Can I Visually ID the amount of ram onboard my zx80?

[AndreSan]

I am unable to power up and test a zx80 I recently came upon. As I sort that out, I wonder if there is any way to ID the ram chips from their markings? This may be googleable, but I don't know where to start a query.

Also, is there any way to visually ID whether this is the US version? My dad traveled to Britain for work in that time. He may well have bought one there and brought it back to the US.

Finally, what is the meaning of the black keyboard overlay (lifted in the photo)?

I want to make this available at a fair price, which means knowing more about what I've got. I found it in my father's stuff after he passed away, so I have no one to ask on this end.

Thank you!

[Moggy]

The labels on the bottom case denote UK/US versions ( barring the swapping of the motherboard or tinkering, mentioned just for pedantic clarity ).
If yours has the black overlay then it looks as if it is a ZX80 to ZX81 conversion, a clear picture of the motherboard would be helpful. The memory chips are two 2114 SRAM chips making 1K of static RAM. The modulator for the TV output is also different on the US version.

The inside of the cases will be sprayed silver for the US version, done as a crude screening measure in order to appease FCC radio emission regulations, the UK cases are plain white internally.

The info found on this site goes into more detail and will make things clearer..

http://blog.tynemouthsoftware.co.uk/201 ... works.html

And the same site with information about the US ZX80.

http://blog.tynemouthsoftware.co.uk/202 ... 0-usa.html

[1024MAK]

SRAM chips on a ZX80

On UK machines, there are two 2114 SRAM chips. These were made by many companies, and in various versions (speed ratings and normal/low power versions) hence the prefixes and suffixes will vary somewhat.

Each 2114 is a 1K chip, but each only handles 4 bits. Hence for 1024 bytes of RAM you need two chips.

2114 SRAM chip

(please ignore me having one in a ZIF (zero insertion force) socket, I use this for testing 2114 SRAM chips).

Mark

[AndreSan]

Do'h - I read the bottom label so carefully that I missed the too obvious "USA" I also have silver spray paint on the cover.

Thanks for the info on the RAM - I misunderstood what I though were options for onboard RAM with what were actually options for the first rampack.

It took me a while to read through the linked articles. Thank you. I may not have delved through enough pages in my searches to find Tynemouth.

I am going to restate my new understanding of the black keyboard zx80 > zx81 conversion based on my read of the Tynemouth blog: Buyers of a zx80 could purchase an 8k ROM chip and replace the 4K chip that was standard in the zx80. It looks like the chip fits into a socket, so no soldering was necessary. Because the 8k ROM had an expanded version of BASIC, a new keyboard was required for correct mapping and the original keyboard was obsolete. I have ID'd that the ROM chip is in fact an 8K chip (Mostek MK36809N), and I have the tell tale sign of a broken lower left corner of the case.
OTOH, it seems like zx80 > zx81 conversion is a bit of a misnomer since the 81 also benefitted from some consolidation of all the TTLs and maybe a few other minor upgrades.

Does that sum it up pretty well?

Thank you!

[1024MAK]

RAM packs for the ZX80 and ZX81 are able to be used on either machine. There are two types: those using static RAM (SRAM) chips and those using dynamic RAM (DRAM).

The type that uses SRAM could have 1K to 3K of extra RAM (in addition to the internal 1K of RAM).
The type that uses DRAM is only available as a 16K unit. If used, the internal RAM in the computer is not used. Although third parties produced larger expansion RAM for the ZX81.

Sinclair offered an upgrade for the ZX80 in the form of a ‘new’ 8K ROM and a new keyboard overlay (because the key words are mapped differently on the keys). The ROM was I think described as 8K BASIC, but it’s exactly the same as the BASIC in the first ZX81 machines.

It does not upgrade the ZX80 to be the same as the ZX81, because the ZX80 lacks the extra circuitry (included in the ULA chip in the ZX81) that allows compute and display mode (also called SLOW mode). The 8K BASIC ROM when fitted to a ZX80 therefore always runs in ‘FAST’ mode complete with the flicker or when running a program, no display.

And yes, the ROM socket can take either the 4K ZX80 ROM or the 8K ZX81 ROM.

The ZX81 chip count is ONLY four or five depending on which SRAM chip(s) are fitted (Z80A, ULA, ROM, RAM).

Mark

[AndreSan]

Thank you!