akin to the cartridge connector you'd find on a console, giving full access to the various CPU buses. It's a strange thing that the Interface 2 was probably intended to work with the 16k Spectrum and once the 48k became the dominant standard a 16k ROM cart simply didn't make sense anymore.

Got to love the UKposts algorithm for recommending this to me: right up my street!

I remember the manual for my Spectrum 48K described the expansion connector as "you can do almost anything here that you can do with a Z80. Sometimes the Spectrum hardware might be in the way." That says a lot about the home computers of the era. I still have my Speccy although I don't know if it works. It is just so much more convenient to use emulation whenever I feel the need.

This reminds me of my KC85/4 (former GDR tech, U880 based), I still have one. And I have lots of modules for it. The whole thing still works. I was lucky enough to find some stuff on ebay, 20 ys ago. if I would have bought all the stuff back in the GDR, others could have bought cars from that money, easily. The community interested in this stuff is fading away ...

James, I was wondering what brand/model of analog oscilloscope you have in the background. I have a Tektronix 422 myself and it looks similar but I think yours may be newer.

Very ambitious! Are you going to expand the functionality by mapping the co-processor to the top of the ROM address space? Also, what kind of co-processor? Maybe a crazy fast piece of modern silicon that can complete complex operations within the space of single ZX clock cycles? A transfer triggered architecture (TTA) that implements floating-point or some kind of SIMD functionality? I’m mainly here for the CPU build but I look forward to seeing what you do with this.

Amazing project. Do you plan to do some kind of "SNES Super-FX Chip" enhancement? something that would be called from the original Spectrum CPU? Or a new CPU that would take control of everything, including the OG Spectrum CPU? Both ways would be amazing, but I don't know what is easier and/or doable :D Thanks for the videos, loving the series!

I had 3 Spectrums over time back in the day. When I moved to the US back in the mid nineties I left them all behind. I do miss it, however, I did signup for the Spectrum Next Kickstarter and really looking forward to getting the reimagined device. FPGA shortages not withstanding.

Now I'm curious if all (old) video game cartridges are like that - just passing wires directly to ROM chips. That was really cool.

Pro tip: start any PCB design by marking the outline, physical constraints (like keepouts, etc.), as well as adding mounting holes if needed... Saves a lot of heart ache later!

Doesn't the Speccy edge connector carry all of the CPU lines? If you do replace the current PCB how about making one with a larger ROM and having a switch to chose between the banks?

Not sure if I misunderstood what you were saying at the end there, but while the cartridge slot in the Interface 2 isn't much use for anything but ROMs, the Spectrum's expansion slot has the connections to allow you to do pretty much anything.

I love retro tinkering like this. I never got to use a Spectrum myself, but I really hope to mess with some early Nintendo stuff myself someday.

I notice the expansion port exposes Y, U and V outputs from the ULA. I wonder what happens if you treat those pins as inputs, and try to feed replacement color data directly into the video modulator? Is there a risk of damaging the ULA?

This might be a crazy thought but we know the following is true if you have a sufficiently powerful co-processor: You can detect memory read addresses You can supply data in response to a memory read You can modify the data to be read from memory by the co-processor asynchronously You could theoretically have code ran by the Z80 that performs memory reads be interpreted by the co-processor as “writes” by looking at the bottom 8 bits of the address of some 256 byte address space and treating it as a data byte that you then write into co-processor memory space to be acted on. You have the ability to write back new strings of Z80 code into the Z80 ROM space by the coprocessor in response to whatever data you read in, including jumping the Z80 wherever you like in ROM space. That’s almost like having infinite memory if you can modify the data coming out of the ROM faster than the Z80 can execute. If you need the Z80 to “wait till something is done” you would just have the code it’s currently executing by a busy loop till you overwrite the byte that jumps back to the loop label. I’m not sure I articulated this thought super well. An RP2040 dual core should be able to do a lot in between the Z80 ticks and obviously can shame the Z80 on all levels but the concept of giving the spectrum “more” and not just trying to replace parts sounds like such an interesting thing to do.

When working on the ZX Interface 2.021, the lack of signals on the cartridge port soon became a problem - my conclusion was that they probably never intended for the format to be anything other than an intermediary product. There's significantly more that can be done from the expansion port and that is probably your best guess, doing regular port read/write as a first step - just ensure that you don't blow up the ULA by using one of its ports (all even numbers at the very least).

I have to admit, I am a little excited as to what coprocessor you end up choosing =) Will it be a faster z80? Perhaps a later ez80 @ 25Mhz or a RPI-Pico. Maybe a 6502 for the trolls, lol's and fans ;) I really appreciate your channel James, got me back into Z80 assembly after quite a hiatus, and I look forward to just more of it =)

i made a little rom board to go in an interface 2(or ram turbo) only really need 2 diodes and a resistor for the eprom/eeprom rom select..and yes the original cartridges can fetch stupidly high prices, they were expensive at the time, from what i remember ... there was a design in an ETI magazine many years ago for a z80 spectrum coprocessor board, which could also run as a 'standalone' computer ..

Was waiting for WeirdBoyJim to get a plug. ;-)

There seems to be a modern-made cartridge for the Spectrum, called the Dandanator. It holds 512KB, and there are a couple games that use it too.

