1024MAK wrote:RetroTechie's web site (http://bitcycle.org/) has been off line for a little while now, but I don't know anything more.
Probably automatic payment for a hosting-related bill went wrong. But so far I haven't bothered to find out what needs doing as I've been busy with more important things. It is/was a pretty low-traffic site, and as said the Internet Archive has most of it for those who care.
The Internet Archive does have a lot of his site here but I don't think it has the zip file to download
There are some time since last post, and my question is how mature is this schematic ?
Is it safe to construct this circuit to replace ULA ? Have someone try to assemble it ?
thank !!!
whilst this TTL version that i built many years ago worked it was not without some small problems, hsync timing not perfect, no back porch, it was built using parts i already had rather than " ideal" parts.
you would be much better off ( if its a schematic your after ) looking at Mark's great work found here. download/file.php?id=1246
RetroTechie wrote: ↑Mon Apr 15, 2013 1:05 am
Why didn't I try this before...
Let's hope the file size isn't an issue for maintainer(s?) of this site - it shouldn't be I think, as this is low-traffic material.
ZX81_ULA-in-a-CPLD_v1.10.zip
Rink wrote:I wonder what the utilisation of the CPLD is like? I'd be tempted to see if I can put the design in an Altera CPLD since I've been playing with those lately.
My design uses 59 macrocells from a 72 macrocell CPLD (Xilinx XC9572). So with a bit of luck it might fit in a 64 macrocell Altera CPLD. But that would be a tight fit. Unfortunately 128 macrocell Altera devices I know off, come in 100 pin QFP packages. Which is just a little too big to fit onto a 40p-DIP footprint (although Andy somehow managed that anyway ).
ZX81 ULA logic isn't very complex - a few counters, a few flipflops, an 8 bit shift register, and some logic tying things together. Understanding how everything works together in a ZX81 is the hard part IMHO. A 64 macrocell will probably do if all you need is a 'vanilla' ZX81 ULA replacement. If you want to add extra's like clock doubling/tripling, a blockswitching mechanism for the RAM etc, then better go for a 128 macrocell device.
You'll also need 5V tolerant inputs, and personally I have a strong preference for not starting new designs using parts that are considered obsolete. So for this application I'd be looking at Xilinx 9500XL (XC9572XL in VQ64 package would be a nice choice), or Altera MAX 3000A family.
also please not that Paul is currently working on a cpld based plug in replacment.
regards
what's that Smell.... smells like fresh flux and solder fumes...
Standby alert 
).