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[https://p.sfx.ms/icons/v2/Large/Pdf.png]<https://1drv.ms/b/s!AgRHXuUu57CegUK87UWfD3EpIfpL> how to make a tpdd cable.pdf<https://1drv.ms/b/s!AgRHXuUu57CegUK87UWfD3EpIfpL> 1drv.ms PDF File ________________________________ From: M100 <[email protected]> on behalf of Geoff Oltmans <[email protected]> Sent: Friday, August 19, 2016 2:10:28 PM To: Model 100 Discussion Subject: Re: [M100] TPDD/TPDD2 boot disk captures (Take 2) 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]<mailto:[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]] 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.
