TxD, or Transmit Data:
This is an output for DTE devices and an input for DCE devices. This is the data channel from the DTE device to the DCE device.

RxD, or Receive Data:
This is an input for DTE devices and an output for DCE devices. This is the data channel from the DCE device to the DTE device.

RTS, or Request To Send:
This is an output for DTE devices and an input for DCE devices. This signal is typically used to gate flow from the DCE device to the DTE device. In other words, the workstation serial port would drop this signal to halt flow from the modem, and then later raise it to resume flow.

CTS, or Clear To Send:
This is an input for DTE devices and an output for DCE devices. This signal typically is used to gate flow from the DTE device to the DCE device. In other words, a modem may drop this signal to halt flow from the workstation, and then later raise it to resume flow.

DSR, or Data Set Ready:
This is an input for DTE devices and an output for DCE devices. This signal is not widely used in UNIX®, except on some DEC machines, which will block it on open if it is not true in some cases.

SG, or Signal Ground:
This is a signal return for all signal lines.

DCD, or Data Carrier Detect:
This is an input for DTE devices and an output for DCE devices. This signal is used to show that there is a valid connection between the DTE and DCE devices. It is typically used to block open on a port before connections, and to generate UNIX "hang up" signals upon loss of a connection.

DTR, or Data Terminal Ready:
This is an output for DTE devices and an input for DCE devices. This signal is typically used in UNIX to show that the port has been activated or "opened".

RI, or Ring Indicator:
This is an input for DTE devices and an output for DCE devices. This signals the DTE device that there is an incoming call. This signal is maintained "Off" at all times except when the DCE receives a ringing signal.

GND, or Chassis Ground:
This is also available on the connector shell.