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 >
