There are some mysteries in life, and one that's all the time puzzled me is why twisted-pair cables can reliably converyance 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. Whatever dealing with Usb, Rs232, Rs485 and Rs422 cables and converters likely has come over that syndrome, and most tend to naturally accept it as other 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 distance of straight-through cables and what enables twisted pairs to do better. Let's start by request 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 closed by sending whether a safe bet or a negative voltage relative to a base signal ground. So if the receiving side sees a safe bet voltage in a safe bet range, it knows it's a "zero." If it sees a negative voltage, it knows it's a "one."
[Parallel To USB Cable]
That seems straightforward 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 thorough 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 distinct approach, one that allows Rs422 (and Ethernet) connections to operate error-free at higher speed and over longer distances. One contrast is that instead of using distinct voltage levels relative to a ground, Rs422 uses what is called a "balanced" signal with negative and safe bet voltages. So now we're dealing with the voltage contrast in the middle of 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 contrast is the use of "twisted-pair" wires.
Twisted pairs are just that, a pair of wires twisted together (often in very accurate and rights 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. straightforward as that! This recipe is not totally foolproof, but it reduces electromagnetic interference adequate to allow for reliable data communication over much greater distances. As the name implies, twisted pairs all the time consist of just two wires twisted colse to each other. However, there can be more than one twisted pair in a cable, and personel 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 allowable choice of cables, protocols, converters and adapters will determine the allembracing success of a serial communications project.
Why Are Twisted-Pair Cables Needed and How Do They Work?