On Sun, 6/26/16, David Riley <fraveydank@gmail.com> wrote:

Agreed on all points. I'd be highly interested in working on that, if only I had more time... a drum emulator should be extremely simple to make in principle, the interface will probably be the hardest part.

I've done a little thinking about that in a different context, and I think the best way to conceptualize the interface is to think in terms of two separate power domains with the interfacing being done with optoisolators. The timing aspects of the interface won't be as difficult as one might guess. The main clocking for the machine comes from the drum, so the whole machine is slaved to the drum for timing. As a result, we wouldn't have to worry about getting the data rates exact as long as simultaneity is preserved. _If_ such a restoration were to ever be attempted, I'd suggest approaching it as four tasks: 1) Develop a full emulation of the machine in such a way that the drum emulation and the rest of the machine are connected through the same interface used on the real machine. For example, using one Raspberry Pi to emulate the machine's logic and another to emulate the drum. That way the drum emulator can be developed against an emulation of the machine and the major functionality of it can be developed before attempting to tie to the real machine. 2) Restore the power supplies. Of course, we're mostly all familiar with that by now from other projects. The good news is that tube electronics of that era generally had relatively simple power supplies when compared with the complex switchers we've been using the last 40 years. 3) Set up a test bench with fixtures for testing all the tubes and all the plug-in modules. Then one-by-one go through every module in the machine, replacing tubes and repairing modules as needed. 4) Restore the original drum. Once Tasks 1, 2, and 3 are completed, we could integrate the drum emulator and would have a running G-15. I'd recommend doing Task 1 first as a proof of feasibility and competence. Then if the machine's owner were amenable, Tasks 2 and 3 could be undertaken concurrently with relatively little risk. Finally, only if it were decided to be worth the risk would any attempt be made to touch the drum. Of course, we'd want to carry out any such effort using the same level of preservation as the CHM PDP-1 project, documenting every step, preserving and labeling replaced components, etc. But I know what you mean about available time. I've been meaning to do an emulator anyway for some time, but other projects keep getting priority. What I should do is force myself to spend a Christmas break doing an emulator. Then if a restoration project were ever approved, I could try to spend most of a summer when I'm not teaching on it. Of course, the same thing applies to restoring George. BLS