>I dunno.  I looked at the specs for the 148x and MAX232 type line
>receivers, and all of them I saw would see 0 and 3.5V as valid RS232
>levels, at least as far as they were concerned.  It surprised me that =
even
>the 1489(88? haven't used one in years) didn't need to see a negative
>voltage.  I seem to recall the cutoff was 0.7V.  Has anyone had an =
actual
>experience with something that would *not* work with the simple =
resistor
>interface, I wonder?  I've had them talking to PCs, modems, and =
terminal
>servers.

The reason for this is a requirement in the RS232 spec where an open =
circuit input should behave as though it has a voltage more negative =
than -3V. The easiest way to do this is set the schmitt trigger =
threshold at a slightly positive voltage. For this reason chips like the =
1489 make excellent TTL line receivers, especially as you can slug the =
frequency response with a capacitor to slow them down. Check the spec =
sheets for this function.=20

it is also possible to adjust the threshold level of the 1489 using a =
resistor to a positive or negative voltage. If the resistor goes to a =
positive voltage then the threshold goes more negative. This is also =
covered in the data sheets.

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