RS-232 electrical levels

[MarioGL]

Hi all,

My question es very short: concerning RS-232 ports, why bit-1 is
represented as a negative pulse if it goes through the transmission or
reception pin, but, it is a positive one if it goes through one of the
control lines? It has any electrical benefit? Common sense says to me
that in both cases the same coding should be used...

Thanks!

 

[John Fields]

On Wed, 21 Nov 2007 03:39:16 -0800 (PST), MarioGL
<mario.garcia@gmail.com> wrote:

Hi all,

My question es very short: concerning RS-232 ports, why bit-1 is
represented as a negative pulse if it goes through the transmission or
reception pin, but, it is a positive one if it goes through one of the
control lines? It has any electrical benefit? Common sense says to me
that in both cases the same coding should be used...

---
I think that was in keeping with the custom that, in TTL, (which
was, as I recall, the predominant logic family when RS-232 was
implemented) data was positive true and control (Chip Select,
Enable, Set, Reset, etc.) was overwhelmingly negative true.


--
JF

 

[mc]

"John Fields" <jfields@austininstruments.com> wrote in message
newsue8k3l0k4moq0abroq139on2mirm4sl2d@4ax.com...

On Wed, 21 Nov 2007 03:39:16 -0800 (PST), MarioGL
mario.garcia@gmail.com> wrote:

Hi all,

My question es very short: concerning RS-232 ports, why bit-1 is
represented as a negative pulse if it goes through the transmission or
reception pin, but, it is a positive one if it goes through one of the
control lines? It has any electrical benefit? Common sense says to me
that in both cases the same coding should be used...

---
I think that was in keeping with the custom that, in TTL, (which
was, as I recall, the predominant logic family when RS-232 was
implemented) data was positive true and control (Chip Select,
Enable, Set, Reset, etc.) was overwhelmingly negative true.

Interesting, but I think RS-232 is slightly older than TTL.

RS-232 replaced Current Loop. How did the latter handle control signals?

 

[Michael Black]

"mc" (look@www.ai.uga.edu.for.address) writes:

RS-232 replaced Current Loop. How did the latter handle control signals?


There were no control signals. It was a single current loop, the keyboard

and the printer in series. Break the loop, and that was the signal.

Michael

 

[mc]

RS-232 replaced Current Loop. How did the latter handle control signals?


There were no control signals. It was a single current loop, the keyboard
and the printer in series. Break the loop, and that was the signal.

That would explain why RS-232 data are upside down (negative for 1, positive
for 0 or idle). And since RS-232 control signals did not inherit from
Current Loop, they are not upside down.