Eric Wolf wrote: > On Sat, Nov 14, 2009 at 8:04 AM, Dave Tweed wrote: > > If I had to guess, I would say that the negative voltage was selected > > for the RS-232 idle state for the same reason that telephone lines use > > a negative supply -- to reduce galvanic corrosion of the wiring from > > leakage currents. > > I always thought it was to cancel out any leftover capacitive voltage > left on the line which could cause bipolar violations if it were to > build up high enough. What on earth are you talking about? I was referring to the -48V "battery" used on analog voice subscriber loops. "Bipolar violation" is a term related to the line coding used on digital trunk lines such at T1, E1, etc. Completely separate worlds; not related at all. -- Dave Tweed -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist