The RS232 connector was originally developed to use 25 pins. In this
DB25 connector pinout provisions were made for a secondary serial RS232
communication channel. In practice, only one serial communication
channel with accompanying handshaking is present. Only very few
computers have been manufactured where both serial RS232 channels are
implemented.
RS232 DB25 to DB9 converter
Testing occurs in a few steps. Data is sent on the Tx line and the
received information on the Rx input is then compared with the original
data.
The signal level on the DTR and RTS lines is also controlled by the
test software and the attached inputs are read back in the software to
see if these signal levels are properly returned. The second RS232 test
plug has the advantage that the ring-indicator RI input line can also be
tested. This input is used by modems to signal an incoming call to the
attached computer.
RS232 null modem cables
The easiest way to connect two PC’s is using an RS232 null modem
cable.
RS232 null modem cables with handshaking can be defined in numerous
ways, with loop-back handshaking to each PC, or complete handshaking
between the two systems. The most common null modem cable types are
shown here.
