Null Modem cables are used for a wide variety of communications applications, basically these are used to link
two PCs together and allow data transfer direct from the serial port of one machine to the serial port of the other.

Some examples of the use of these cables are PC Games, Doom, Quake etc all have the ability to use a null
modem cables for communications, Data transfer utilities such as Interlink, certain peripherals such as cash tills etc
also utilise these cables.

To simplify this overview the overview will discuss only the 25 -25 way cable.

As can be seen in the diagram below the cable basically consists of three crossovers and a direct connection.

The direct connection is the Ground (Pin 7), if this ground reference connection is not made the Serial Port may
have problems deciphering the correct data as this is used as a reference.

The First crossover connects the Transmit of each PC (Pin 2) to the Receive of the other (Pin 3).

The Second crossover connects the Clear To Send (Pin 5) of each PC to the Ready To Send (Pin 5) on the
other.

The Third crossover connects the Data Set Ready (Pin 6) of each PC to the Data Transmit Ready (Pin 20).