• SMS Paddles on Sinclair ZX computers

    07/23/2019 at 04:50 0 comments

    Sinclai ZX computers (ZX81/Spectrum) don't ship with a joystick port but they can count on the ubiquitous Kempston interface to provide 4 directional lines plus 3 buttons and that is more than enough to connect a Sega Master System HPD-200 paddle controller.

    Paddle detection and reading code provided by SMS POWER. I/O address changed to match Kempston (0x1F), though.

    A simple hardware adapter is necessary in between the Kempston interface and the HPD-200 paddle

    An alternative for the HPD-200 is to build a DIY SMS/MarkIII paddle controller by Raphaël Assénat. In such case the wiring can be modified to route the  signals from  pins 5,7 and 9 thus dispensing the use of the adapter.

    Example code (work still in progress) available at my github repository.

  • CP297s

    07/02/2019 at 00:37 0 comments

    PCB for ZX97 lite on a CP200s case. Kicad files available at github

  • Backporch for ZX97 lite

    06/21/2019 at 23:24 0 comments

    I believe it is possible to add a backporch for ZX97 (original) using three available gates from U25.

    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)