Tauno Voipio wrote:
....
Is the idle line up or down?

Down, which I think you are saying means that the signal I see is the
same polarity as full-blown RS232 levels, but the amplitude is reduced
to only 4V p-p. (0 to 4V).

Obviously, the trip point of the receiver of such a reduced amplitude
signal would have to be at about 1.5V (TTL like).

Thanks,