Sinclair ZX80 / ZX81 / Z88 Forums

The relevant time delay is "address stable AND chip selected" until "output data read". AFAIK address lines are stable roughly at rising edge of Z80 T1 cycle, and RAM chips are selected with /MREQ going low (=shortly after falling edge T1). Grant's scope investigations showed that the character code...

As for original question: true, it's really that simple. That resistor hangs on an ULA pin which has a (weaker) internal pull-up. With resistor missing (PAL / 50 Hz), the ULA pin goes to logic high (1), with resistor present (NTSC / 60 Hz) the ULA pin is pulled to logic low (0). The resistor (R30) i...

Very impressive feat indeed! Knew there were many similarities between ZX81 and Jupiter ACE, didn't know they were similar enough to allow squeezing the logic side-by-side into a moderate size CPLD. Although I suspect much of the logic will be shared. And the ability to switch machine type without r...

Time for an update... I've been testing loads, loads, loads of games, demo's and utilities on my ZX81 with CPLD-based ULA. This test-ZX81 has internal 16K (S)RAM expansion, which makes true Hi-Res software work (WRX scheme, basically: RAM outputs data during refresh - should be the same as external ...

It is simple: in UV erasable EPROMs (also one-time programmable versions, which use same internal technology), an 'empty' memory bit reads as a 1. And you can program a 0 into each bit. This is done by using the programming voltage (25/21/12~12.5V) to pull a bunch of electrons onto an isolated 'isla...

Hehe...

http://hardware.slashdot.org/story/12/0 ... 1-lives-on

Oh that poor ZX81 Museum server...

One of my zx81's has a UV EPROM chip instead of a mask-programmed ROM - this looks original, as it has "ZX81" actually printed on it next to the type number (2564) plus several pins bent out and soldered to wires going to various pcb pads...???? Anyone else seen one of these?? I read in Wikipedia a...

Issue 1 boards are with the curvy, hand-drawn traces. I have one which has green soldermask, not sure if other colors soldermask were used for this issue board. Issue 2 boards are nowhere to be found in the wild, so this would be either non-existent or extremely rare prototype / pre-production versi...

Nice! I love the 'fits in a rampack' factor ;) Considered that for Flash-based mass storage - I wouldn't feel sorry about butchering an original ZX81 RAMpack to bring a ZX81 into the 21st century (well perhaps with RAM pack wobble all over again :lol: ). Don't have much use for those RAMpacks anywa...

Hi guys! After my TV broke down it took a while to get replacement (1st candidate broke down within hours :shock: didn't even get to hook up my ZX81). And have been busy with other things. Anyway picked up development a few days ago. First that Rezurrection demo - couldn't get it to work properly, b...