1. PREFACE

Diagrams and tables below explain parallel, serial, and null-modem configurations.  Pay close attention to the "female" versus "male" plugs when considering pinouts.  Usually the male connector will be an exact opposite of the female, note that I underlined "usually"

2. BASIC CONCEPTS OF COMPUTER CABLING

Parallel ports consist of only 25 pin port adapters called "DB25" where serial ports can consist of either a 25 pin port adapter called a "DB25" or 9 pin adapter called a "DB9" port adapter. Whether the port is a 9 pin or 25 pin it can accomplish all of the same tasks that serial port communications have been designed for.

Each adapter can be a male type connector with pins or a female type adapter with tiny holes.  Generally a PRINTER port, or "LPT" on the back of a workstation is female type adapter and a male DB25 pin cable on it for PRINTER connection.

"COM" ports on devices are usually male for the serial ports but it may not be depending upon the hardware. Below are diagrams of a DB25 and DB9.

3. IBM DB25 and DB9 diagrams.

Note! Diagrams show the view you see if your were to look into the connection from the outside

DB25 Connector: DB9 Connector:

 

 

 

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4. IBM DB25 and DB9 numbering scheme. (Pin numbers)

Each pin has a number assigned to it. When connecting null modem, for example, it is important to know these numbers in order to select the correct cables, or when making your own cables.

 

DB25 Connector
Chart #1 (Female)
Chart #2 (Male)
DB9 Connector
Chart #3 (Female)
Chart #4 (Male)

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5. IBM Serial port Pin Descriptions (Wire name) for each pin number.

For following all connections we may use these chart for transmission of Data via Serial Port.

Pin assignments
DB25
2 TD Transmit Data
3 RD Receive Data
4 RTS Request to send
5 CTS Clear to send
6 DSR Data Set Ready
7 SG Signal Ground
8 DCD Data Carrier Detect
20 DTR Data Terminal Ready
22 Ring Ring Indicator
Pin Assignments
DB9
1 DCD Data Carrier Detect
2 RD Receive Data
3 TD Transmit Data
4 DTR Data Terminal Ready
5 SG Signal Ground
6 DSR Data Set Ready
7 RTS Request to Send
8 CTS Clear to Send
9 Ring Ring Indicator

Note: In DB25 serial cables the following numbered pins are NOT USED:
    1, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 23, 24, & 25

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6A. IBM Parallel port Pin Descriptions (Wire name) for each pin number.

Data transmissions via Parallel Port

Pin Assignments of Parallel Port (LPT1)
Female DB25 on PC
<= In
or
=> Out		 DB25 Pin Centronics Pin Signal Registry Bit Function Notes
=> out 1 1 -Strobe C0- Set Low p1ulse >0.5 us to send
=> out 2 2 Data 0 D0 Set to least significant data
=> out 3 3 Data 1 D1 ...
=> out 4 4 Data 2 D2 ...
=> out 5 5 Data 3 D3 ...
=> out 6 6 Data 4 D4 ...
=> out 7 7 Data 5 D5 ...
=> out 8 8 Data 6 D6 ...
=> out 9 9 Data 7 D7 Set to most significant data
<= in 10 10 -Ack S6+ IRQ; Low Pulse ~ 5 uSeconds, after accept
<= in 11 11 +Busy S7- High for Busy/Offline/Error
<= in 12 12 +Paper End S5+ High for out of paper
<= in 13 13 +Select In S4+ High for printer selected
=> out 14 14 -Auto Fd C1- Set Low to auto feed one line
<= in 15 32 -Error S3+ Low for Error/Offline/Paper End
=> out 16 31 -Init C2+ Set Low pulse > 50uS to init
=> out 17 36 -Select C3- Set Low to select printer
== 18-25 19-30,33,17,16 Ground - Do not connect any of these grounds to a shield

Note:  Some cables, ports, or connectors may not connect all grounds. Centronics pins 19-30 and 33 are "twisted pair return" grounds, while 17 is "chassis ground" and 16 is "logic ground"

"<= In" and "=> Out" are defined from the viewpoint of the PC, not the printer. The IRQ line (-Ack/S6+) is positive edge triggered, but only enabled if C4 is 1.

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6B. IOMEGA ZIP DRIVE / Any scanner for Parallel Port Cable Pinouts
(Also called Straight-through PP Cable)

Most external parallel port devices like Iomega ZIP-Drives and most scanners use straight-through parallel port cable with straight pinouts like 1 to 1, 2 to 2, 3 to 3 and so on up to 25 to 25.  The straight-thru cable is also used for printer data switches between motherboard and the data-switch.

