How to Wire a DB9 Null Modem Cable

Written by norm dickinson Google
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How to Wire a DB9 Null Modem Cable
A female DB9 connector can be used to link two computers. (serial adaptor image by Paul Moore from Fotolia.com)

A DB9 null modem cable, often called a LapLink cable, can be fabricated by soldering a few connections onto DB9 connectors on opposite ends of a cable. Null modem cables are used to connect two computers together that do not have network access or USB ports or computers running old operating systems such as various versions of MS DOS. The later versions of MS DOS even included utilities that specifically supported file transfers and printing across null modem connections between two machines by designating one machine as the host and the other machine as the client.

Skill level:
Moderate

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Things you need

  • Wire stripper
  • Cable with at least seven 28 gauge or larger insulated copper conductors
  • 2 DB9 female D-sub solder-type connectors with pins
  • Solder
  • Electronic soldering iron
  • 2 DB9 plastic hoods with connectors
  • Multimeter

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Instructions

    Soldering and Assembly

  1. 1

    Strip about an inch of cable back from each end and remove the insulation from the end of seven conductors for about a quarter inch, being sure to use the same conductors on each end of the cable as indicated by the colour coding on the conductors. Cut any additional conductors back flush with the stripped insulation on the cable.

  2. 2

    Solder one conductor to Pin 3 on the first DB9 connector, and solder the other end of the same conductor to Pin 2 on the second DB9 connector at the other end of the cable. Solder one conductor to Pin 2 on the first DB9 connector, and solder the other end of the same conductor to Pin 3 on the second DB9 connector at the other end of the cable. At this point, there should be a connection from Pin 2 on one end to Pin 3 on the other end of each connector, known as a crossover, which is another word for this type of cable. These two pins are used for transmit (TR) and receive (RD) signals.

  3. 3

    Solder one conductor to Pin 6 on the first DB9 connector, and solder the other end of the same conductor to Pin 4 on the second DB9 connector at the other end of the cable. Solder one conductor to Pin 4 on the first DB9 connector, and solder the other end of the same conductor to Pin 6 on the second DB9 connector at the other end of the cable. At this point, there should be a connection from Pin 6 on one end to Pin 4 on the other end of each connector. These two pins are used for data set ready (DSR) and data terminal ready (DTR) signals.

  4. 4

    Solder one conductor from Pin 5 on the first DB9 to Pin 5 on the second DB9 to complete the cable connection. This pin is used for the signal ground (SG).

  5. 5

    Solder one conductor to Pin 7 on the first DB9 connector, and solder the other end of the same conductor to Pin 8 on the second DB9 connector at the other end of the cable. Solder one conductor to Pin 8 on the first DB9 connector, and solder the other end of the same conductor to Pin 7 on the second DB9 connector at the other end of the cable. At this point, there should be a connection from Pin 7 on one end to Pin 8 on the other end of each connector. These two pins are used for request to send (RTS) and clear to send (CTS) signals.

  6. 6

    Attach the DB9 hoods to the cable and test the connections with a multimeter to ensure proper wiring and connectivity.

Tips and warnings

  • MS DOS Interlnk and Intersvr applications are included with DOS versions 6 and above installations, and these files can be copied to computers using earlier versions of DOS. Newer operating systems also support direct cable connections with various utilities and third-party software is available to use this type of connection.
  • Use caution when soldering, as the soldering iron gets very hot and it is easy to get burnt or to melt the plastic around the pins when soldering conductors to them.

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