> Brian, don't assume just because one PC Serial Port will work with TTL
> Levels that they all will.
>
> I have two different manufacturer's PCs here that both work differently and
> both need different levels of negative voltage before they accept an RS-232
> "Mark" (1 or negative voltage).

That is interesting; are they laptops, or desktop models?  Is the serial
I/O on the motherboard or on a card?  Are the receiver chips identifiable?

On one of my PIC projects where I wanted to be able to receive RS-232 but
also be able to tell whether it was plugged in or not (i.e. so an unplugged
cable would read as a "break") I wired a 4.7K resistor from RB0 to the input
and another 4.7K resistor from RB0 to VDD.  Perhaps your PC is doing some-
thing similar?

> If you're designing for yourself and it works with TTL levels, great.  Just
> don't assume that it will work for everybody.

It's still useful to know how common machines are on which something like
that will break.  For example, in some cases it can be EXTREMELY useful to
use the four control wires on a printer port as inputs, even though this
technique will fail on Toshiba laptops.