How to use the serial port pins as GPIO

I have a project where I need one input and one output pin. Using the power plug GPIO pins is not an option due to highly complicated (and expensive) cable assembly. The by far easiest way would be to use on of the handshake signals on the serial port as GPIO. Is it possible? Some hints on how?

The good parth though, is that the serial port is not need in the field.

Thanks in advance.


BTW, I know about the ISEM IO card, but using it would drive the target price through the roof.