Using one of the now 3 Global Village modems in my possession (after first verifying I could communicate with it unmodified of course), I cobbled together the following test unit:
Good news: There's no fancy Modem / PowerBook initialization communication that needs to be duplicated, nor is there (assuming flow control is disabled) anything unique happening on CTS / RTS or DTR. Tx/Rx are also completely inert from boot, through connection open, and until input is sent via the terminal.
Building the above (which I also expect to come in handy during the forthcoming InterSlip portion of this project), was a worthwhile exercise though, as I found the problem preventing communication almost immediately. Pin 6 (Rx) and 8 (Tx) with no modem present are logic-low and seemed to be DOA. The modem itself, however, pulls both high once a connection is opened. The modem grounds Rx and the PB grounds Tx respectively when asserted. Seeing the Tx pin low on the PowerBook side by default (with no modem present and a connection established), I'd incorrectly assumed I was dealing with inverted logic, but this makes more sense.
I'll reconnect my test cable, pull them both high via pull-up resistors, and see what happens...
The same thought crossed my mind while I searched for more information about them. Sounds like the best recommendation right now is to buy replacement ram?
Thanks for including the additional information (reference guide, calculation and example). I tried a bunch of iterations but the only change I got was snow-like static going back and forth across the screen and the same glitchiness from the original (suspected) bad ram.
Adding photos — I also have this ram laying around, but maybe it's OP for this Macintosh?
And this is what the "glitchiness" looks like which I described earlier.
Reach out to Inertial Computing (they're active on the forum). They may or may not still be looking for beta testers for a potential v5.5-based replacement to the out-of-stock (out of production?) previous PowerBook edition. Such a device, hypothetically speaking of course, might appear as pictured below within my 180c, where it may have been working flawlessly for the past week
Alternately, @360alaska is making a nifty sled and adapter for use with the standard v5.5 and has them listed for sale on the auction site.
I would never have thought of putting one of those bags in the washer, and its so obvious!
Alcohol with white cloth/paper towels works well as a first line tool for getting out permanent marker, but sometimes you have to go farther. I had a SCSI to ethernet box that was terribly, marked up. I had to use acetone mixed with alcohol to get that out, but a few words of caution:
1) If you do go that route, mix it with the smallest amount of acetone that will do the job, mix it well in a glass/metal or ceramic cup, and test the mixture in an inconspicuous area, or on a test piece, if you have one.
2) Acetone WILL dissolve plastic, so do not use it undiluted and be careful to only use it sparingly.
3) DO NOT use straight acetone.
This article outlines several options, always try the least dangerous first. https://www.wikihow.life/Remove-Permanent-Marker-from-Plastics