mirror of
https://github.com/mahjongg2/ZX81plus38.git
synced 2026-02-13 19:35:48 +00:00
25 lines
2.7 KiB
Markdown
25 lines
2.7 KiB
Markdown
# ZX81plus38
|
|
Mij latest ZX81 clone without ULA built from discrete pin through hole components
|
|
It features a PCB with the exact same size as a normal ZX81, (142 x 100mm) and features drill holes and connectors at the same locations (with the 3.5mm power jack replaced
|
|
with a modern Type B power jack, so you can power this ZX81 clone with a simple USB power supply, and with the RF output modulator replaced with a composite video
|
|
output connector, that produces a video signal that modern composire monitors should be able to accept (yes with a real front porch signal).
|
|
There is no ULA needed for this clone, as all the ULA logic is replicated with modern DIP compatible logic IC's. The software (ZX-81 BASIC) fits into a cheap OTP
|
|
(one time programmable) 27C256 ROM, and the bit-for bit same code as a real ZX-81 code can be programmed into a 27C256 OTP ROM, and will work exactly the same as a real ZX-81. but with a few jumper changes you can alos use an original 8K ZX81 ROM. The use of a 4K ZX80 ROM is also supported.
|
|
Its also possible to fit several (different) code images into the 32K ROM, and it is possible to add switches to select one image to boot with.
|
|
Extra logic was added to simulate the same cassette loading screen patterns as a real ZX81 has.
|
|
The cassette loading input logic has been eenhanced to give perfect cassette loading behaviour, even whith cassette signals generated with a laptop, for easy cassette loading.
|
|
The ULA replacement logic is perfect enough to enable the use of games which use various enhanced highres graphics techniques to work flawlessly.
|
|
In fact the ZX81+38 clone is easy to build, and a joy to use.
|
|
|
|
Warning: The revision 1.4 version of my ZX-81+38 contains some errors, and will NOT work as is. The main reason being that the WAIT generator flip/flop idea doesn't work, and must be replaced with a working wait circuit. As revision 1.4 is not working, and has now been replaced by revision 1,5, I have deleted revision 1,4
|
|
|
|
ZX81+38rev1,5.zip contains all gerber and drill files for the working updated and layout checked files to produce a revision 1,5 version PCB.
|
|
|
|
The file ZX81+38rev1,5.pdf is the schematic of the corrected revision 1.5 (in black and white)
|
|
|
|
We found one more error, that I have fixed in revision 1,6:
|
|
the keyboard matrix is connected to eigh Address lines, A8 to A15, but the error is that for A8 a version is used that comes from the signal line passing through a 10K resistor (R1) so it is signal A8' , that is not correct, the keyboard needs unattenuated signals, so A8, not A8'. otherwise the resulting dataline does not go low enough to reliably register as "low".
|
|
|
|
|
|
And I will upload kiCad files after I know revision 1.6 is working well.
|