Electronics

Pin Assignments

D-Subminiature Connector, 25 Contacts (DB-25)					D-Subminiature Connector, 9 Contacts (DE-9)
Pin	Name	Function	Source		Pin	Name	Function	Source
1	FG	Frame Ground	---		1	DCD	Data Carrier Detect	DCE
2	TXD	Transmitted Data	DTE		2	RXD	Received Data	DCE
3	RXD	Received Data	DCE		3	TXD	Transmitted Data	DTE
4	RTS	Request To Send (= DTE ready)	DTE		4	DTR	Data Terminal Ready	DTE
5	CTS	Clear To Send (= DCE ready)	DCE		5	GND	Signal Ground	---
6	DSR	Data Set Ready	DCE		6	DSR	Data Set Ready	DCE
7	GND	Signal Ground	---		7	RTS	Request To Send (= DTE ready)	DTE
8	DCD	Data Carrier Detect	DCE		8	CTS	Clear To Send (= DCE ready)	DCE
9	---	Positive Test Voltage	DCE		9	RI	Ring Indicator	DCE
10	---	Negative Test Voltage	DCE
11	QM	Equalizer Mode	DCE
12	SDCD	Secondary Data Carrier Detect	DCE
13	SCTS	Secondary Clear To Send	DCE
14	STD	Secondary Transmitted Data	DTE
15	TC	Transmitted Clock	DCE
16	SRD DCT	Secondary Received Clock Divided Clock, Transmitter	DCE DCE
17	RC	Receiver Clock	DCE
18	DCR	Divided Clock, Receiver	DCE
19	SRTS	Secondary Request To Send	DTE
20	DTR	Data Terminal Ready	DTE
21	SQ	Signal Quality Detect	DCE
22	RI	Ring Indicator	DCE
23	DRS	Data Rate Selector Data Rate Selector	DTE DCE
24	SCTE	Serial Clock Transmit External	DTE
25	BUSY	Busy	DTE

DTE (Data Terminal Equipment): Computer or Terminal
DCE (Data Communications Equipment): Modem

Hardware Handshaking Connections

Null Modem Connections

Timing Diagram of Asynchronous Signal

Bi-directional Transmission

Null Serial Adapter

(for Testing the Serial Port)

Other Resources

• Serial Port Central
• Mac Serial Port Interfacing
• Fil's FAQ-Link-In-Corner: Serial Port/Modems
• Interfacing the PC

DISCLAIMER