Re: RS-232 levels to computer
- From: budgie <me@xxxxxxxxxxx>
- Date: Sun, 12 Mar 2006 13:47:17 +0800
On Sun, 12 Mar 2006 04:57:33 GMT, Mac <foo@xxxxxxx> wrote:
On Sat, 11 Mar 2006 19:45:27 +0800, budgie 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.
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.
I don't think you correctly understood what I wrote.
I believe I did.
The poster to whom I
was responding said many poorly designed interfaces will work swinging
from ground to +5V. I said that many well-designed interfaces will work,
with that swing, too, but that the spec doesn't guarantee it.
Is there some part of that that you disagree with? Is it your contention
that the spec DOES guarantee operation with a swing from 0 to 5V?
Quite the opposite, BUT ...
The reality lies in what the spec - and I would argue quite intentionally - does
NOT guarantee or state.
It is years since I actually had a copy of the 232 tome before me, so these
numbers are from memory. The four pivotal requirements for interfacing were:
1. A transmitter (driver) had to deliver a mimum of 5V (either polarity) into
IIRC a 1K load.
2. A transmitter had to withstand an indefinite short to ground or any other
signal line (i.e. includes driver-driver contention). This is the one that made
trial-and-error interfacing non-destructive in compliant hardware.
3. A receiver had to respond correctly to +/-3V applied.
4. A receiver had to withstand +/-25V.
Armed with those, one went forward into the jungle. Note that the spec doesn't
state where in that +3v<->-3v zone the receiver threshold should lie.
Compliant real world receivers invariably used a positive threshold, responding
to 0v as low and +3v as high. As a result, TTL swings would ALWAYS drive a
real world 232 receiver.
The spec doesn't guarantee it. That is true. But the real world devices do.
I'm open to someone showing me documentation of a receiver IC with a negative
threshold. But as an example of a receiver, have a look at the input circuit of
the ubiquitous 1489: http://www.national.com/ds/DS/DS1489.pdf
.
- Follow-Ups:
- Re: RS-232 levels to computer
- From: Don Bowey
- Re: RS-232 levels to computer
- References:
- RS-232 levels to computer
- From: pdrunen
- Re: RS-232 levels to computer
- From: Richard Henry
- Re: RS-232 levels to computer
- From: Mac
- Re: RS-232 levels to computer
- From: budgie
- Re: RS-232 levels to computer
- From: Mac
- RS-232 levels to computer
- Prev by Date: Re: RS-232 levels to computer
- Next by Date: Re: Any transmission line experts?
- Previous by thread: Re: RS-232 levels to computer
- Next by thread: Re: RS-232 levels to computer
- Index(es):
Relevant Pages
|
