RS232 has two equipment types Data Terminal Equipment (DTE) and Data Connect Equipment (DCE). Your computer is a DTE and has a male connector as all DTE equipment has, that is if the RS232 standard has been followed. All equipment intended to connect to a computer RS232 port is DCE and has a female connector. In the early days null modems were intended to be used to connect two computers and therefore a null modem cable was female to female. This was before networks were invented. The problem now is that we have DCE to DCE equipment which requires a male to male connector, which are hard to find and easy to make. So if the two pieces of equipment have the same gender a null modem is required, RX and TX are crossed. If one is male and the other female then a straight serial cable is needed.

Unless of course you have a piece of equipment that has not followed the RS232 convention and there are some trust me.

Clear as mud?

Steve