Rob Roy wrote: > I was wondering if anyone has wrestled with trying to eliminate rs232 > line drivers and their associated capacitors from a dongle like device > attached to a PC or similar serial port You wouldn't dare confess on this list to building dongles, would you? > I was wondering since resistors and diodes are cheaper and my supply > voltage is +10v & -10v If you have those voltages available, use whichever is cheaper; op- amp or 14C88 (current-conserving CMOS version). > whether I could condition my input serial line with a resistor and a > zener like so: Sounds great; but the Zener should be 4.7V. At 5.1V the PIC would shunt first, 3.3V sounds just a tad close to the 2.5V threshold. You should load the RS-232 line though - a 4.7K resistor to ground at the input will suppress ringing and provide a default state, while as Mike Keitz mentions, a higher value of series resistor will then be fine. I'd probably go for 33K with a Zener, 100K without (using the PIC diodes. That latter doesn't feel right, but it does seem that the designers very deliberately engineered the protection diodes not to cause problems). Mike Keitz wrote: > Most PC's will accept a 0-5V level directly from a PIC output. But > some won't, so a driver circuit of some sort is a very good idea. I'd be VERY worried if I had a PC which didn't accept 5V CMOS levels on a short cable (or none; Rob did say "dongle-like"), as knowing all purpose-built RS-232 interface devices DO, it suggests that machine contains *very* non-standard components! My experience is strictly limited, but every time I thought there was a problem with a PC serial card, I was overlooking something else! Regarding RS-232 drive: > This circuit won't quite work. Two transistors are required. Usually > the first is an NPN driven by the PIC in common-emitter or common-base > mode to provide a high-voltage open-collector output. This output > drives a PNP transistor with the emitter connected to the +10V supply > and the collector outputting to the PC and pulled down to -10V. But to be RS-232 compliant and safe, it also requires a series resistor in the output line, and may require slew-limiting capacitors. We then have: R in first base circuit, R = first collector pull-up, R = second base current limit, R = second collector pull-down to -10V and R = output current limiting. The tally is five resistors and two transistors, one possible capacitor (in addition to the supply decoupling caps you must have anyway), against a quarter IC alone using the proper driver. Using an op-amp requires somewhere from one to three resistors, but they are cheap. How much do you value PCB space and hole count? Cheers, Paul B.