ZX81 ROM ?
Denis M
I'm re-studying 8bit technology - using ZX80,81 etc as a basis for
this......... Going to build Grant Searle's home built ZX80... :)
(second time around for me, I got my first ZX80 in 1980 - still have it
........)
I've come across a ZX81 (issue 3) with a 28 pin EPROM, obviously fitted as
an after thought. The extra pins connected with wirewrap.
The chip has markings;
64K ZX81
JAPAN
(TexasInstrumentsLogo) RT18248
Any idea what this might be ?
While Sinclair provided for 28 pin ROMS (or eproms. maybe) , I didn't think
they ever issued any.
The ZX81 is not working - due to a dud ULA - I may swap in one from another
ZX81 , just to see.
( dud ULA = The ULA is being clocked by the crystal oscillator, but no clock
signal is being issed by it at pin 14 )
Chris Johnson
<dma...@iol.ie>
>Any idea what this might be ?
>While Sinclair provided for 28 pin ROMS (or eproms. maybe) , I didn't
>think they ever issued any.
If you wanted to replace the ZX81 ROM with a pin compatible EPROM you
used a 2532 (the 2732 had a different pin configuration). When we were
developing alternative ROM's for ZX81 we sometimes used a 2764 and
kludged the wiring. Your example might have been something similar.
I still have an impressive collection of ZX81's with cut-away cases and
EPROM carriers plus ZIF sockets towering above them!
Regards
Chris
--
Chris Johnson mobile:+44 (0)7785 302122
EDIMatrix Ltd work: 0845 126 0680 or +44 20 8501 3097
fax: +44 (0)20 8501 3096
EDI website http://www.edimatrix.co.uk
Kryten
> If you wanted to replace the ZX81 ROM with a pin compatible EPROM you
> used a 2532 (the 2732 had a different pin configuration).
Erm, that might work for the ZX80's 4K masked ROM (2332),
but the ZX81 used the 8K masked ROM (2364).
I guess you cold use two stacked 2532 chips with separate CS lines.
There may have been 2564 chips but if so they were not often seen.
Distributors sold 27xx EPROMs mostly,
and the odd 2532.
Peter de Vroomen
I have a few of those, they do exist :). I needed them for my ZX81, and
could buy 4 through EBay. But when I got them, it turned out my eprom
programmer couldn't program them :+). It could program 2532's (which are
very common), so I assumed it could also program 2564's. But nope. So I
ended up kludging a 2764->2564 adapter with two sockets and some wire. it
didn't really matter anyway, because it fit perfectly in the case. I didn't
even need low-profile sockets.
PeterV
Wilf Rigter
burner that will program them. Pin for pin compatibility makes them ideal
for replacing a bad ZX81 ROM but with a little more effort you can install a
lower power 28 pin 27C512 EPROM programmed with a bunch of DIP switch
selectable ZX81 operating systems.
Or if you rather turn on than switch, glue a 74HC173 Quad latch on top of
the EPROM and you can use the ZX81 keyboard on power up to select 1 of 7
operating systems you wish to boot from.
Mmmmmm..... multiple operating systems
"Peter de Vroomen" <peterv[at][spamblock]jaytown[dot]com> wrote in message
news:42dfd531$0$90381$e4fe...@news.xs4all.nl...
Chris Johnson
Kryten <kryten_droid...@ntlworld.com>
>Erm, that might work for the ZX80's 4K masked ROM (2332), but the ZX81
>used the 8K masked ROM (2364).
True. The replacement EPROM was for the 4k Aszmic assembler, and was
originally developed for ZX80. I can remember coming over to the UK to
demonstrate it at Sinclairs, who promptly plugged it in to the very
newly-released ZX81, where it became apparent that the keyboard mappings
were not quite the same as for a ZX80. Very embarrassing.
Denis M
I get the impression I'm in the presence of those who know ...........from
way back.:)
I replaced the ULA with a good one.
The zx81 now boots OK to the [K] cursor.
Seems like the eprom is a simple 8K replacement for the original ZX81 ROM -
maybe the original became defective.
Tho' I've heard it said a ROM would need to be put in a vice to damage it :)
I'm still going to attempt to construct an original ZX80 as per Grant
Searle's design, and all of the investigation of various dud ZX81
is helping me to study the area.
This is 8 bit second time round for me . 1979 and 1980 were exciting times.
Damn LSI, surface mount etc for virtually killing electronics as a hobby.
Wilf's Zx look very tempting .........
> I can remember coming over to the UK to demonstrate it at Sinclairs, who
> promptly plugged it in to the very newly-released ZX81, where it became
> apparent that the keyboard mappings were not quite the same as for a ZX80.
Chris,
does the quote above mean you were actually involved in the
manufacture/development process of the ZX 81 ?
Wow, I'm impressed :)
It almost embarasses me , in this company to admit I specialised in 6502
baxk then ....
Regards
Denis .
Chris Johnson
<dma...@iol.ie>
>does the quote above mean you were actually involved in the
>manufacture/development process of the ZX 81 ?
>Wow, I'm impressed :)
No. I lived abroad then and did work for the Swedish distributor for
Sinclair products; initially because the home computers needed to be
customised slightly for the different character set. Having painfully
worked out how a ZX80 operated (which included disassembling the ROM) I
just went on to make other products for ZX80/1; the ASZMIC ROM (sold
about 3000 of those), the Prestel Adapter (sold about 100 - technically
very clever, but a commercial disaster) and the SP ROM (an extended
Basic with hi-res graphics, windows and sprites, which unfortunately
relied upon an external mounting card the main component of which hit a
world shortage - another clever idea which was thus never able to be
marketed very hard)
>
>It almost embarasses me , in this company to admit I specialised in
>6502 baxk then ....
Nothing to be ashamed of. The 6800/6502 were programmers' CPU's (they
had more sophisticated addressing modes), the 8080/Z80 were better
suited for engineering applications (they were more adept at
bit-diddling).
Kryten
"Chris Johnson" <chr...@edimatrix.demon.co.uk> wrote in message
news:fHhpWkVj...@edimatrix.demon.co.uk...>>It almost embarrasses me, in this company to admit I specialised in 6502
>>back then ....
>
> Nothing to be ashamed of. The 6800/6502 were programmers' CPU's (they had
> more sophisticated addressing modes), the 8080/Z80 were better suited for
> engineering applications (they were more adept at bit-diddling).
Agreed.
I think I'm fonder of the 6502 because of its regular memory access cycles
and the fact it had about the same horsepower as a Z80 for about half the
silicon area.
I can't recall seeing the 6502 on sale in hobbyist stores for a long time,
but
you can still buy it commercially from WDC or small numbers from
www.6502.org.
Partly because of this, it has invisibly crept into many far-eastern
microcontroller cores for rice-cooker/microwave applications.
Likewise, the Z80 has evolved into the Rabbit core.