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.

Full RS232 mode, six available pins are assigned to the most used RS232 signals. Pins 1 and 6 are however reassigned to lines A and B when the port is used in RS485 mode.
These pins are normally used for the DTR, the terminal ready data and the DSR, willing to establish data signal.

On some models of the PLC, it is possible to mix the RS232 and RS485 communication. When the DSR and DTR signals are not used, the two pins can be used for simultaneous RS485 communication.

RJ11 to RS232 adapters are technically simple.

They are a little expensive, found on the Web or in the specialized shops.