Re: RS-232 levels to computer

On Sat, 11 Mar 2006 05:37:04 GMT, Mac <foo@xxxxxxx> wrote:

On Fri, 10 Mar 2006 19:10:42 -0800, Richard Henry wrote:


<pdrunen@xxxxxxx> wrote in message
news:1142015379.013518.240620@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi All,

I understand that standard RS-232 levels must be bipolar with at least
a -3V for a logic "1" and +3 volts for logic "0".

The micocontroller 232 level is TTL level, I use this signal to drive a
NPN so the the signal is inverted and use +12 on the collector to get
the level. The lowest voltage out of the NPN will be near zero. I am
using this for just collecting some data sent over a small cable lengh
and I don't plan on this being part of a design.

The question is in regards to the computer RS-232 input, would the near
zero voltage be taken as by the com port as the correct level?

Don't count on it. However, many poorly-designed, non-spec-compliant
interfaces will work with those voltages.

Sometimes.

Intermittently.

Many well-designed ones will work, too. It's just not guaranteed by the
spec to work.

You too are misunderstanding the spec. The spec mandates minimum receiver
threshold performance. Any receiver that can reliably respond *inside* those
+/-3V specified minima is compliant.

This is similar to the distance "limits" that people often read into the spec.
The spec requires that the devices shall communicate reliably over certain
distances at certain data rates. I can't count the number of times this has
been regurgitated in this and other forums as a MAXIMUM distance over which it
can work.
.

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