The cable requires at minimum a few diodes, besides the necessary pin to pin wiring. The real original cable appears to use resistors and transistors, and there is a schematic someone came up with a long time ago that uses diodes and resistors, and at least one person has claimed since then that they made a cable that works fine with only the diodes.
The complicating issue is that the PDD is TTL (0-5v) while the serial port is rs232 (-12v-+12v), and the fact that you can usually get away with a lot when interfacing with rs232 and techgnically violate the spec and yet it still "works", and the fact that part of the scheme in our case is particular to the actual wiring of the rs232 plug in the M100 where there are pull-down (or pull-up, I forget which) resistors on some of the flow control pins in the M100, which ends up causing the signal to be valid high or low for rs232 merely by blocking it, without having to actually DO the proper electrical conversion you otherwise would have to in the cable. IE, the unofficial cable schematic merely one-way-blocks some signals with a diode, which technically does NOT produce a valid rs232 signal level (-3v to -25v or +3v to +25v), but, because of the internal pull-downs in the M100 on those pins, merely blocking the signal in the cable ends up hving the result that the pull-down in the M100 pulls the signal down to a valid level. So, a cable that works between a PDD and M100/102/200, is not guaranteed to work between a PDD and any other pc serial port or usb serial adapter, although so far, they do seem to be working everywhere. Links to the unofficial schematic, and the service manual which shows the pinout for the PDD end of the cable: http://tandy.wiki/TPDD On Tue, Feb 27, 2018 at 4:38 PM, Scotty Brown <[email protected]> wrote: > Hey all, > > I've just got back into my M100 over the last few months - have been > blogging about it a little here: https://scottyau.blogspot.com.au/ > > I've pulled a lot of stuff out of the mailing list archives - thank you > all so much. > > I've managed to get my hands on a Portable Disk Drive 2 - however it > didn't come with the serial cable. > > Chances are I can probably find a pin out and make one - but I wondered > if anyone on list had an original cable they would be willing to part > with (for a fee + postage of course). > > Cheers! > > Scotty > > -- bkw
