Serial Connection Pin Assignments

Quincy Center for Technical Education
Computer Technology Department

**Serial Connection Pin Assignments**
Pin Number	Designation	Activity
1	Carrier Detect (CD)	Indicates a connection is established.
2	Recieve Data (RD)	All incoming data is recieved on this pin.
3	Transmit Data (TD)	All outgoing data is sent on this pin.
4	Data Terminal Ready (DTR)	The host device (such as the PC) is ready to communicate.
5	Signal Ground	Not used on PC systems.
6	Data Set Ready (DSR)	The connected device (such as a MODEM) is able to communicate.
7	Request To Send (RTS)	Host device wants to communicate.
8	Clear To Send (CTS)	Connected device is ready to communicate.
9	Ring Indicator (RI)	The telephone is ringing.

The serial connector's pins are used to send signals to create what amounts to a conversation (the handshaking) between two serial devices. Picture that each pin is a light that's turned on to indicate the next of a sequence of events. Handshaking accomplishes the hardware flow control (the most common method) between the PC and the modem, as follows.

The DTE (Data Terminal Equipment), or PC, turns on the DTR (Data Terminal Ready), indicating that it's good to go.
The DCE (Data Communications Equipment) acknowledges this message by turning on the DSR (Data Set Ready) that says, "Me too."
The DTE turns on its RTS (Request To Send) signal to let the DCE know it is ready to receive data.
The DCE acknowledges this request with a CTS (Clear To Send) that replies, "Here it comes."
The data flows one bit at a time until one of the devices needs to stop it. This stopping is indicated by either the RTS or CTS being turned off. The flow starts again when the applicable indicator is turned back on.

Quincy Center for Technical Education Computer Technology Department

Quincy Center for Technical Education
Computer Technology Department