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
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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...