RJ11 - RS232
A symmetrical pin layout allows to connect two DTE, data terminal ()) directly without using a modem or other DCE data communication equipment.
The original wiring RS232 was developed for 25 pins.
The vast majority of applications RS232 use only 3 links : TX, RX and GND.
9 pins RS232 connectors are commonly used.
In mixed applications, a 9-to-25 converter can be used to plug in different sizes.
Two other contacts of signal, RTS and CD are sometimes used when interfacing to modems or RJ11.