Why Are Twisted-Pair Cables Needed And How Do They Work?

There are some mysteries in life, and one that's always puzzled me is why twisted-pair cables can reliably transport information over much longer distances than a neat and straight serial or parallel cable where all the just go from pin to pin without any twisting. Anyone dealing with USB, RS232, RS485 and RS422 cables and converters likely has come across that syndrome, and most tend to simply accept it as another one of those physics things that you need a degree in electrical engineering to understand.

But since it is nice to know how and why things work, let's take a look what limits the length of straight-through cables and what enables twisted pairs to do better. Let's start by asking ourselves what a cable needs to do. That's simple: it needs to reliably let the other end know whether it has just transmitted a logical "zero" or a logical "one," also known as "space" and "mark." In RS-232 that's accomplished by sending either a positive or a negative voltage relative to a common signal ground. So if the receiving side sees a positive voltage in a certain range, it knows it's a "zero." If it sees a negative voltage, it knows it's a "one."

That seems simple and error-proof, but it's not. That's because wires pick up noise. Noise can enter a wire from a noisy power supply, which is called conductive noise. Common-impedance noise can happen when two circuits share a ground. Magnetic and electromagnetic noise occurs via external or internal electric and magnetic fields. Capacitive and inductive interference happens when the voltage in one wire affects the voltage in another. What it all boils down to is that the receiving end may no longer know with certainty that a received voltage is a one or a zero, which renders the signal useless.

This is why standard RS-232 serial cables have speed and distance limits (in general, the higher the speed, the shorter the distance). And it is also why RS-422 uses a different approach, one that allows RS422 (and Ethernet) connections to operate error-free at higher speed and over longer distances. One difference is that instead of using different voltage levels relative to a ground, RS422 uses what is called a "balanced" signal with negative and positive voltages. So now we're dealing with the voltage difference between two wires as opposed to voltage compared to notoriously unstable ground. But that alone would not allow for the much higher speed and stability. That's because the other difference is the use of "twisted-pair" wires.

Twisted pairs are just that, a pair of wires twisted together (often in very precise and proprietary ways). This way, if one wire is subjected to electromagnetic interference, on the next twist, it's the other wire that's affected and cancels out the noise in the first wire. Simple as that! This method is not totally foolproof, but it reduces electromagnetic interference enough to allow for reliable data communication over much greater distances. As the name implies, twisted pairs always consist of just two wires twisted around each other. However, there can be more than one twisted pair in a cable, and individual pairs and the whole cable can be shielded or unshielded, depending on the application. As a result, cables are rated by impedance and maximum bit rate. What it all means is that the proper choice of cables, protocols, converters and adapters will determine the overall success of a serial communications project.

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About The Author: Chris Robertson is an author of Majon International, one of the worlds MOST popular internet marketing companies. For tips/information, click here: RS232 Converters
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