RS-232 is widely used for connections between data acquisition devices and computer systems. RS-232 devices are defined as either DTE (usually a computer) or DCE (usually an interface device). When wiring a DCE device to a computer (DTE), a straight-through connection is required. However, not all interface devices or data acquisition systems are DCE, therefore, require a null-modem cable, which 'crosses over' the necessary signal wires.
Most RS-232 devices will operate with only 3 signal wires: Transmit (TX), Receive (RX) and Ground (GND). In order for two RS-232 devices to communicate, you must connect the TX from one instrument to the RX of the second instrument, and vice versa. The Ground pins must be connected together. Keep in mind that a 25 pin RS-232 port on a PC trasmits on pin 2 and receives on pin 3, and Ground is pin 7. A 9-pin RS-232 port on a PC transmits on pin 3, receives on pin 2 and Ground is pin 5. You cannot simply connect two devices with a serial cable simply because the connectors fit. You must verify the functions of each pin on each device, as well as verify whether or not the cable is a straight-through or null-modem cable.
The following diagrams illustrate a 9-pin and 25-pin RS-232 connector on a PC. Keep in mind that these connectors are always Male.