>As I remember the RS-232 spec said:

>At the receiver:

>Anything below -3v is a mark.
>Anything over +3v is a space.

Yes, but

There is also a requirement that when the input is open circuit (i.e. the plug
has been pulled out) the receiver chip should behave as though it is receiving a
constant mark signal. This stop your processor getting continuous interrupts due
to an inadvertent break condition  ;)

Implementation left up to the circuit designer. The normal means of doing this
is the way the 1489 series do it by having the threshold level at a slightly
positive voltage.

I cannot quote chapter and verse for the paragraph with this requirement
unfortunately, and do not have the time to look for it.

P.S. have you ever seen an RS232 receiver with 6V hysteresis centered on 0V?

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