9 Pin Serial Port Connector

Connector may be reversed depending on which side is viewed. All pins are numbered.

Pin No. Function
1 DCD (Data Carrier Detect)
2 RX (Receive Data)
3 TX (Transmit Data)
4 DTR (Data Terminal Ready)
5 GND (Signal Ground)
6 DSR (Data Set Ready)
7 RTS (Request To Send)
8 CTS (Clear To Send)
9 RI (Ring Indicator)
Voltage sent over the pins can be in one of two states, On or Off. On (binary value "1") means that the pin is transmitting a signal between -3 and -25 volts, while Off (binary value "0") means that it is transmitting a signal between +3 and +25 volts.
An important aspect of serial communications is the concept of flow control. This is the ability of one device to tell another device to stop sending data for a while. The commands Request to Send (RTS), Clear To Send (CTS), Data Terminal Ready (DTR) and Data Set Ready (DSR) are used to enable flow control.