In article <Pine.SOL.3.96.990928171737.12297F-100000@marcy.nas.nasa.gov>,
Bill Studenmund <wrstuden@nas.nasa.gov> wrote:
>On Tue, 28 Sep 1999, Grey Wolf wrote:
>
>> On Tue, 28 Sep 1999, John Refling wrote:
>> 
>> # 
>> # >> discussion on adding a resistor on the RX line to pull voltage down
>> # >> to -10 volts, when inactive.........  proposed
>> 
>> Just a minor nit:  I thought voltages for RS232 were supposed to be
>> between 0 and 5, since RS232 originally ran on TTL.
>
>Nope, a bit-0 is a positive voltage, and a bit-1 is a negative voltage.
>positive and negative voltages are from 3 to (I think) 25 volts. Also
>don't forget that, back in the day when this was developed, logic boards
>commonally had -5 V and some 12 V volatages around. :-)
>

I think that Grey Wolf is right here. RS232 was defined for TTL voltages.
RS422 which is the most common form we see today is differential.

christos