Schematic of a GM ALDL -> PC converter

[Stephen Dubovsky]

...
>The spec for RS232 is (i believe) a logic 1 for any signal below .2v, and 
>a logic 0 for any voltage above 2.7v. Therefore, even though RS 232 is 
>meant to be -15 and +15, the ALDL signal of 0v (logic 0) and 5v (logic 1) 
>will work if the signal is "inverted".
...

Used to be that -5 to +5 (or +-7) was invalid for RS232 inputs back when
everyboy went from -12 to +12.  (The invalid area was a hysteresis band so
you HAD to cross both boundries to toggle the bit)  Now with all the 3.3V
chips around, they use a charge pump to get almost +6.6V and -6.6 (usually
+6.2 and -6.0 in my exp) and the hysteresis band is reduced to something
like +-2V.  I believe it is still bipolar, so if you only go from 0V to 5V
the bit may get 'stuck' in one direction.  Hope this helps...  You might
also have a serial interface that doesn't truly follow the RS232 spec and
toggles around 0.7-2.7V though.  

SMD
--
Stephen Dubovsky
dubovsky at vt.edu

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