com port problem: it works on an Apple2, but not on a PC, why?

[wlsmith@valve.heart.rri.uwo.ca (Wayne Smith)]

We have an old Searle ISO-CAP Gamma counter that outputs data through
a serial line to an Apple ][.
A basic program does the data collection, and the Apple's serial port is
opened as 110 baud, 7 data bits, even parity, 1 stop (generic stuff).

There are only 3 wires used: pins 1, 3 and 7.  Now, if this is standard
RS-232, then pins 1 and 7 are ground and 3 is the receive data line.

Now, I wired in a female 25-pin connector in parallel and connected it to
a PC's serial port.  But with both the PC and the Apple running, the
Apple gets data but the PC doesn't register anything. (it also doesn't
work when connected to only the PC). 
A diagnostic program verifies that the
PC's serial port is working (although I haven't
done an exernal loop-back test).  When a mouse is connected to the serial
port I do get "garbage" when the mouse is moved, so the port seems to 
function.

Are there any differences between Apple ][ and PC serial ports that 
can explain this situation?  ie does the Apple use a current-loop 
rs-232?  I temporarily moved the wire on pin 3 to pin 2 on the PC, 
but that didn't help.

So, what do I have to do to get this to work on the PC?
-Wayne-