New post from an old member

[Lee Hart]

Hi all! I joined around 2010 when I designed and built a ZX80 clone. With the help of the talented members on this group, I was successful.

So what have I been doing since: I've been designing various vintage pocket-size computer kits, using classic CPUs such as the 1802, 6502, 8080, and Z80 (see www.sunrise-ev.com).

At VCF-East, I was asked an interesting question -- can I design a ZX80/81 "badge" for next year's show; that is, a tiny ZX80 clone small enough to be worn as a "badge of honor" at computer shows. It should be battery operated, and offered in kit form so any reasonably skilled hobbyist can build it.

Now that's a challenging idea!

Obviously, one could use an Arduino or other modern micro to just emulate a ZX80. But that feels like cheating -- it isn't really a vintage computer, is it?

The ZX81 got the chip count down to just 4 chips; but the ULA is "unobtainium". However, maybe it can be duplicated with parts I can actually get? I've seen various ULA-clone circuits, but they take a lot of chips (more than the original ZX80 design). Are there ways to do it with programmable logic parts that were available in the 1980's (to keep it "vintage")?

Thought and ideas on such a project would be most welcome!

Happy hacking,
Lee Hart

[Paul]

Hello Lee, Long Time No see
It all depends on how time accurate you want to be.
There were no PAL or GAL chips in 1980, but PAL was available in 1985. I doubt you find the chips nowadays.
My suggestion is a SMD XC9572XL. It's smal enough to be hidden under a Z80 or ROM or RAM.
It is cheating but still just replaces the ULA with a commonly available chip. In case of space issues you could try two XC9548XL chips as the functions in the ULA work block wise and can easily be separated.
Cheers
Paul

[1024MAK]

Lee, what kind of screen are you thinking of using?

The ULA handles various things but a significant amount of it is taken up with the shift register and timing for the video output.

It also handles the keyboard and the tape input.

The question is, how much functionality is wanted?

Mark

[Lee Hart]

1024MAK: It seems there would be two options. First, stick with NTSC/PAL video. Use a pocket TV (Sony Watchman etc.) as the monitor. They accept NTSC or PAL video, and are fairly available on ebay at reasonable prices. Or, there are lots of cheap LCD monitors sold for baby monitors, vehicle backup cameras, etc.

Second, give up on video, and use a modern LCD. A black-and-white graphic LCD with parallel interface could be driven by the Z80 to simulate the ZX80/81 screen. Digikey has examples for about $30, and I'm sure there are others. The hardware would be considerably simpler, but the ZX80 ROM would have to be rewritten. BASIC could be made to work, but I fear it would break many of the more interesting ZX80/81 programs.

Paul, a CPLD could certainly handle the logic. The XC9572XL could work, but it is a 3.3v chip. Digikey has it in stock, but lists it as "obsolete". Being surface mount also means it's not something the average kit builder could install. I'd like to stick with 5v through-hole parts if possible.

There are quite a few early programmable chips, including PROMs, EPROMs, PALs, GALs, etc. that can still be found surplus in 5v through-hole packages. They're at least 1980's vintage. My VIP2K uses a 74HC4040 counter, 27C32 EPROM, and 74HC374 latch "state machine" that generates video much like the ZX80. The whole package (ASCII keyboard and all) fits in an Altoids tin. I'm scheming at something like that, only a ZX80.

Also, if I go modern, wouldn't it be the same as just using an Atmel micro to simulate the whole thing?

Lee

[Paul]

The trick of the XL serie is they are 3.3V, but 5V tolerant! And Chinese PCB companies solder the SMD parts.
The average user orders pcb with soldered SMD and soldiers the through hole parts.

[1024MAK]

Well, the various ULA replacements use CPLDs. There have been at least three designs over the years. Currently available is the vLA81. Details here.

Yes, it is possible to use 74 series logic. There are at least three different boards that do this to create a ZX81 clone board. One SMD, one mostly through-hole and one completely through hole. This doesn't include replica ZX81/TS1000 boards. There may be others as well.

I think if you want to avoid CPLDs, and use only through-hole, maybe use a mixture of 74 series logic and one or more GALs.

Mark

[David G]

Hi Lee, i have enjoyed your most insightful emails on the evlist


A ZX80 back in 1980 didn't come with a display, so in my view using a small, wearable LCD screen doesn't detract from the retro-ness of a ZX-80 badge. Back then we used whatever we had (old TV, surplus CCTV monitor, etc)


Then the shirt pockets will be available for a pocket cassette player

Also, if I go modern, wouldn't it be the same as just using an Atmel micro to simulate the whole thing?

[Lee Hart]

I talked to Charlie at <https://vdrivezx.com/vla81/>. It sounds like his replacement for the ULA could indeed make ZX81 "badge" in just 4 chips! Lots of details to work out, though.

Leave out the RF modulator, and just provide NTSC output. There are lots of cheap NTSC monitors on ebay etc. for $20 or less.

Leave off the audio cassette I/O. Is there a way to provide serial I/O so a USB-serial adapter can be used for power and to download/upload data and programs?

Use a 32k RAM and 32k EPROM as they are cheap and available. Could they be bank-switched in some fashion to provide a selection of pre-loaded programs? Could a CMOS Z80, RAM, and ROM be used, so it all runs on a single 3.6v cell?

I can use 1/4" tactile switches for the keyboard, like my VIP2K. A paper overlay can provide the key labels.

A big unknown is price. What are people willing to pay for such a thing?

Any other ideas?

[1024MAK]

Without the passive components of the normal ZX81/TS1000 design, the ULA pin used for the tape input ( "ear") may be able to be used to receive bit bashed serial input. But there is no support for this, so a custom ROM would have to be written.

Serial output is a bit more tricky, as the tape output (mic) is actually taken from the video output pin of the ULA. But may be possible. Otherwise it means adding extra chips for an output port.

The size of the ROM and RAM chips is not that important, it's the address decoding that determines the memory map. You can have 16K bytes of ROM in a normal ZX81/TS1000 with minimal modification. Although you would need to check if the vLA81 supports that as I can't remember. The vLA81 includes provision for a 32K byte SRAM chip.

Bank switching is possible if the extra circuitry (chips) to provide it is included and some ROM code is written for it. Bank switching on a ZX81/TS1000 is not something you will find many references to. One advanced clone did provide this. Although manual switches to swap between different ROMs is/was done by some.

Note that the normal ROM code clears the normal RAM area at power up or reset (0x8000 to 0xBFFF) but ignores any other RAM. NEW also clears most of the RAM but only up to the point recorded by a system variables (which can be changed). Loading a program from ROM therefore requires either the user typing in a BASIC line to jump to some code in the extra ROM space or a modified BASIC ROM code.

Keep in mind that the top part of the memory map is special. If using the vLA81 you don't need to worry about this.

If the vLA81 will work from a 3.6V supply, I don't see why a whole board won't if you use modern CMOS parts that are also happy with this voltage.

Price is a difficult thing. There are plenty of members here that have either built a clone or who have modified their existing ZX81/TS1000.

But building a 'badge' is something different compared to making something that could fit in an original case...

I'm not the best person to answer (I have a 6502 badge from however many years ago that was made available as a kit).

But how many, IDK.

Mark