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RS232 DB25(RS-232C)

DB25: View looking into male connector

DB25: View looking into female connector

Pin No.	Name	Dir	Notes/Description
1	-	-	Protective/shielded ground
2	TD	OUT	Transmit Data (a.k.a TxD, Tx) (ASYNC)
3	RD	IN	Receive Data (a.k.a RxD, Rx) (ASYNC)
4	RTS	OUT	Request To Send (ASYNC)
5	CTS	IN	Clear To Send (ASYNC)
6	DSR	IN	Data Set Ready (ASYNC)
7	SGND	-	Signal Ground
8	CD	IN	Carrier Detect (a.k.a DCD).
9	-	-	Reserved for data set testing.
10	-	-	Reserved for data set testing.
11	-	-	Unassigned
12	SDCD	IN	Secondary Carrier Detect. Only needed if second channel being used.
13	SCTS	IN	Secondary Clear to send. Only needed if second channel being used.
14	STD	OUT	Secondary Transmit Data. Only needed if second channel being used.
15	DB	OUT	Transmit Clock (a.k.a TCLK, TxCLK). Synchronous use only.
16	SRD	IN	Secondary Receive Data. Only needed if second channel being used.
17	DD	IN	Receive Clock (a.k.a. RCLK). Synchronous use only.
18	LL	-	Local Loopback
19	SRTS	OUT	Secondary Request to Send. Only needed if second channel being used.
20	DTR	OUT	Data Terminal Ready. (ASYNC)
21	RL/SQ	-	Signal Quality Detector/Remote loopback
22	RI	IN	Ring Indicator. DCE (Modem) raises when incoming call detected used for auto answer applications.
23	CH/CI	OUT	Signal Rate selector.
24	DA	-	Auxiliary Clock (a.k.a. ACLK). Secondary Channel only.
25	-	-	Unassigned

RS232 DB9(EIA/TIA 574)

DB9: View looking into male connector

DB9: View looking into female connector

Pin No.	Name	Dir	Notes/Description
1	DCD	IN	Data Carrier Detect. Raised by DCE when modem synchronized.
2	RD	IN	Receive Data (a.k.a RxD, Rx). Arriving data from DCE.
3	TD	OUT	Transmit Data (a.k.a TxD, Tx). Sending data from DTE.
4	DTR	OUT	Data Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised only when RI arrives from DCE.
5	SGND	-	Ground
6	DSR	IN	Data Set Ready. Raised by DCE to indicate ready.
7	RTS	OUT	Request To Send. Raised by DTE when it wishes to send. Expects CTS from DCE.
8	CTS	IN	Clear To Send. Raised by DCE in response to RTS from DTE.
9	RI	IN	Ring Indicator. Set when incoming ring detected - used for auto-answer application. DTE raised DTR to answer.

출처 : http://www.zytrax.com/tech/layer_1/cables/tech_rs232.htm