There are two types of serial cable connection in the RS232 standard, DTE connections and DCE connections. DTE stands for Data Terminal Equipment and DCE stands for Data Communications Equipment. DTE connections are what are on the serial port of a PC. A DCE connection is similar to a DTE connection except that the Transmit & Receive, DTR & DSR, and RTS & CTS pin connections are switched.

	25 Pin Connector on a DTE device (PC connection)
Male RS232 DB25
Pin Number	Direction of signal:
1	Protective Ground
2	Transmitted Data (TD) Outgoing Data (from a DTE to a DCE)
3	Received Data (RD) Incoming Data (from a DCE to a DTE)
4	Request To Send (RTS) Outgoing flow control signal controlled by DTE
5	Clear To Send (CTS) Incoming flow control signal controlled by DCE
6	Data Set Ready (DSR) Incoming handshaking signal controlled by DCE
7	Signal Ground Common reference voltage
8	Carrier Detect (CD) Incoming signal from a modem
20	Data Terminal Ready (DTR) Outgoing handshaking signal controlled by DTE
22	Ring Indicator (RI) Incoming signal from a modem

	9 Pin Connector on a DTE device (PC connection)
Male RS232 DB9
Pin Number	Direction of signal:
1	Carrier Detect (CD) (from DCE) Incoming signal from a modem
2	Received Data (RD) Incoming Data from a DCE
3	Transmitted Data (TD) Outgoing Data to a DCE
4	Data Terminal Ready (DTR) Outgoing handshaking signal
5	Signal Ground Common reference voltage
6	Data Set Ready (DSR) Incoming handshaking signal
7	Request To Send (RTS) Outgoing flow control signal
8	Clear To Send (CTS) Incoming flow control signal
9	Ring Indicator (RI) (from DCE) Incoming signal from a modem

Null modem cables and Null modem adapters:

If you connect two DTE devices (or two DCE devices) using a straight RS232 cable, then the transmit line on each device will be connected to the transmit line on the other device and the receive lines will likewise be connected to each other. A Null Modem cable or Null Modem adapter simply crosses the receive and transmit lines so that transmit on one end is connected to receive on the other end and vice versa. In addition to transmit and receive, DTR & DSR, as well as RTS & CTS are also crossed in a Null Modem connection.

A Null Modem adapter is a small device that plugs in the line of a standard serial cable that converts the connection to a Null Modem connection.

Null Modem adapters are available at most computer and office supply stores for under $5.

Related Links

Sartorius Cable Pinouts
Pinouts for other common types of cable