At 03:34 PM 8/23/2004 +0100, you wrote:


>(Having typed that I've just remembered that RS232
>"idles" at -V, so perhaps you treat this as the base
>state and anything trying to send would take it to +V ?
>You'd have to have a pull-down on the wire somewhere, I
>think?)

RS232 receivers typically have a 5K (nominal) resistor to ground on the
inputs. (it's between 3 and 7K on the MAX232). They also change state
above 0V by a couple of volts (800mV to 2 or 2.4V guaranteed on the MAX
series).

Note that when the line goes to -V you are depending on the resistor
to discharge the wire and other stray capacitance to less than this
(poorly defined) threshold voltage within a fraction of the bit time or
there will be an error*. Also the line impedance (against noise) has gone from
a few hundred ohms with perhaps 8-10V of noise immunity up to 5K or so
with maybe 1/5 that noise immunity. It's really an ugly hack, but usable for
some limited applications.

For example, suppose you have a 12V driver, and you're unlucky enough to
have a MAX receiver that triggers at 800mV and has a 7K resistor to ground.

v(t) = 11.4*exp(-t/(7K*C))

For error-free communication at 19200 baud, with a wee bit of margin, we'd
like it to drop to, say, 400mV within 1 bit time or 52usec.

Cmax = 52usec/(ln(11.4/0.4)*7K) = 2200pF

At, say, 100pF/m, it will work up to 20m or so.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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