Null modem

Thread Starter


Joined Oct 7, 2011
My null modem scheme is:

As you can see, there are 3 wires. I opened connector shell and I saw one more wire. That wire connects metal parts of connectors. (picture below)

I was wondering, what is purpose of that wire? Should my cable work without that wire?
Last edited:


Joined Mar 24, 2008
It is ground, and is there for noise suppression. It will likely not work without it, since it is there as a conductor also.


Joined Oct 2, 2009
Presumably, the reason you are using a NULL modem is because you want to connect one computer to another and the cable is less than 12 feet for example. The bare wire is connected to the shield of the cable and connected to the metal casing of the connector. This is called "chassis ground" which is different from "signal ground" (pin-5).

The chassis ground and shielding in the cable provides protection from EMI (electro-magnetic interference). The cable should work fine without this connection. You should have at least one end of the bare wire connected.