Yep, that makes sense. The inputs to the TPDD should already take care of this on the input side... they are clamped to the Voutput low/high of the buffer. On the flipside, the outputs on the TPDD have diode protection to prevent damage if something is mis-wired, but no way to generate the normal +/- 12V that RS-232 requires. IIRC, +5V is not much trouble since it would be outside the dead zone of around +/-3V, but 0V is a problem... so that level needs to be translated.
On Tue, Jan 12, 2016 at 2:04 PM, John R. Hogerhuis <[email protected]> wrote: > On Tue, Jan 12, 2016 at 11:40 AM, Geoffrey Oltmans <[email protected]> > wrote: > > That's useful, and I must confess I was looking at the wrong part of the > > schematic! ;) > > > > I suspected that it must not be a straight through cable, because that > would > > be too easy. From his description it sounds like he ran across three > > transistors, SOT-23 packages I'm guessing. According to a cross reference > > most likely a PNP transistor with two built-in resistors, similar to > what is > > shown in the schematic on those same lines. > > > > I'll have to try and construct a cable, but the next hurdle is that I > have > > no disks! > > > > > > I might be wrong but I don't think anyone has successfully created a > TPDD cable other than one that used to be sold on EBay and that guy > didn't publish his design. > > The theory is to sap power from flow control lines to convert from TTL > levels supported by the TPDD to the minimum RS232 levels necessary for > the Model T. > > -- John. >
