IBM PC connectors & pinouts

IBM PC (and compatibles) Pin-Outs, Connections and Cables
CONTENTS

Serial Ports
    Serial Port Mapping
    9-Pin Serial Port
    25-Pin Serial Port
    Null modem Connections
    Serial Loop-back Plugs

Parallel Printer Port
    Parallel Printer Port Mapping
    25-Pin Parallel Port
    Parallel Loop-back Plug for Checkit®
    Parallel Loop-back For Norton® Utilities
    Parallel Loop-back For PC-Certify®
    Parallel Transfer Cable for Interlnk
    36-Pin Centronics Printer Port

Power Supply Unit Connectors
AT Motherboard Power Connectors
Device Power Connectors

Rear Panel Connectors
    AT Keyboard Connector
    PS/2 Keyboard/Mouse Connector
    PS/2 Mouse Port to 9-Pin D-Type Adapter
    15-Pin Game Connector
    MDA and CGA Adapters
    EGA Adapter
    VGA Adapter

10BaseT Ethernet LAN
    T568A RJ45 pin assignments
    T568B RJ45 pin assignments
    Cross-Over Cable pin assignments

Common Modem to UK Telephone Socket
    Typical Hayes Compatible Modem Lead
    Typical UK Modem Lead (e.g. Pace)

Serial Ports

The IBM PC uses a DTE (Data Terminal Equipment) Connection standard presented on 9 or 25-pin male D-type connectors. Most Modems are DCE (Data Communication Equipment), fitted with 25-pin female D-type sockets.

I/O Mapping
COM1:	$3F8-3FF
COM2:	$2F8-2FF
COM3:	$3E8-3EF
COM4:	$2E8-2EF

9-Pin Serial Port
9-Pin Male D-Type Connector (Figure 1)
Pin	In/Out	Signal	Meaning	Pin	In/Out	Signal	Meaning
1	In	DCD	Data Carrier Detect	6	In	DSR	Data Set Ready
2	In	RD	Receive Data	7	Out	RTS	Request to Send
3	Out	TD	Transmit Data	8	In	CTS	Clear to Send
4	Out	DTR	Data Terminal Ready	9	In	RI	Ring Indicator
5	-	SG	Signal Ground

Figure 1

25-Pin Serial Port
25-Pin Male D-Type Connector (figure 2)
Pin	In/Out	Signal	Meaning	Pin	In/Out	Signal	Meaning
1	-	FG	Frame Ground	14	Out	STD	{Secondary TD}
2	Out	TD	Transmit Data	15	In	TC	{Transmitter Clock}
3	In	RD	Received Data	16	In	SRD	{Secondary RD}
4	Out	RTS	Request to Send	17	In	RC	{Receiver Clock}
5	In	CTS	Clear to Send	18	-	-	Unassigned
6	In	DSR	Data Set Ready	19	Out	SRTS	{Secondary RTS}
7	-	SG	Signal Ground	20	Out	DTR	Data Terminal Ready
8	In	DCD	Data Carrier Detect	21	In	SQ	{Signal Quality}
9	In		{+ve dc test voltage }	22	In	RI	Ring Indicator
10	In		{-ve dc test voltage}	23	Out		{Data Rate Selector}
11	-		Unassigned	24	Out	TC	{External Transmitter Clock}
12	In	SDCD	{Secondary DCD}	25
13	In	SCTS	{Secondary CTS}	*Functions in { } are not normally* supported.**

Figure 2

Null modem Connections (All Connectors Female)
Signal			D-25	D-9		D-25	D-9
SG	ó	SG	7	5	ó	7	5
TD	ó	RD	2	3	ó	3	2
RD	ó	TD	3	2	ó	2	3
RTS	ó	CTS	4	7	ó	5	8
CTS	ó	RTS	5	8	ó	4	7
DSR	ó	DTR	6	6	ó	20	4
DTR	ó	DSR	20	4	ó	6	6
DCD	ó	DCD	8	1	ó	8	1

Serial Loop-back Plugs (Female) As used by CheckIt®, PC Certify® and Norton Utilities® etc.
Signals to Link			D-25			D-9
TD	ó	RD	2	ó	3	3	ó	2
RTS	ó	CTS	4	ó	5	7	ó	8
DSR	ó	DCD	6	ó	8	6	ó	1
DSR	ó	DTR	6	ó	20	6	ó	4
DCD	ó	RI	8	ó	22	1	ó	9

Parallel Printer Port

The IBM PC uses a Centronics compatible parallel printer output presented on a female 25-pin D-type socket.