I actually have a few ideas about the Spectrum. First one is an external extension of the ULA to allow for use of sprites, using the Composite out for Sync and the RGB from the cartridge port and switching in our sprite data when needed. This could read the RAM address being accessed by the ULA when displaying and place the sprite in the right pixel position, though it might need a little timing to get it below 8 horizontal bits. Instead of the Speccy's limited 2-bit colour per attribute square, I'd go for 4 bits of colour to allow for a transparent layer. Using the standard Speccy pallete of course, but one of the black colours can serve as transparent. Oh dear this has become too long, let me comment further below

When you said the cartridge replaces the internal ROM, one of the first things I thought was in fact CP/M-80, given that one of the requirements is "16k of RAM starting at address 0"; to run it, should be almost as easy as replacing the ROM with a 16K RAM chip, figuring out some way to set the RAM RW for writing to it despite the limited cartridge, writing a bootloader, rewriting the display drivers, and somehow connecting to some sort of disk for loading programs. Should even work on a 16k Spectrum I think The tough part really is setting the RAM RW signal though...

It's interesting one of the roms failed cause it was actually a tape image, I wonder what the difference is between the two, and if tape images can be converted to rom images? I have a C64 I've been meaning to do something with for a while. It wasn't my childhood computer, that was an Amstrad CPC464, but the C64 has a simpler processor instruction set, so I'm hoping it'll be easier to hack/mod/play about with. I want to pick apart the data on on one of the many tape games it came with and see if I can extract out the code and convert that to something that'll run on a rom. I'll be sure to stick that on the discord when I start though :D

An interesting project! It was certainly possible to add 2nd processors via the standard connector, this is effectively how development systems squirted the data into the Speccy memory, but this did need the full bus. My first thought was to split the 16K chunk into 2, with the lower half being (as it must be) the Speccy Boot ROM, and the upper being 8kB of static RAM for the external processor to dump data into. Sadly, there is no R/W line to allow the Speccy to write data back. I guess you can fake this by using the low 8 address bits as data bits (plus possibly a clock/latch bit) and maybe having 1 or 2 byte-wide latches instead of SRAM. A boggo Rasp Pi would probably be overkill as a slave CPU, but interfacing would be easy, and the development environment would be faster than a headless RP2040 or whatever. Shame there is no interrupt pin available.

I am wondering if you were to get a whole MEMOTECH MTX512 plus the MEMOTECH HDX512 or the MEMOTECH FDX512 you might be able to do more with it than the SPECTRUM could ever do and using a special adapter called the SPECULATOR would let you play SINCLAIR SPECTRUM Software on the MEMOTECH.

Have a ZX Interface 2 with the original Sinclair box in nice complete condition. But no ROM cartridges for it such as Jetpac. Would be great to have the original Jetpac ROM cartridge. To use it on my ZX Spectrum 16K.

I've been "having a crisis" recently about obtaining Lead/Tin solder... I was trying to zoom in on your reel in this video to see what you had but couldn't quite get there... the "long-term, health hazard" warning symbol looks promising. ;) Love the video-overlay / x-ray cam effect. There's a history project... "Why don't the power and ground rails match the other breadboard holes?" There must be some reason for the cluster of five arrangement! Well, maybe. I wonder if it's even possible to find out.. Yeah... I was wondering if there were any logic chips that just pull down any floating pins. Having two pins called "ROMCS" and "ROM_CS" is... erm... not helpful.

Can't see in the video (ageing eyes) but you might want to chamfer the edge connector, straight cut card edges can bend the fingers in the edge connector.

I wonder if it would be possible to connect a Spectrum to the internet with a 48k modem. Transmit could be the beeper pin, and receive could be a pin in the joystick port. After that, a modem to USB adapter or old router should be easy to get, and some stuff like the Twitter API can be accessed pretty easily I think. But perhaps a humble serial port would be more realistic with this bit-banging setup. Sorry if these many comments are annoying...

What about putting a smd rom on the board and using your own cpu to make a rom programming routine? That could be cool.

It's like proto-Subnautica! ;)

You set you will like to create some coprocessor. I'm looking forward what kind of coprocessor you are planing to implement. Anyhow; you have plenty place in original ROM to add your own routine and use write into ROM space positions for communication with your cartridge. you can in say mode 0 react just on specific address range with your routine, in mode 1 implement your own ROM all 16KB with bank switching and use in ROM address space 4KB for transfer buffer to load into upper 48KB. you can even listen on bus and make backup from standard upper memory space to by able to restore original content. In any case I'm waiting what for coprocessor you would like to create. From me 👍

Respectfully adding an FPU isn’t the obvious choice. You’ll need to write an single instruction + data to the memory then read the data out. That’s slow on an instruction by instruction basis. What would be better is a GPU. Upload a block of code once and read / write buffers as needed. The bottle neck of writing in and out of memory isn’t as big a deal then.

For a moment I thought the interface was a modern zx spectrum

a memory mapped device will do just fine.

Maybe the PCB isn't going to be published, so might just be a moot point but perhaps through hole caps may have been easier for the average joe bloggs.

I went back to my hometown recently, contacted an old friend and he gave back my old Zx Spectrum 48k he borrowed over 30 years ago. Membrane busted though.

maybe add a rpi pico with sd card reader to emulate roms ...

When electronics design also includes mechanical design.

You could be really silly and put a FPGA in a cartage. then have spectrum code that reads the FPGA and displays the graphics. then during the vertical refresh the keyboard and joystick would be read. At that point it's just a display and keyboard to connect to an external device to do all the more modern stuff. But this would kinda be like the guy that displays video on a PET.

There is enough room in all the ROM space for thousands of flip flops, you just can't read them back that easy.

I hate that these cool machines are just niche and a hobby today.

Booo! AutoRouter 😊

thanks for job. but this is pain in ass. better put sd card.