DB-25 Connector

The DB-25 connector (named for its "B"-size "D"-shaped shell and 25 pins) is practically ubiquitous in the electronics industry. The DB-25 connector is used for a variety of purposes. Two common applications are RS-232/EIA-232 (serial) connections, and the parallel printer interface on the IBM PC. The DB-25 connector is also used for SCSI connections.

Here's a good set of figures for DB-25 male and female connectors, as viewed from the pin side (not the solder side).

DB-25 Male

DB-25 Female

Pins commonly used for RS-232/EIA-232 (serial):

DB-25	DB-9	Signal Direction	Signal Name
1		x	Protective Ground
2	3	DTE-to-DCE	Transmitted Data
3	2	DCE-to-DTE	Received Data
4	7	DTE-to-DCE	Request To Send
5	8	DCE-to-DTE	Clear To Send
6	6	DCE-to-DTE	Data Set Ready
7	5	x	Signal Ground
8	1	DCE-to-DTE	Received Line Signal Detector (Carrier Detect)
20	4	DTE-to-DCE	Data Terminal Ready
22	9	DCE-to-DTE	Ring Indicator

Click here to see the pin out of D-sub 9 connectors.

Common Null-Modem Connection

Signal Name	DB-25 Pin	D-sub 9 Pin		D-sub 9 Pin	DB-25 Pin
FG (Frame Ground)	1	-	X	-	1	FG
TD (Transmit Data)	2	3	-	2	3	RD
RD (Receive Data)	3	2	-	3	2	TD
RTS (Request To Send)	4	7	-	8	5	CTS
CTS (Clear To Send)	5	8	-	7	4	RTS
SG (Signal Ground)	7	5	-	5	7	SG
DSR (Data Set Ready)	6	6	-	4	20	DTR
CD (Carrier Detect)	8	1	-	4	20	DTR
DTR (Data Terminal Ready)	20	4	-	6	6	DSR
DTR (Data Terminal Ready)	20	4	-	1	8	CD

Click here to see the pin out of DB-9 connectors.

DB-25 pins commonly used for the IBM-PC parallel port:

DB-25 Pin	Centronics Pin	Signal Description	Signal Direction (at the PC)	Signal Function
1	1	STROBE	Output	Clocks data
2	2	DATA Bit 0	Output	Data line
3	3	DATA Bit 1	Output	Data line
4	4	DATA Bit 2	Output	Data line
5	5	DATA Bit 3	Output	Data line
6	6	DATA Bit 4	Output	Data line
7	7	DATA Bit 5	Output	Data line
8	8	DATA Bit 6	Output	Data line
9	9	DATA Bit 7	Output	Data line
10	10	ACKNLG	Input	Acknowledge receipt of data
11	11	BUSY	Input	Printer is busy
12	12	POUT	Input	Printer is out of paper
13	13	SEL	Input	Pinter is online
14	14	Auto Feed XT	Input	Autofeed
15	32	FAULT	Input	Indicates printer fault (or when printer is offline)
16	31	Input Prime or INIT	Output	Resets printer, clears printer buffer and initializes it
17	36	SLCT IN	Output	TTL high level
18-25	16, 17, 19-30, 33	Ground	N/A	Ground reference for signal pins 1-12, in most cables as twisted pairs.

Click here to see the pin out of Centronics connectors.

DB-25 pins commonly used for SCSI

At one time, Apple used a DB25 connector for SCSI (narrow), but this doesn't allow twisted pairs and is not compliant to SCSI standards. Some companies like Iomega still use this connector on new devices. Such connectors have been noted to have problems on SCSI busses using faster devices such as UltraSCSI (Iomega itself doesn't recommend using more than one DB25 device, but still uses it for Zip drives). The HD50 connector is preferred.

DB-25 Pin	LD-50 Centronics Pin	Signal Description	Signal Function
1	49	REQ	Request
2	46	MSG	Message
3	50	I/O	Input/Output
4	45	RST	Reset
5	44	ACK	Acknowledge
6	43	BSY	Busy
7, 9, 14, 16, 18, 24	1-11, 15-25, 35, 36, 40, 42	GND	Ground
8	26	D0	Data bit 0
10	29	D3	Data bit 3
11	31	D5	Data bit 5
12	32	D6	Data bit 6
13	33	D7	Data bit 7
15	48	C/D	Command / -Data
17	41	ATN	Attention
19	47	SEL	Select
20	34	DP	Data Parity
21	27	D1	Data bit 1
22	28	D2	Data bit 2
23	30	D4	Data bit 4
25	38	TermPwr	Termination Power (+5V)

Click here to see the pin out of Centronics connectors for SCSI.

You can find more SCSI info here.

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