The RS-232 signals are serial data communication. The RS-232 line has two states: either high or low. According to the standard, the high level can range between -15 and -3 V and the low level can range between 3 and 15 V. The voltage level of the RS-232 transmitter significantly impacts the total transmission length of the line, so the closer the line signal is to 15/-15 V the longer the transition cable can be – and the more immunity it will have to external interference.
Below you see an image that shows the relationship between the data that you send and the
RS-232 line level.
Line levels are just one of many aspects used by the RS-232 to transmit data. Characteristics
such as speed, parity and stop bit play important roles as well.
The pin out of the 9-pin D-sub connector
Remember the importance of crossing Rx and Tx (pin 2 and 3) because the receiver needs to listen on the transmitting part from the counterpart and vice versa.
As shown below in the two illustrations: