The host system is DTE (Data Terminal Equipment) and the peripheral system is DCI (Data Circuit–Terminating Equipment)

A low level (–3 to –15 volts) is defined as a logic 1. Asserting a logic 1 or turning on a signal is historically referred to as “marking”.

A high level (+3 to +15 volts) is defined as a logic 0. Releaseing to logic 0 or turning off the signal is referred to as “spacing”.

DB25 PIN	DB9 PIN	MNEMONIC	NAME*	DIRECTION	TYPE
7	5	AB	Signal Common	–	Common
2	3	BA	Transmitted Data (TD)	To DCE	Data
3	2	BB	Received Data (RD)	From DCE	Data
4	7	CA	Request to Send (RTS)	To DCE	Control	Request to Send (RTS): When the host system (DTE) is ready to transmit data to the peripheral system (DCE), RTS is turned ON. In simplex and duplex systems, this condition maintains the DCE in receive mode. In half-duplex systems, this condition maintains the DCE in receive mode and disables transmit mode. The OFF condition maintains the DCE in transmit mode. After RTS is asserted, the DCE must assert CTS before communicationcan commence. Hard-wire to CTS on connector to bypass this function
5	8	CB	Clear to Send (CTS)	From DCE	Control	Clear to Send (CTS): CTS is used along with RTS to provide handshaking between the DTE and the DCE. After the DCE sees an asserted RTS, it turns CTS ON when it is ready to begin communication. Hard-wire to RTS on connector to bypass this function
6	6	CC	DCE Ready (DSR)	From DCE	Control	Data Set Ready (DSR): This signal is turned on by the DCE to indicate that it is connected to the telecommunications line. Hard-wire to DTR on connector to bypass this function
20	4	CD	DTE Ready (DTR)	To DCE	Control	Data Terminal Ready (DTR): DTR indicates the readiness of the DTE. This signal is turned ON by the DTE when it is ready to transmit or receive data from the DCE. DTR must be ON before the DCE can assert DSR. Hard-wire to DSR on connector to bypass this function
22	9	CE	Ring Indicator (RI)	From DCE	Control	Ring Indicator (RI): RI, when asserted, indicates that a ringing signal is being received on the communications channel.
8	1	CF	Received Line Signal Detector** (DCD)	From DCE	Control	Data Carrier Detect (DCD): This signal is turned ON when the DCE is receiving a signal from a remote DCE which meets its suitable signal criteria. This signal remains ON as long as the a suitable carrier signal can be detected.
		CG	Signal Quality Detector	From DCE	Control
		CH	Data Signal Rate Detector from DTE	To DCE	Control
		CI	Data Signal Rate Detector from DCE	From DCE	Control
		CJ	Ready for Receiving	To DCE	Control
		RL	Remote Loopback	To DCE	Control
		LL	Local Loopback	To DCE	Control
		TM	Test Mode	From DCE	Control
		DA	Transmitter Signal Element Timing	from DTE To DCE	Timing
		DB	Transmitter Signal Element Timing from DCE	From DCE	Timing
		DD	Receiver Signal Element Timing From DCE	From DCE	Timing
		SBA	Secondary Transmitted Data	To DCE	Data
		SBB	Secondary Received Data	From DCE	Data
		SCA	Secondary Request to Send	To DCE	Control
		SCB	Secondary Clear to Send	From DCE	Control
		SCF	Secondary Received Line Signal Detector	From DCE	Control