New memory boards for the P2

  • As a D flip flop has tow outputs and the second one is the first complimentary, the addition of an or gate allows to address the lower, unbanked 16k.


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    So if a '138 is fed with "-SysRegion" select signal...



    Four controlled 2k regions can be selected independently: three ROM select lines and a RAM select line. Later I'll deal of how this is tied against SRAM and ROM, but the following is the goal to achieve.



    There's overlap on the MOS memory and also on the ROM region. That's more or less what's wanted: overlap must be controlled, mainly by switches. Most people (including me) doesn't have material to read a TMS27xx, even less writing it. By using an standard 27c64 or 2864 (I think it was this reference) and three select lines, ROM can be selected in 2k chunks, allowing both partial (if > 0 but < 3 system roms failing) and full ROM replacement.

    When I tried to list all retro systems I have at home, the "The message is too long, must be under 500 characters" error appears! :lol:


  • I've tied the previously generated control lines. The EPROM is also included in the diagram. All overlapping on the 16k is controlled. At this point the memory mode is not selectable and stuck at 64K mode. Once redesigned the switch logic, I'll start optimizing the diagram, placing references (Ux) at the ICs, specifying power supply pins and this kind of stuff. Still, counting gates and other components, this board should be buildable with 11 ICs.


    I have to put this project on hold again: I have an exam in 19/06 so active development will temporally cease (but I'll follow this and other threads) until later in that same day or until the following one.


    That being said, I'd like to know if you (especially who are experienced with electronics) can find points where this design may fail. After all, It's my first project and, while I've soldered a lot of kits, I've never created a board from scratch.


    Thank you very much!

    When I tried to list all retro systems I have at home, the "The message is too long, must be under 500 characters" error appears! :lol: