Henner, I have taken both approaches. I have a few comments: 1) RAMRST serves to protect RAM during power events. So, I think you can for sure use it to disable the internal ram, but then you need to take some precautions on whatever RAM you add. I don't think it's purpose was to disable ram for any other reason than protection. RAMRST is switched using a number of different conditions, including auto-power down. I don't have a good recommendation on such a protection ckt.
2) A* can be pulled high. This has the effect of killing the chip selects to the ram modules. This is more "painful" but it can be done. You could for example add a switch and control which RAM you want to use. I would recommend using a RAM with 2 chip selects, and then using RAMRST to protect the secondary ram. I would recommend manipulating A* at the system bus. Steve On Sun, Aug 21, 2022 at 4:56 PM Henner Zeller <[email protected]> wrote: > On Sat, 20 Aug 2022 at 14:44, Henner Zeller <[email protected]> wrote: > >> Hi, >> I enjoy playing with my new/old T100 and want to start doing some >> hardware hacking with its system bus. >> >> According to the docs, REXCPM can disable >> <https://bitchin100.com/wiki/index.php?title=REXCPM#Overview> the >> built-in RAM and shadow the internal ram with its own banked RAM. Since I >> consider doing some RAM banking without pulling out the original RAM or >> sneaking in an extra wire or resistor and wondering how that can be >> achieved. >> >> It would mean that we need to switch off the internal RAM. But looking at >> the schematic <https://github.com/hzeller/trs80-100-schematic>, it looks >> like options would be to >> >> * Either make sure that none of the chip enable signals for the RAM >> ever go to zero, but they are under control of M3 and M4 and this looks >> infeasible >> * We could force pull the RAMRST signal to high, but I suppose that >> would make transistor T9 not very happy. So also not feasible >> * The option that I missed :) >> > > To answer my own question: pulling RAMRST is indeed what needs to be done > and it is intended for that purpose. > The service manual states it on page 4-13. > > Measuring, it turns out that pulling up RAMRST to VDD only takes about > 1.2mA, so the transistor T9 will not have trouble with it. > > -h > >>
