Would the 1K and 2K not be able to be done with a ROM change and maybe holding the 1 or 2 key down at power on?
ZX21 Project ZX81 Replacement PCB
Re: ZX21 Project ZX81 Replacement PCB
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
To limit RAMTOP isn't sufficient. You need the correct RAM-Shadows as well. Which means you have to disconnect the unused RAM addresslines and use pull-ups or pulldowns instead.
In theory, there is no difference between theory and practice. But, in practice, there is.
Re: ZX21 Project ZX81 Replacement PCB
The best idea I can come up with at the moment is an adapter plugged into the RAM socket to provide the 1K, 2K functions. It is early days however and I may come up with a better plan for a future PCB issue!
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
A looooong way to go.
Constructive feedback always welcome!
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
A6SC1008 works great with TTL, don't hestitate to use it. It also supports 3.3V as 5V supply depending on your hardware. This chip is used in several memory attachments, at least in my ZXmore or ZXblast. And yes, little extra logic is necessary to get it to 56k RAM. 32k RAM is often used but run out of memory fast when using different modules/drivers, HRG software and so on.fordp2002 wrote: ↑Wed Nov 25, 2020 9:56 pm The AS 6C1008-55PCN is CMOS only so not great for a TTL Computer. You can use the 512K Byte big brother however and I do have some of those.
Is there much software that needs more than 32K?
You would, of course, need to add additional logic to get more than 16K working.
628128 would be also fine but source more current, AS6C1008 is a direct replacement. Just in case.
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Re: ZX21 Project ZX81 Replacement PCB
Yes, that datasheet for AS 6C1008 contradicts itself. It states it is compatible with "all" in one part, while only claiming CMOS levels in others.
My PCB will take that chip, however.
I have added the 1K, 16K switchable feature as an option I worked out how to do it. You need to add an additional IC for it.
There comes a time when you cannot support every request, however.
Let's see if I can finish routing it!
My PCB will take that chip, however.
I have added the 1K, 16K switchable feature as an option I worked out how to do it. You need to add an additional IC for it.
There comes a time when you cannot support every request, however.
Let's see if I can finish routing it!
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
No mounting holes as original PCB?
Re: ZX21 Project ZX81 Replacement PCB
I deleted the mounting holes by mistake after a reimport of the circuit diagram. Luckily I did not delete the construction lines on the user layer so it only took a few seconds to put them back in.
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
I have put that version on Github!
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
The ZX81 is a hard board to route (for me anyway) but I am getting there with less than 100 connections to go and I think the hardest bits are done.
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!