Reference pinout diagrams for DB9 connector (sometimes known as DE9 connector or D-sub connector) for RS232 serial communication ports. RS232 is a serial data communication signalling protocol. D-sub 9pins or 25pins connector is commonly use for RS232. These days RJ45 (or 8P8C) modular connector is also commonly used for RS232 purpose.

Note:
For 3 wires RS232 serial communication (non-hardware-handshaking or asynchronous data communication) wiring, you only need pin 2, 3 (RX, TX) and pin 5 (Gnd)

Typically the TX pin of a RS232 D-sub connector represent data flowing out of the connector, flowing into the RX pin of its mating connector. This is a communication channel between two devices.

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