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What is a null modem and why is it needed?

What is a null modem and why is it needed?
A null modem is a device that basically turns communications into two way communications through a straight wired connection. This is for half-duplex environments where you cannot send and receive on the same cable pairs at the same time (full-duplex). Basically, it swaps the send and receive pairs on one end so that the communication travels on one send pair, gets looped and travels back on the receive pairs.
This was last published in January 2005

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