Looks like the issue may have been in the M100 RS232 interface circuit. A look at the M100 versus T102/T200 service manuals would indicate a design change of the circuit between the models. The M100 is using a single CMOS 4584 hex Schmidt trigger as the interface circuit with no VEE and the T102/T200 is using dual 4584’s with dual triggers on the input lines and the GND’s tied to a VEE.
The changes appear to have been introduced with the T102. A design flaw in the M100 RS232 circuit maybe? From: M100 [mailto:[email protected]] On Behalf Of John R. Hogerhuis Sent: Friday, 19 August 2016 6:40 PM To: Model 100 Discussion <[email protected]> Subject: Re: [M100] TPDD/TPDD2 boot disk captures (Take 2) On Thu, Aug 18, 2016 at 10:21 PM, Gary Hammond <[email protected] <mailto:[email protected]> > wrote: Errr…no I wasn’t. I have not seen it mentioned in any of the documentation that I have read so far and there’s no reference to it on the TPDD circuit diagram. The pin-outs on the circuit diagram for the 8pin DIL connector on the back of the drive do not show any VCC for a circuit housed in the DB25 shell, unless it derives voltage via one of the handshaking lines. I have checked for continuity between the DIL8 end of the lead and the DB25M end of the lead. Pins 1 (GND), 2 (CTS), 5 (DSR) and 7 (RXD) on the DIL8 are directly connected to their equivalents on the DB25M. Pins 3 (DTR), 4 (RTS) and 6 (TXD) on the DIL8 show resistance to their equivalents on the DB25M so there is circuitry of some sort in there. I can’t tell if it’s passive or active. Thanks for the tip! Yes there are active electronics in the cables. It is believed to draw power from handshaking lines, probably DTR/DSR It does level conversion. No one knows for sure what's in there. If you're snooping a TPDD connection you definitely want to use a standard cable. -- John.
