inverted voltages

Just as a clarification, RS-232 has a 6V dead band around 0V (+ and -

3V about 0), which isn't either a defined '1' or '0'. The MARK state uses the negative (relative to ground) voltage value and the SPACE state uses the positive (relative to ground) voltage value.

Why the relative-negative voltage was specifically chosen for the MARK state, rather than the relative-positive, I don't know. It probably does have meaningful historical and electronic reasoning, though, appropriate for the time of the standard's development.

Jon

Archeology, chemistry, habit! The "ancient" IT of telephony found that it was more "technical" from corrosion standpoint to ground the positive terminal of batteries supplying the grid. So an agreed "standard" developed and the spider "web" of copper wires could connect different sectors to each other. On this backgroung the interpreting of what is "true" and "false" in binary terms was accepted and is with us till now.

HTH

Stanislaw Slack user from Ulladulla.

Hi, Mike. My understanding was that, when RS-232 was first developed as a standard, germanium ruled the earth. For various reasons, PNP transistors were more popular, so power supplies were typically negative, and PNP inputs to switching circuits required an active low voltage to turn on the transistor (pulling current out of the base of the transistor). | | GND GND | | | | .-. | | | | | | | | | | '-' | | ___ | |< | In o-|___|-o-| PNP Ge | |\\ | | Out | o------->

| | | .-. | | | | | | | '-' | | | V | -12VDC (created by AACircuit v1.28.6 beta 04/19/05

I can't find a reference on this (it's a bit before my time), but my hunch (or WAG, depending on your level of cynicism) might be a place for you to start looking.

Good luck Chris

put a negative potential on a conductor an you get anodic protection same reaspon why negative voltages are used in phone lines as I understand it.

--

Bye.
   Jasen