inverted voltages

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It seems that in RS-232, the voltages for data bits are inverted (a low
voltage is 1 and a high voltage is 0). Anyone know the motive for
this?

Thanks.

 

[Jonathan Kirwan]

It seems that in RS-232, the voltages for data bits are inverted (a low
voltage is 1 and a high voltage is 0). Anyone know the motive for
this?

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

 

[Stanislaw Flatto]

It seems that in RS-232, the voltages for data bits are inverted (a low
voltage is 1 and a high voltage is 0). Anyone know the motive for
this?

Thanks.

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.

 

[Chris]

It seems that in RS-232, the voltages for data bits are inverted (a low
voltage is 1 and a high voltage is 0). Anyone know the motive for
this?

Thanks.

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 www.tech-chat.de)

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

 

[jasen]

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.

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.