07/02/2019 at 00:37 •
PCB for ZX97 lite on a CP200s case. Kicad files available at github
06/21/2019 at 23:24 •
D11 discharges the capacitor at each negative horizontal pulse coming from /CSYNC and causes pin 11 of IC25 to go LOW, which blanks the video output signal at U25C.
After the sync pulse leaves R4 start to charge the capacitor. While the voltage is below positive threshold of U25 inputs the video stays blank, thus generating the backporch level. After that the output of U25D goes HIGH and the video content coming from U5B pin 8 can reach the output.
It might be necessary to fiddle with the value of R24 according with the technology of U25 (LS, HC, HCT, etc)
The resistors R1,R2,R3 were calculated to provide full compatibility with RS170: 1Vpp @ 75 Ohms load, 70% Video, 30% sync, DC coupling at output (the latter is a de facto standard).
This circuit have not been tested yet on the ZX97 but works like a charm on TK85 (a ZX81 clone)
06/13/2019 at 01:24 •
One of the users at msx.org tech forums asked if was possible to use Atari 7800 joypads in MSX.
The 7800 controllers are very similar to 2600 and therefore to MSX. The main difference is that to maintain compatibility with 2600 joysticks whilst providing two buttons, the 7800 used active high buttons on pins 5 and 9 which are used in 2600 for paddle inputs.
The straightforward solution is to invert the signals and that can be accomplished by a pair of NPN transistors.
Controller and adapter schematics, side by side, for better understanding.