Quatech RS-232 user manual External Connections

7 External Connections

RS-232-C devices are classified by their function as either Data Terminal Equipment (DTE) or Data Communication Equipment (DCE). Generally, data terminal equipment is defined as the communication source and data communication equipment is defined as the device that provides a communication channel between two DTE-type devices.

Terminal

DTE

Terminal

DTE

RS-232-C

RS-232-C

Modem

DCE

Modem

DCE

Telephone

line

Figure 13 --- Use of DTEs and DCEs in a communications link

DTE- and DCE-type devices have complementary pinouts to allow terminals and modems to be connected directly using a one-to-one cable as shown in Figure 13. In many applications, DCEs are unnecessary, and in these cases a cable called a "null modem cable" or "modem eliminator cable" is used to directly connect two DTE-type devices. A typical null modem cable is also shown in Figure 14.

(3)	RxD	TxD	(3)	(3)	RxD	RxD	(3)
(2)	TxD	RxD	(2)	(2)	TxD	TxD	(2)
(4)	RTS	CTS	(4)	(4)	RTS	RTS	(4)
(5)	CTS	RTS	(5)	(5)	CTS	CTS	(5)
(20) DTR		DSR	(20)	(20)	DTR	DTR (20)
(6)	DSR	DTR	(6)	(6)	DSR	DSR (6)
(8)	DCD	DCD (8)		(8) DCD		DCD (8)
(22)	RI	RI	(22)	(22)	RI	RI	(22)
(7)	GND	GND (7)		(7) GND		GND (7)
	Typical DTE-to-DCE cable			Typical DTE-to-DTE null modem cable

Figure 14 --- Cabling requirements for RS-232-C devices

(cables using 25-pin connectors shown)

ESC(LP)-100 User's Manual 27