A difference in voltage between two conductors is how a receiver sees a signal, a potential difference. As a result, it takes two wires to make a lane for a signal to travel on. In fiber optic, light only needs the one glass tube to make its journey. As electric signals travel through conductors, the electrons that make the signal generate magnetic fields around their conductors. This “radiation” of magnetic fields affects electrons in nearby conductors causing a false signal in those nearby conductors. This problem is referred to as “crosstalk”. Ways to conquer crosstalk are shown below. In fiber optic, there is NO crosstalk, light traveling through the glass tube doesn’t get out of the cable and can’t effect adjacent cables because photons don’t generate magnetic fields like electrons do.
RS232, Centronics parallel, IEEE 1284, USB, Thinnet, Audio, Video, and “single end” SCSI are just a few examples of Unbalanced signals. Most unbalanced signals travel Coax cables but a few do not. As mentioned before, it takes two wires to make a lane for data. In the case of the coax, one wire runs the middle while the other wire is the foil or braid that wraps around the outside. The two signals of an unbalanced connection are referred to as “signal” and “ground”. The ground is the zero reference while the signal has a voltage level that is above or below zero. This voltage level determines weather the signal is a 1 or a 0 (VGA, Audio & Video are Analog signals, the analog signal can have a voltage level anywhere between the high and low voltage levels). In RS232, the “ground” for all signals is shared on one conductor; this is also true of some other Unbalanced signals. RS232, Parallel and SCSI are more vulnerable to crosstalk since they have more than one signal in the same jacket and in some cases, sharing the same ground. Parallel and Single End SCSI suffer the worst, We usually don’t guarantee a parallel cable (Centronic or IEEE 1284) to work any longer than 40 to 50 feet. Because the signal is stronger for RS232, it can usually reach lengths of 300 feet, but as the data or BAUD rate is increased, the range is shortened. In coax cables, the “radiation” generated by the signal on the center wire gets absorbed by the foil or braid on the outside. Coax cables are the way to fight against crosstalk for Unbalanced signals.
RS422, Differential SCSI, Ethernet 10Base-T, and Ethernet 100Base-T are just a few examples of balanced signals. Balanced signals have often been called “current loop” signals and travel on “twisted pairs” (UTP for Unshielded Twisted Pair or STP for Shielded Twisted Pair). The two signals in a balanced pair are like opposite charges of each other. What that means is if one wire has 12 volts, the other wire will have -12 volts. As the signal travels the pair, one wire radiates a magnetic field but its partner wire generates an opposite field, the two fields cancel out. This canceling is how balanced signals conquer crosstalk. Since Twisted pair wire is usually cheaper then Coax wire, balanced signals are more popular. Ethernet used to travel on a Coax (10Base-2 or Thinnet) but lately has been traveling on twisted pairs (10Base-T, 100Base-T). 100Base-T is so fast that the wire has to meet some specifications to carry it. The Cat 5 cable requires that the pairs be twisted tight enough to continue the canceling of the radiation. Also if the pairs are untwisted too much, the canceling won’t happen and you’ll get that dreaded crosstalk.
The impedance is a value determined by the capacitance and attenuation effects that a wire has on the signal traveling through it. It isn’t necessary to explain the full details. Each signal has it’s own rating (Arcnet has 93 ohm, Audio and Video have 75 ohm, and Thinnet has 50 ohm). Using a cable with the wrong rating will distort the signal. Twisted pair cables also have an “impedance” but most of the time, their name usually determines the signal they carry (a Cat 5 is made for 100Base-T).
What is the Balun?
Some users have Unbalanced signals like Arcnet or Thinnet, which use coax cables, but they do not want to pull expensive coax cables. The Balun allows them to string twisted pair in place of coax. Baluns are used in pairs, one at each end of the twisted pair wire. The coax side of the Balun has an impedance rating and you would need to find the balun with the impedance rating to match the signal. The word “Balun” comes from combining the words “BALanced” with “UNbalanced”. PLEASE NOTE: Putting a Balun on a Thinnet port doesn’t convert it to 10Base-T, a converter would be needed for that.