zx80 ROM confusion.

[DoktorDoris]

hey there,
yet again i would care for some advice please.
i have soldered nearly all of the bits to my zx80 PCB but i am kind of stuck with the ROM. according to my BOM i need 8832 or 2364 chips. well i can't find them anywhere. i have read elsewhere that i can use 27C512 ROM, but these have different pin arrangement. i have also found on ebay adapter jobbies to turn one of them into something suitable to use with a C64 is this what i need? And i still haven't got a clue about writing to them and the gubbins i need to do so.
i would be extremely grateful for any advice.

doris

[Moggy]

A 2732 EPROM is the easiest way to do it should you have one and possess the means to program it or find someone local who can do this for you.

More information here in this thread...

https://www.sinclairzxworld.com/viewtop ... zx80+eprom

The two pictures are of the the 2732 and the much harder to find 2532, showing the mods required to use either one.

[1024MAK]

All EPROM chips need a special programmer to program them. They cannot be programmed "in system" ( meaning in the target system).

Some types of the later technology, EEPROM / E²PROM or flash memory chips can be programmed "in system". But, you need a working system to start with, which means you need a working, programmed EPROM, EEPROM or ROM chip... (and with a ZX80/ZX81/ZX Spectrum etc. an expansion board for the chip to be programmed to be fitted in...).

But the smallest DIP/DIL EEPROM comes in a 28 pin package.

When the ZX80 was designed, there were a number of different pin-outs in the market. Plus some differences between EPROM chips and mask ROM chips (the chips used by manufacturers that are made with fixed data permanently made as part of the manufacturing process).

Over time, all the older types of EPROM chips have become obsolete. Hence the types that are pin compatible are extremely hard to find.

27C256 and 27C512 one time programmable (OTP) EPROM chips are still made, but are overkill, being 32K bytes and 64K bytes respectively. Plus these come in a 28 pin plastic package and will need the otherwise unused address pins to be tied to a suitable logic level. Where as the ZX80 only needs 4K bytes.

So I do recommend you take the advice from Moggy. As you don't have a programmer, you will need to ask for someone to program (or "blow" as it's known) an EPROM for you.

It's easier if you can say whereabouts in the world you are, then someone that is hopefully not too far away may be able to help you.

Mark

[DoktorDoris]

wow. you have both been terribly helpful. i live in Leicestershire, in a fly speck of a village nr Melton.
i now have another problem. i just got 2 z80s from amazon, of all places. for a start i just want to see them do something, anything. to know they are okay. i have given them 5v to 11, ground to 29 and used my func gen to give a ttl to 6. and then scoped all the address pins but nothing was going on. then i read some more and pulled reset to ground upon powering up, then gave it V with a 1kOR. but still nothing on any address or anywhere else. are my chips knackered? am i doing something terribly wrong? i just want to see a z80 do something on my breadboard? sorry for being so full of questions.

and whilst i am happily rambling i bought a zx81 t'other day. 40 quid from ebay with the zx81 basic programming guide, the warranty postcard, and a Sinclair 16 K ram pack. i love to peer inside things so i soon took the ram pack apart (it was fairly manky. flux everywhere, and so many supporting ICS). i haven't powered the zx81 up yet. i lack a PSU, but i can use a bench supply for that, and i don't own an RF cable yet, I have a 1976 Binatone B and W portable, the kind mums used to have in the kitchen that i use with my ay 8 3500 pong chips, so the tv is sorted..
hmm, i am kind or waffling. sorry, it is weekend, so sauce night. what i meant to say is the zx81 looks perfect, and i haven't wanted to violate it. but i need to look inside. if i remove the 4 original screw covering pads to see inside can i re-attach them?

also i would like to say it is lovely being here. i had almost forgotten the internet could provide this sort of thing. talking with peers, and sharing knowledge.

Doris

[1024MAK]

i now have another problem. i just got 2 z80s from amazon, of all places. for a start i just want to see them do something, anything.

I suggest you build a NOP test circuit. The Z80 needs various control lines to be at the correct logic level and a suitable clock signal before it will reset correctly and then start to execute code. See this link for one example of how to build a NOP tester. NOP is the name of the machine code instruction that means, do nothing, but actually does more than that!

There is an error in the text on that page: "The 555 timer is connected via the label CLOCK to the RESET pin on the Z80" is incorrect. It should say "The 555 timer is connected via the label CLOCK to the CLOCK pin on the Z80". Which is pin 6.

I should point out that with the NMOS version of the Z80, the clock input needs to have a signal that gets fairly close to the supply voltages (+5V and 0V), it's not a TTL logic level input.

if i remove the 4 original screw covering pads to see inside can i re-attach them?

The ZX81 feet often fall off on their own! They may stick back on...
Otherwise use double sided sticky tape. Or buy new feet (such as these).

It's safe to open a ZX81 case and have a look inside. But I don't recommend disturbing the keyboard membrane tails, as they are likely to be brittle and fragile.

Oh, set your bench PSU to between 8V and 9V. Tip is +, ring is - (negative). Current consumption of a ZX81 on it's own should be less than 450mA.

Mark

[DoktorDoris]

thanks so much for your detailed reply.

the link you furnished me with is amazing. i love building something and going to my stash of little drawers and laying my hand upon a required thing.

sorry, another question. i have happened upon these PCB on ebay - https://www.ebay.co.uk/itm/284625146300 ... R7LZxJuKZQ

would this serve my needs?

-update-
now I am sober on the day after i can clearly see the ebay thing is the same. i followed the instructions on the linked page and made the tester on BB. one of my z80 happily lit the leds, and gobbled up 50 or so mA. the other does nothing, now i am not sure if i killed it last night, or if it arrived knackered. i suppose i will never know. but what i do know now is how to make a tester, and that i have one good CPU. so yet again thanks very much. i am off to make a more permanent tester on veroboard.

doris

[1024MAK]

Yes, those NOP tester PCBs work fine.

Here’s mine https://youtube.com/watch?v=ZMJ9ywbyzqI

I got the 40 pin ZIF socket from this seller or if you prefer, their eBay listing.

Mark

[Lardo Boffin]

Here is a good supplier of retro bits:

https://www.retroleum.co.uk/zx81-components

As are Dataserve retro (they have their own website as well):

https://www.ebay.co.uk/itm/126672704403 ... media=COPY

Or

https://www.sellmyretro.com/category/re ... clair-zx81

Saves making Jeff Bezos any richer…

[DoktorDoris]

i am chuffed to bits with that retroleum shop. ceramic resonators, and cheap tested z80s! thanks for drawing my attention to it.

doris