I/O Mapping
LPT1:	$378-37A
LPT2:	$278-27A

25-Pin Parallel Port
25-Pin Female D-Type Connector (figure 3)
Pin	In/Out	Signal	Meaning	Pin	In/Out	Signal	Meaning
1	Out	STROBE	Strobe	14	Out	AUTO FD	Auto Feed
2	In/Out	D0	Data Bit 0	15	In	ERROR	Error
3	In/Out	D1	Data Bit 1	16	Out	INIT	Initialise Printer
4	In/Out	D2	Data Bit 2	17	Out	SLCT IN	Select Input
5	In/Out	D3	Data Bit 3	18	-	Gnd	Ground
6	In/Out	D4	Data Bit 4	19	-	Gnd	Ground
7	In/Out	D5	Data Bit 5	20	-	Gnd	Ground
8	In/Out	D6	Data Bit 6	21	-	Gnd	Ground
9	In/Out	D7	Data Bit 7	22	-	Gnd	Ground
10	In	ACK	Acknowledge	23	-	Gnd	Ground
11	In	BUSY	Busy	24	-	Gnd	Ground
12	In	PE	Paper Out	25	-	Gnd	Ground
13	In	SLCT	Select

Figure 3

Parallel Loop-back Plug for Checkit®
25-Pin Male D-Type Connector (figure 4)
Busy	11	ó	17	Select Input
Acknowledge	10	ó	16	Initialise
Paper End	12	ó	14	Auto Feed
Select	13	ó	1	Strobe
Data Bit 0	2	ó	15	Error

Figure 4

Parallel Loop-back For Norton® Utilities
25-Pin Male D-Type Connector (figure 4)
Data Bit 0	2	ó	15	Error
Data Bit 1	3	ó	13	Select
Data Bit 2	4	ó	12	Paper Out
Data Bit 3	5	ó	10	*Acknowledge
Data Bit 4	6	ó	11	Busy

Parallel Loop-back For PC-Certify®
25-Pin Male D-Type Connector (figure 4)
Data Bit 2	4	ó	15	Error
Data Bit 3	5	ó	17	Select Input
Data Bit 4	6	ó	13	Select
Data Bit 5	7	ó	12	Paper Out
Data Bit 6	8	ó	10	Acknowledge
Data Bit 7	9	ó	11	Busy

With INTERLNK and INTERSVR DOS V 6.0 introduced the ability to use either a 7 wire "Null Modem" cable or a parallel cable to connect two PCs via the serial or parallel ports respectively. The "Null Modem" cable was covered earlier. The pin outs for the parallel cable are shown here:

Interlnk Parallel Transfer Cable
25-Pin Male D-Type Connector (figure 4)
25-D Male		25-D Male
2	ó	15
3	ó	13
4	ó	12
5	ó	10
6	ó	11
10	ó	5
11	ó	6
12	ó	4
13	ó	3
15	ó	2
25	ó	25

36-Pin Centronics Printer Port
36-Pin Female Amphenol Connector (Printer) (figure 5)
Pin	In/Out	Signal	Meaning	Pin	In/Out	Signal	Meaning
1	In	STROBE	Strobe	19	-	Gnd	Ground
2	In/Out	D1	Data Bit 1	20	-	Gnd	Ground
3	In/Out	D2	Data Bit 2	21	-	Gnd	Ground
4	In/Out	D3	Data Bit 3	22	-	Gnd	Ground
5	In/Out	D4	Data Bit 4	23	-	Gnd	Ground
6	In/Out	D5	Data Bit 5	24	-	Gnd	Ground
7	In/Out	D5	Data Bit 6	25	-	Gnd	Ground
8	In/Out	D6	Data Bit 7	26	-	Gnd	Ground
9	In/Out	D7	Data Bit 8	27	-	Gnd	Ground
10	Out	ACK	Acknowledge	28	-	Gnd	Ground
11	Out	BUSY	Busy	29	-	Gnd	Ground
12	Out	PE	Paper Out	30	-	Gnd	Ground
13	Out	SLCT	Select	31	In	INIT	Initialise Printer
14	In	AUTO FD	Auto Feed	32	Out	ERROR	Error
15	-	-	n/c	33	-	Gnd	Signal Ground
16	-	0V	Logic Ground	34	-	-	n/c
17	-	CHASSIS	Printer Frame	35	Out	LOGIC 1
18	-	-	n/c	36	In	SLCT IN	Select Input

Figure 5

Power Supply Unit Connectors

A typical AT power supply might have 8 Connectors: 2 x 6 pin for Motherboard Power, 4 large 4 pin and 2 smaller 4 pin connectors for disk, CD ROM and tape drive power.

AT Motherboard Power Connectors
P8		P9
Pin	Signal	Pin	Signal
1	Power Good	1	GND
2	+5v (or N.C.)	2	GND
3	+12v	3	-5v
4	-12v	4	+5v
5	GND	5	+5v
6	GND	6	+5v

A typical ATX power supply might have 7 Connectors: A 20 pin connector for Motherboard Power, 4 large 4 pin and 2 smaller 4 pin connectors for disk, CD ROM and tape drive power.

