Re: RS-232 levels to computer

On Sat, 11 Mar 2006 10:30:56 -0600, John Fields <jfields@xxxxxxxxxxxxxxxxxxxxx>
wrote:

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


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.

---
I think _you've_ got it wrong in that a data '1' is guaranteed
betwen -3V and -15V and a data '0' is guaranteed between 3V and 15V.

That's why I deliberately used the terms 'high' and 'low' rather than 0 and 1.

In the no man's land between 3V and -3V, nothing is guaranteed.

There are two elements here - what the spec mandates a device must do, and what
a device must do to meet the spec requirement.

In the no-man's-land between, the spec does not mandate a specific response.
But to meet the spec, a device needs to have both positive-going and
negative-going thresholds (assuming hysteresis has been used) within that
no-man's-land. Clearly those thresholds aren't going to be at +/- 2.999999v.

As Spehro's footnote asked:

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

I for one certainly don't. JimT, as the 1488/1489 man, would know about this,
and a quick viewing of the data *** for those early devices will say the same.
.

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