Attic 81's - My collection and attempts to repair

[Moggy]

Quote.
"I'd quite like to try one of the modded / rewritten ROMS I've been reading about. I'd idly assumed I could get some compatible EPROMS and burn some, but I think that's no longer possible in 2021. Can anyone recommend a way that I might easily switch ROM images or flash them into something?"

Really? I have no problem finding suitable EPROMS(ebay is a good source). I use 2764's which only require two wires attached and two pins bent out then program them using a ZX Spectrum powered burner and only last week finished converting all my Zeddies to the X2 ROM.

I also have Forth ROMS done in the same manner. I know there are probably easier solutions but I just like messing around with old kit.

[mrtinb]

The reason I use EEPROMs instead of EPROMs is that I don't have an EPROM-burner and don't have an EPROM eraser.

The EEPROM "burner" I have is just a simple shield for an Arduino UNO.

[1024MAK]

There are different ROM like chips / technology:

EPROM chips need to be programmed in a programmer. If you want to erase them, you need a UV light box operating with the correct UV light frequency/wavelength. EPROM chips are supplied in a ceramic package that has a window on top.

A cheaper version of EPROMs is still being manufactured, these are in fact normal EPROM chips, but in a plastic package with no window. So they cannot be erased. These are known as One Time Programmable (OTP).

EEPROM (or E²PROM) are electrically erasable. They don’t need a UV light to erase them. There are various types, some still need a +12V programming voltage, but others can be programmed at +5V. In a suitable system, they can even be programmed in the device they are fitted in.

Flash is similar to EEPROM but normally sections at a time have to be programmed in one go (the chip stores the data in a buffer). If you are using a programmer, this is not an issue, but it makes it harder for a device to program a flash memory in circuit.

Two common EEPROMs used by hobbyists are the 8K byte AT28C64 and the 32k byte AT28C256. Here’s a link to Farnell (they are also available from other suppliers, but be careful if buying from trading sites as fakes are common).

Mark

[sheddyian]

If it’s working, definitely don’t change TR2. And unless you think TR1 is actually faulty, again don’t change it. ZTX313 transistors have a fast switch off time and I have still not found an in-production good direct equivalent.

The top line bending over is not uncommon. It’s generally assumed to be caused by the ULA.

Noted. I've no reason to suspect either TR1 or TR2 are faulty, I just wondered if there might be some improvement to be had by upgrading them.

If you’ve seen any (all?) of my heatsinking ULA rants, err, discussions, you may want to say unless you really want more fun from me
Yes, the earlier ULA versions do run hot. It’s normal. If it’s not in a small case, I don’t see a problem with a heatsink being fitted.

I'd seen some strong opinions from both sides, which is why I was careful to note that the Zeddy won't have a case fitted Thanks for clarifying that the earlier ULAs run hotter, this was my concern as the other 81 with the later ULA gets warm but not uncomfortably hot like this one does.

I did post up how I did a Why Wait modification. I’ll try to find it.

That would be very useful, thank you. I've not got a drawer full of transistors to try, I'll be ordering anything I need, so a recommendation as to what actually works would be great.

The mod. to fit a 2764 or 27C64 (8 kbyte), or a 27128 or 27C128 (16k byte) EPROM is not very complex. Alternatively RWAP on Sell My Retro does sell this which is a plug in module that needs no wiring changes.

From reading (and perhaps misunderstanding) a LOT of Retro ZX81 stuff over the past few months, I'd assumed that compatible EPROMS weren't available any more, so thanks for this clarification. More on this in subsequent replies.

Ian

[sheddyian]

Quote.
"I'd quite like to try one of the modded / rewritten ROMS I've been reading about. I'd idly assumed I could get some compatible EPROMS and burn some, but I think that's no longer possible in 2021. Can anyone recommend a way that I might easily switch ROM images or flash them into something?"

Really? I have no problem finding suitable EPROMS(ebay is a good source). I use 2764's which only require two wires attached and two pins bent out then program them using a ZX Spectrum powered burner and only last week finished converting all my Zeddies to the X2 ROM.

I also have Forth ROMS done in the same manner. I know there are probably easier solutions but I just like messing around with old kit.

Thank you for this, that looks a very simple mod. I had misunderstood the availability of compatible EPROMS. Its been a good few years since I burnt an EPROM, and have never had the facilities to do it at home (used to do them at work for 486 motherboards). So I would be interested to know a simple / easy / cheap (AND/OR!) way to do this at home and what might be best to buy.

Thanks

Ian

[sheddyian]

The reason I use EEPROMs instead of EPROMs is that I don't have an EPROM-burner and don't have an EPROM eraser.

The EEPROM "burner" I have is just a simple shield for an Arduino UNO.

