The RS-232 standard specifies 20 different singal lines.
Many of them are rarely implemented and only two signal wires plus a common ground wire are required as a minimum.
The DB-9 connector had become the commonly used standard in recent years.
Due to the asymmetrical specification of the interface, there are two different connector layouts, DTE (Data Terminal Equipment, originally used by teletype writers) and DCE (Data Communication Equipment, originally used by modems). Usually, a male connector is available on the DTE device, while a female connector is provided at the DCE.
The signal names refer to the DTE device.
|1||DCD||DTE <- DCE||Data Carrier Detect (when connected).|
|2||RxD||DTE <- DCE||Received Data (sent from DCE).|
|3||TxD||DTE -> DCE||Transmitted Data (sent from DTE).|
|4||DTR||DTE -> DCE||Data Terminal Ready (for connection).|
|6||DSR||DTE <- DCE||Data Set Ready (to connect).|
|7||RTS||DTE -> DCE||Request To Send (or DTE ready to receive).|
|8||CTS||DTE <- DCE||Clear To Send (or DCE ready to receive).|
|9||RI||DTE <- DCE||Ring Indicator (with incoming call).|
Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous ground loop.