There are different types of serial cables. However, we will discuss about the two most common types for our purposes which are null-modem cables and standard (“straight”) RS-232 cables.
Null Modem Cables
A null-modem cable passes some signals, like “signal ground”, straight through, but switches other signals. For example, the “send data” pin on one end goes to the “receive data” pin on the other end
RS-232 Cable
All the RS-232C signals are passed straight-through in a standard serial cable which means the “send data” in on one end of the cable goes to the “send data” pin on the other end. This is the type of cable that is used to connect a modem to the system, and is also suitable for some terminals. It is an accepted industry standard for serial communication connections. Adopted by the Electric Industries Association, this Recommended Standard (RS) defines the specific lines and signal characteristics used by serial communication controllers to standardize the transmission of serial data between devices. The letter 'C' here signifies the third in a series.
