Sinclair ZX80 / ZX81 / Z88 Forums

No wonder I had so much trouble pinning down the exact moment to grab the data - this would be so much easier if original ULA is gone & you can determine yourself what happens on the bus, and when. Also these new scope pix seem to suggest that /RAMCS indeed uses /MREQ signal. Big thanks for digging ...

If /RAMCS uses /MREQ (it does in my current setup, and I'd think most ZX81 clones do it like that), access time wouldn't start until shortly after falling edge T1. Rising edge T2 would then be around 90~100 ns in (?) which might be fine for a modern SRAM (half-used 32Kx8, 120 ns SRAM here) but too s...

Yeah I know - this is one part which would have been easier without original ULA running side-by-side. Point is I'm inputting 6.5 MHz into that ULA, but can't say for sure what ULA does with it internally until it comes out as /CPUCLK - read: the clock signals I have available inside the CPLD aren't...

Progress! I've managed to replace ULA's A3' output. :D I've been looking a lot at the scope picture on this page by Grant , but vexed by the question of when exactly ZX 81 ULA grabs character code from the databus. You see, in the ZX80 RAM sits on the other side of resistors that separate databus si...

Veeeerrrrrryyy nice! :D Didn't know about this page. As for the project, I have some remarks: While I understand the reason for using loose turned pins, I doubt it's necessary. If you have a turned pin socket & push it into circuit board, part of the pins stays above the board & you can solder onto ...

Very nice work Andy! 8-) But I have to wonder: why such a big device (package-wise), couldn't find anything smaller to do the job? I'd be thinking about something like Xilinx XC9572XL in 64p TFQP package, the Altera MAX in the pix looks a bit too much (for my taste, at least) in terms of package siz...

You think your CPLD can do the job with the 180 Ohm resistor for long time ? Ehm... YES. I have some experience with this CPLD family by now, read the datasheets, application notes (which basically say the thing to watch is overall device heat dissipation), and I've done the math. This is high spee...

I'm currently using a ZX Spectrum dk'tronics keyboard. Schematics show that ZX81 and ZX Spectrum keyboard matrix is the same. Unfortunately the molex connectors are wired a bit different, so just plugging it in mixes up the keys. Unsoldered the connectors on the board, made a simple wiring adapter &...

Bill H wrote:Call me stupid (or maybe naive), but what exactly is an NPN transistor?

All you need to know is that it's cheap.

Btw original source here: http://www.zx81.de/english/video_e.htm

Yes, welcome Mark - the more souls the better! :D I am mostly interested in hardware topics but also I am also interested in continuing my education of getting better at assembly language. Good to know. For hardware-only ppl it's mostly about repairing things, building things that others designed et...