How to make a null-modem cable

From ArticleWorld


Although they are becoming less common today, serial connections between two computers used to be rather popular some time ago. This was the case when Ethernet cards were too expensive and, in some cases, not necessary.

Nowadays, serial connections have been supersed by Ethernet connections. Nevertheless, they may be useful in some cases:

  • Getting diagnostic messages from computers with damaged video cards. Some operating systems broadcast messages to their serial ports, so that you can intercept them with a "serial console" (another computer, connected through a serial link).
  • Using a computer without its own monitor and keyboard, but also without running an additional service like SSH. It will not be exactly "remote", but it will save you the hassle of attaching monitors and keyboards. You can just use your laptop.

Guides concerning the configuration of the computers involved freely float around the Internet and depend on the operating systems being used, so we will not get into details here. What we will explain though is how to make a null-modem cable.

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Steps

You will need a serial cable and two serial connectors. You can get this at most computer shops.

1. Start by de-insulating the ends of the serial cable. Be careful not to cut the wires as well (they contain 9 or 25 wires).

2. Open the serial connectors and attach the wires to the ping, according to the following diagram: 

Pin of port #1....goes to....Pin of port #2
2                            3
3                            2
4                            6,1
5                            5
6,1                          4
7                            8
8                            7

or, for 25-pin serial connectors: 

Pin of port #1....goes to....Pin of port #2
2                            3
3                            2
4                            5
5                            4
6,8                          20
7                            7
20                            6,8

3. Close the serial connectors and secure the cable.

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