Are TS1000 <-> ZX81 parts interchangeable?

[1024MAK]

Cool! So I was going to ask how the ROM would know about how much addressable space there is if I put in a 2k chip in place of a 1k but, duh, it must figure it out somehow whenever a ram pack is attached on the back...

The ROM fills every location of the 16k address space allocated to RAM with a number. It then counts down from the highest of these addresses and changes the number stored as it goes. This enables it to work out how much useable RAM there is. But this is not a full test, so if the RAM chip is only partially working, the machine can be fooled, it will then crash.

Mark

[Dukes909]

Also, forgot to say all chips on my issue one are socketed.

[1024MAK]

Ok well I don’t have one yet but the TS1000 seem to go for less than the ZX81’s. I may buy one now knowing I can do this (unless there is another source for replacement components???)

Direct like for like components, or rather, the chips is a bit tricky. New old stock/secondhand/pulled/used NMOS Z80A microprocessors are available. Brand new CMOS Z80s are still being manufactured and work fine.

Most of the various SRAM chips should also be available, again secondhand/pulled/used. Or brand new 32k byte CMOS SRAM chips can be fitted, but do need some wiring modifications.

Original ROM chips are tricky to find, but can be replaced with EPROM and EEPROM chips, but need a minor bit of wiring modification to get to work (can’t be too hard, a cat can do it). Although the required EPROM chips are also obsolete, they are available secondhand as well. The EEPROM chips are still being manufactured.

Replacement original ULA chips are now fairly rare. It’s been some years since the last box of new old stock appeared. But a VERY limited supply of ULA replacement modules is available.

So, yes, it is possible to bring a ZX81/TS1000 back from the dead.

Mark

[1024MAK]

I should point out that most users wire up a 32k SRAM chip to only operate as a 16k byte memory. Because the modification to use it as a 32k byte memory is a bit more complex. And not much original software actually needs more than 16k bytes.

Mark

[Moggy]

QUOTE...

"Can't be too hard a cat can do it"



Cheeky young sod!!

[Dukes909]

Ok well I don’t have one yet but the TS1000 seem to go for less than the ZX81’s. I may buy one now knowing I can do this (unless there is another source for replacement components???)

Direct like for like components, or rather, the chips is a bit tricky. New old stock/secondhand/pulled/used NMOS Z80A microprocessors are available. Brand new CMOS Z80s are still being manufactured and work fine.

Most of the various SRAM chips should also be available, again secondhand/pulled/used. Or brand new 32k byte CMOS SRAM chips can be fitted, but do need some wiring modifications.

Original ROM chips are tricky to find, but can be replaced with EPROM and EEPROM chips, but need a minor bit of wiring modification to get to work (can’t be too hard, a cat can do it). Although the required EPROM chips are also obsolete, they are available secondhand as well. The EEPROM chips are still being manufactured.

Replacement original ULA chips are now fairly rare. It’s been some years since the last box of new old stock appeared. But a VERY limited supply of ULA replacement modules is available.

So, yes, it is possible to bring a ZX81/TS1000 back from the dead.

Mark

Should I be looking at eBay or are there specialty places for secondhand/pulled/used chips?

[1024MAK]

There are some. Being in Europe, I only know of Europe based businesses. For example www.retroleum.co.uk/, www.bytedelight.com, www.mutant-caterpillar.co.uk, www.dataserve-retro.co.uk and www.sellmyretro.com/. There are more, but you get the idea.

There is at least one company in the USA that sells obsolete parts, but their name escapes me at the moment.

The other main place is eBay. But, buying chips from China based sellers is a bit of a gamble. As over there, there is a trade in selling parts that have been remarked. So for example, if you search for a CMOS Z80, there is a chance that you will get a chip marked with a Zilog CMOS part number for a CMOS Z80, but the chip is actually a NMOS Z80 and may not be even have been made by Zilog. Also the speed rating of it is likely to be different to the marking. A ZX81 will be happy with either a NMOS or CMOS Z80, from any manufacturer, as long as it’s a 4MHz or faster version.

The SRAM chips are less of a problem, in that they are less valuable, so you are less likely to get a remarked one. But if your machine does not work with a RAM pack, it’s less likely that the problem is the RAM chip in your machine. In fact, you can prove this, with the power off, remove the RAM chip from your board. Connect the 16k byte RAM pack, and try it.

One of the companies/traders above has a spare ROM chip listed, but you would have to ask the others.

If you do try to buy from eBay, only type in the main part of the part number, not the full part number. As different manufacturers use different long part numbers a lot of which are a superset of the common shorter part number. But there are also equivalent parts with completely different part numbers. For example, 6116 SRAM chips also work. But also watch out with searching, Zilog used ‘Z80’ as a prefix for various different chips which although part of the Z80 family, are not CPUs. And some manufacturers used a completely different number, NEC used µPD780C (often shortened to D780C) on their Z80 CPU chips. The speed rating is important, so with NEC chips, you would want a D780C-1 for example.

If in doubt, ask on here before buying.

I have some spare RAM (definitely some 1k byte chips, I may have some 2k byte chips) and some spare NMOS Z80 chips, but shipping/postage costs to America are expensive, and BREXIT has made things more complex. so it may be easier, cheaper and faster to buy within your own country.

Mark

[Moggy]

Another thing to look for if going the used EPROM route.

Whilst I have been blessed insofar as the chips purchased all worked once erased(not bad for a stupid cat), there has been odd occasions when I have seen listings for 2764 21volt EPROMs only but had 12volt 27c64's mixed in with them, which is ok providing you have a programmer that can handle both programming voltages.