Re: RS-232 levels to computer



Spehro Pefhany wrote:
On Sat, 11 Mar 2006 19:45:27 +0800, the renowned budgie
<me@xxxxxxxxxxx> wrote:


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.



Indeed, one could argue that practice in the PC and other industries,
with their huge installed base, trumps whatever 232 spec is current,
and any new implementation that acted only minimally compliant with
the spec (eg. thresholds at +/-2.6V nominal) would be 'broken'.

One major advantage of the way the receivers generally work** is that
the output state is known for a disconnected cable.

Current practice, in effect, imposes *tighter* constraints than the
standard. This is not an uncommon situation.

** does anyone know of a commercial RS-232 receiver IC that *doesn't*
have a threshold in the +500mV~ +2.5V range?


Best regards, Spehro Pefhany

That's why the damned things act up when the peripheral and computer are plugged into different line circuits with significant voltage difference between GNDs- not enough noise margin. For high reliability, the bipolar drive should be used, and it should be as large as possible, +/-12V if feasible.

.

Relevant Pages

  • Re: RS-232 levels to computer
    ... I understand that standard RS-232 levels must be bipolar with at least ... The lowest voltage out of the NPN will be near zero. ... You too are misunderstanding the spec. ...
    (sci.electronics.design)
  • Re: RS-232 levels to computer
    ... I understand that standard RS-232 levels must be bipolar with at least ... The lowest voltage out of the NPN will be near zero. ... You too are misunderstanding the spec. ...
    (sci.electronics.design)
  • Re: RS-232 levels to computer
    ... I understand that standard RS-232 levels must be bipolar with at least ... The lowest voltage out of the NPN will be near zero. ... You too are misunderstanding the spec. ...
    (sci.electronics.design)
  • Re: RS-232 levels to computer
    ... I understand that standard RS-232 levels must be bipolar with at least ... The lowest voltage out of the NPN will be near zero. ... For high reliability, the bipolar drive should be used, and it should be as large as possible, +/-12V if feasible. ...
    (sci.electronics.design)
  • Re: RS-232 levels to computer
    ... I understand that standard RS-232 levels must be bipolar with at least ... The lowest voltage out of the NPN will be near zero. ... For high reliability, the bipolar drive should be used, and it should be as large as possible, +/-12V if feasible. ...
    (sci.electronics.design)
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