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Null Modem Cable

 

A null modem cable will enable you to connect two computers together for serial communications. It is a particularly designed cable that permits anyone to connect two computers directly to each other via their communications ports called RS-232 ports. Serial communications with RS232 is one of the oldest and most widely spread communication methods in computer realm. Null modems are specially helpful with portable computers because they enable the portable computer to exchange data with a bigger machine. Null modem cables have the TD (Transmit Data) and RD (Recieve Data) lines crossed over, permiting information to be sent from one computer to the other. They are similar to a crossover cable where it is the CTS (clear to send) and RTS (ready to send) lines are crossed over. Null-modem cables are particularly used for serial port connections.

9-pin to 25-pin Null Modem Cable Pinout

9 pin female connector

9 pin D-Sub Female

25 pin female connector

25 pin D-Sub Female

Signal

D-Sub 9

D-Sub 25

Signal
 

Receive Data

2

2

Transmit Data
 

Transmit Data

3

3

Receive Data
 

Data Terminal Ready

4

6+8

Data Set Ready + Carrier Detect
 

System Ground

5

7

System Ground
 

Data Set Ready + Carrier Detect

6+1

20

Data Terminal Ready
 

Request to Send

7

5

Clear to Send
 

Clear to Send

8

4

Request to Send

25-pin Null Modem Cable Pinout

25 pin female connector

25 pin D-Sub Female

 

Signal

D-Sub 1

D-Sub 2

Signal
 

Receive Data

3

2

Transmit Data
 

Transmit Data

2

3

Receive Data
 

Data Terminal Ready

20

6+8

Data Set Ready + Carrier Detect
 

System Ground

7

7

System Ground
 

Data Set Ready + Carrier Detect

6+8

20

Data Terminal Ready
 

Request to Send

4

5

Clear to Send
 

Clear to Send

5

4

Request to Send

9-pin Null Modem Cable Pinout

9 pin female connector

9 pin D-Sub Female

 

Signal

D-Sub 1

D-Sub 2

Signal
 

Receive Data

2

3

Transmit Data
 

Transmit Data

3

2

Receive Data
 

Data Terminal Ready

4

6+1

Data Set Ready + Carrier Detect
 

System Ground

5

5

System Ground
 

Data Set Ready + Carrier Detect

6+1

4

Data Terminal Ready
 

Request to Send

7

8

Clear to Send
 

Clear to Send

8

7

Request to Send

This data was contributed by the team at The Hardware Book.

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