Ah. I'm familiar with the burning and UV erasing of EPROMS (from many years past) and also that EEPROMS are erased electrically, but it's around there that my knowledge ends.

Do you have any pointers to a newbie guide to EEPROM programming and erasing, and what works/doesn't work for a ZX81?

Thanks

Ian

[sheddyian]

There are different ROM like chips / technology:

EPROM chips need to be programmed in a programmer. If you want to erase them, you need a UV light box operating with the correct UV light frequency/wavelength. EPROM chips are supplied in a ceramic package that has a window on top.

A cheaper version of EPROMs is still being manufactured, these are in fact normal EPROM chips, but in a plastic package with no window. So they cannot be erased. These are known as One Time Programmable (OTP).

EEPROM (or E²PROM) are electrically erasable. They don’t need a UV light to erase them. There are various types, some still need a +12V programming voltage, but others can be programmed at +5V. In a suitable system, they can even be programmed in the device they are fitted in.

Flash is similar to EEPROM but normally sections at a time have to be programmed in one go (the chip stores the data in a buffer). If you are using a programmer, this is not an issue, but it makes it harder for a device to program a flash memory in circuit.

Two common EEPROMs used by hobbyists are the 8K byte AT28C64 and the 32k byte AT28C256. Here’s a link to Farnell (they are also available from other suppliers, but be careful if buying from trading sites as fakes are common).

Mark

Thank you for this explanation. I like the prices of the EEPROMS, with that and with what I understand so far, that may well be the route I'll go.

Where is a good place to read up on programming such devices, ie what hardware is needed, and what programs to do it with (I'm assuming the hardware will connect to a computer via USB?). Software can be WIndows or Linux. And also what is needed to make the AT28C64 or AT28C256 from your example compatible with the ROM socket on the ZX81?

Thanks

Ian

[Moggy]

Quote.
"I'd quite like to try one of the modded / rewritten ROMS I've been reading about. I'd idly assumed I could get some compatible EPROMS and burn some, but I think that's no longer possible in 2021. Can anyone recommend a way that I might easily switch ROM images or flash them into something?"

Really? I have no problem finding suitable EPROMS(ebay is a good source). I use 2764's which only require two wires attached and two pins bent out then program them using a ZX Spectrum powered burner and only last week finished converting all my Zeddies to the X2 ROM.

I also have Forth ROMS done in the same manner. I know there are probably easier solutions but I just like messing around with old kit.

Thank you for this, that looks a very simple mod. I had misunderstood the availability of compatible EPROMS. Its been a good few years since I burnt an EPROM, and have never had the facilities to do it at home (used to do them at work for 486 motherboards). So I would be interested to know a simple / easy / cheap (AND/OR!) way to do this at home and what might be best to buy.

Thanks

Ian

Well for the erasing side of things I bought one of those cheap and cheerful ebay jobbies from China about five years ago for a tenner and it's served me very well.

For the burner I acquired one that works with either the ZX81 or Spectrum but only have the Spectrum software. I hold on to this as it burns the 21/25 volt 2764,2532, 2732, 2716 etc devices I pick up here and there though in all honesty you if you only want to program for the ZX81 you would be better off with a cheap Chinese burner that will burn the later 27C64 chip as used on the early Zxpand interfaces which has a 12 volt programming voltage, as some modern programmers struggle with the 21 volt devices.

Having said all of this it has to be said that for £20 or so you can buy the chip all ready mentioned earlier in the thread, from Sell My Retro programmed with a ROM image of your choice all of which are readily available.

I only stick with the old stuff because I like soldering bits of wire to things plus the fun of reviving an EPROM and the anticipation of whether the old bugger will program or not! Not everyone is interested in going this route I know but I like it.

[1024MAK]

Why Wait - see this topic

Mark

[1024MAK]

Any of the cheap Chinese programmers will program (blow) a 27C64, or a 27C128 EPROM. But note that they are unlikely to be able to program the older 2764 or 27128 EPROMs, as these normally require a 21V programming supply. All of these EPROMs are obsolete.

OTP 27C256 EPROMs are still available from the major electronic supplier companies and are around £1.62 + VAT (plus the small order charge / postage) (link to an example).

The programmer that I recommend is theTL866II Plus, but the prices are not as cheap as they once were. Here’s an example. Note if you are only going to be using DIL / DIP parts, you don’t need any of the adapters.

These can also program 28C64 or 28C256 EEPROM chips.

I’m going to blow a chip with the latest version of the BASIC ROM later this month. When I do, I will document it in a new topic. I’ve not decided which chip to use yet, other than it will be a 32k byte type (so either a OTP 27C256 or a 28C256).

Mark