I haven’t tried building one yet, but my hunch is that based on descriptions of the guts of the TPDD cable, I think it has to do with the fact that there are no RS-232 line drivers on the handshaking signals from the TPDD, TTL levels as indicated here, 0 or 5V. These are only partly compatible with RS-232 as the 0V level falls in the dead zone between +/- 3V where the line drivers can’t tell the difference between mark and space. IIRC there are some pull down resistors for these signals in the 100/102 so that when nothing is connected there is a known value for these signals. The description of the guts of the cable seem to indicate that the active components are a couple of digital bjt transistors (i.e. transistors with built-in bias resistance) so that a logic high will pass 5V and a logic 0 will output high impedance, thus “stealing” the -V from the 100/102.
That’s my theory anyway. ;) > On Aug 19, 2016, at 3:40 AM, Gary Hammond <[email protected]> wrote: > > 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] > <mailto:[email protected]>] On Behalf Of John R. Hogerhuis > Sent: Friday, 19 August 2016 6:40 PM > To: Model 100 Discussion <[email protected] > <mailto:[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.