ATX Motherboard Power Connectors
20 Pin Female
Pin	Signal	Colour	Pin	Signal	Colour
1	+3.3V *	Orange	11	+3.3V *	Orange
2	+3.3V *	Orange	12	-12V	Blue
3	GND	Black	13	GND	Black
4	+5V	Red	14	PS_On	Green
5	GND	Black	15	GND	Black
6	+5V	Red	16	GND	Black
7	GND	Black	17	GND	Black
8	Power_Good	Grey	18	-5V	White
9	+5V_Standby	Purple	19	+5V	Red
10	+12V	Yellow	20	+5V	Red
Pins marked * are optional

Device Power Connectors (HDD/CD/TAPE etc.)
4 Pin Female
Pin	Signal	Colour
1	+12v	Yellow
2	Gnd	Black
3	Gnd	Black
4	+5v	Red

Rear Panel Connectors

AT Keyboard Connector
5-pin DIN Connector (female) (figure 6)
Pin	Signal
1	KBD CLK OUT
2	KBD SERIAL OUT
3	KBD RESET IN
4	KBD GND
5	KBD +5v

Figure 6

PS/2 Keyboard/Mouse Connector
6-pin Mini-DIN Connector (female) (figure 7)
Pin	Signal
1	Data
2	Reserved
3	Ground
4	+5V
5	Clock
6	Reserved

Figure 7

PS/2 Mouse Port to 9-Pin D-Type Adapter
PS/2 Mouse Plug (female)	9-Pin Male D-Type
1	9
3	5
5	1
4	8
For FULLY MS Mouse Compatible devices

15-Pin Game Connector
15-Pin Female D-Type Connector (figure 8)
Pin	Signal	Pin	Signal
1	+5v	9	+5v
2	Button 1-1	10	Button 2-1
3	X1	11	X2
4	GND	12	GND
5	GND	13	Y2
6	Y1	14	Button 2-2
7	Button 1-2	15	N/C
8	N/C
Buttons - Connect from GND to Button inputs
X and Y pots connect from +5v to X-Y inputs.

Figure 8

Display Adapter Connectors

Monochrome (MDA), Hercules, Colour (CGA) Adapters
9-Pin D-type Female Connector (figure 9)
Pin	Signal	Pin	Signal
1	GND	6	Intensity
2	GND	7	Video
3	N/C or RED	8	Horizontal Drive
4	N/C or GREEN	9	Vertical Drive
5	N/C or BLUE

Figure 9

EGA Adapter
9-Pin D-type Female Connector (figure 9)
Pin	Signal	Pin	Signal
1	GND	6	Secondary Green / Intensity
2	Secondary Red	7	Secondary Blue / Mono Video
3	Primary Red	8	Horizontal Drive
4	Primary Green	9	Vertical Drive
5	Primary Blue

VGA Adapter
15-Pin HDD-Type Female Connector (figure 10)
Pin	Signal	Pin	Signal
1	Red Video	9	[KEY]
2	Green Video	10	Sync GND
3	Blue Video	11	Monitor ID - Bit 1
4	Monitor ID - Bit 2	12	Monitor ID - Bit 0
5	GND	13	Horizontal Sync
6	Red GND	14	Vertical Sync
7	Green GND	15	N/C (Reserved)
8	Blue GND

Figure 10

10BaseT Ethernet LAN

Most Twisted Pair networks will use (EIA) T568A or T568B assignments.

T568A RJ45 pin assignments (Figure 11)
Pin	Pair	Colour (PRIMARY/trace)
1	3	WHITE/green
2	3	GREEN/white
3	2	WHITE/orange
4	1	BLUE/white
5	1	WHITE/blue
6	2	ORANGE/white
7	4	WHITE/brown
8	4	BROWN/white

Figure 11

T568B (aka 258a) RJ45 pin assignments (Figure 11)
Pin	Pair	Colour (PRIMARY/trace)
1	2	WHITE/orange
2	2	ORANGE/white
3	3	WHITE/green
4	1	BLUE/white
5	1	WHITE/blue
6	3	GREEN/white
7	4	WHITE/brown
8	4	BROWN/white

A Cross-Over jumper can be made by terminating one end of a cable as T568A and the other as T568B (i.e. crossing pairs 2 & 3) – Useful for ‘daisy-chaining’ hubs and back-to-back PC links.

10BaseT Cross-Over Cable pin assignments (Figure 11)
End A	ó	End B
1	ó	3
2	ó	6
3	ó	1
6	ó	2

Common Modem to UK Telephone Socket

Typical Hayes Compatible Modem Lead (US Source) (Figure 12)
BT Line Plug (BT431a or 631a)	ó	RJ11Plug (4/6 or 6/6)
1	ó	n/c
2	ó	3
3	ó	2
4	ó	5
5	ó	4
6	ó	n/c

BT631a and RJ11 6/6 6 way line plugs
Figure 12

Note: The plugs illustrated are the BT 631a and RJ11 (6/6) Line Plugs. The more common BT 431a and RJ11 (4/6) plugs are identical apart from the fact that Pins 1 and 6 are omitted; making the first and last usable connections Pin 2 and 5.

Typical UK Modem Lead (e.g. Pace) (Figure 12)
BT Line Plug (BT431a or 631a)	ó	RJ11Plug (4/6 or 6/6)
1	ó	n/c
2	ó	5
3	ó	4
4	ó	3
5	ó	2
6	ó	n/c