> -----Original Message----- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Mouse > Sent: 19 August 2016 13:39 > To: cctalk@classiccmp.org > Subject: Re: SWTPC 6800 > > > If you have two serial devices on the same line and one is just > > listening while you work with the other, *can* that work, or would it > > just confuse things? > > It depends on what you mean by "the same line". > > For ease of language here, I'm going to assume that the devices are a > computer, C, and two terminals, T1 and T2. > > If you connect all the pins, it will work fine for the signals that T1 and T2 are > driving to the same state. Signals driven to opposite states may register as > being in one of the two states or they may fall into the undefined > intermediate zone (between -3V and +3V, IIRC), depending on the voltages > T1 and T2 are trying to drive them to and the exact impedances of the > drivers. (It shouldn't fry anything, though; one really nice feature of RS-232C > is that the spec requires that any pin or combination of pins can be shorted > together and/or to any voltage source within the allowed range (-25V to > +25V, IIRC) indefinitely without damage. I'm not sure this applies to ground > pins, though; it certainly doesn't in practice - I've seen ground loops.) > > However, the terminal-driven data line (the one that T1 and T2 use to send > to C) is one of those signals. I would suggest using a breakout box, or two > connectors wired by hand with that signal omitted, to isolate C from one of > the two terminals on that pin. (I would actually go as far as to connect only > two pins, signal ground and C-to-T data, to one of the two terminals.) It will > mean you can't type on both T1 and T2 (or, rather, typing will be ignored on > one of them); if you want that to work, you will need at least a few active > components between them - two diodes and a pullup resistor strikes me as > the bare minimum, and even then you may have to play with the resistor > value to get the voltages within the correct ranges. > > Another nice feature of RS-232C is that it is electrically very simple. > You can throw together serial-line stuff with alligator clip leads and discrete > components like diodes and resistors. You don't have to worry about things > like modulation schemes and lower-level protocols, the way you do with > things like USB or Ethernet.
If you feel really lazy you can get serial switches in both 9 and 25-pin varieties for almost less than the parts... http://www.ebay.com/itm/131887801860 Dave G4UGM > > /~\ The ASCII Mouse > \ / Ribbon Campaign > X Against HTML mo...@rodents-montreal.org > / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B