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7A. Parallel Port Laplink Cable Pinouts

Needed to make cable:

1. Two DB25 male sockets.
2. Shielded cable with 9 pairs of wire.

DCC Parallel Laplink Cable Pinouts.
Male DB25 ==>> Male DB25
1 Both Not used
2 to 15
3 to 13
4 to 12
5 to 10
6 to 11
7 Both Not used
8 Both Not used
9 Both Not used
10 to 5
11 to 6
12 to 4
13 to 3
14 Both Not used
15 to 2
16 Both Not used
17 to 19
18 to 18
19 to 17
20 Both Not used
21 to 21
22 to 22
23 to 23
24 Both Not used
25 to 25
Pinbody* to Pinbody

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8. IBM Null-Modem Cable pinouts. (DB25 to DB25), (DB9 to DB9) and (DB25 to DB9).

Only eight of the nine pins are used in a null modem even though there are 25 pins on DB25 sockets and 9 on DB9's.  You need 8 or 9-wired cable with two DB25-Female pin socket or DB9 Female pin socket or one of both.

Null-Modem Pinouts
DB25 to DB25
2 to 3
3 to 2
4 to 5
5 to 4
6 and 8 to 20
7 to 7
20 to 6 and 8
Null-Modem Pinouts
DB9 to DB9
1 and 6 to 4
2 to 3
3 to 2
4 to 1 and 6
5 to 5
7 to 8
8 to 7
Null-Modem Pinouts
DB25 to DB9
2 to 2
3 to 3
4 to 8
5 to 7
6 and 8 to 4
7 to 5
20 to 6 and 1

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9. IBM Normal-Modem Cable pinouts. (DB25 to DB25) and (DB25 to DB9).

IMPORTANT : Both pin sockets must be of DIFFERENT type. This means that for modem-side of cable we need to use MALE type of DB25 pin. And on CPU side of cable we need to use FEMALE type of DB25 or DB9 pin.

Chart #9 Chart #10
Normal-Modem Pinouts
DB25 to DB25
2 to 2
3 to 3
4 to 4
5 to 5
6 to 6
7 to 7
8 to 8
20 to 20
22 to 22
Shield * to Shield *
Normal-Modem Pinouts
DB25 to DB9
2 to 3
3 to 2
4 to 7
5 to 8
6 to 6
7 to 5
8 to 1
20 to 4
22 to 9
Shield * to Shield *

* Shield is OPTIONAL. We easily get cable with 9-wires in it, but not 10 then the shield can be attached with shielded wire.

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10.The pin-configuration of Internal Serial Port Cables inside Cabinet.

The pin-configuration of internal serial cables (the one that connects from the motherboard to the case) are different for different motherboards, which come in two types. One with straight pins as in old boards like 286/386/486. Another is twisted style cable like in latest Pentiums. The male DB9 pin is connected to serial port on motherboard with 9 wired cable in following two types. Both Serial ports on motherboard have 10 wires but only 9 of them to connect to male DB9 Serial Pins.

DB9 Pinouts of
STRAIGHT SERIAL PORT
on 286/386 systems
Cable Wire number
on motherboard		 Wire Connected
to male DB9 pin
on cabinet.
1 Red Wire 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
 
DB9 Pinouts of
TWISTED TYPE SERIAL PORT
on modern systems
Cable Wire number
on motherboard		 Wire Connected
to male DB9 pin
on cabinet.
1 Red Wire 1
2 6
3 2
4 7
5 3
6 8
7 4
8 9
9 5

The MALE DB25 pin is connected to serial port on motherboard with 9 wired cable in following diagrams. Both serial ports on motherboard have 10 wires to connect to Male DB25 serial pins as follows

DB25 Pinouts of
STRAIGHT SERIAL PORT
on 286/386 systems
Cable Wire number
on motherboard		 Wire Connected
to male DB25 pin
on cabinet.
1 Red Wire 8
2 3
3 2
4 20
5 7
6 6
7 4
8 5
9 22
 
DB25 Pinouts of
TWISTED TYPE SERIAL PORT
on modern systems
Cable Wire number
on motherboard		 Wire Connected
to male DB25 pin
on cabinet.
1 Red Wire 8
2 6
3 3
4 4
5 2
6 5
7 20
8 22
9 7

Note: #10 wire from motherboard is an optional line which can be connected to pin 1 of DB25 with no harm.  Above are the exact pinouts of the all four types of working serial ports with me for IBM Machines.