ground
I was reading about Serial communication in computers and embedded systems and found the following statement –
Computers connected by wires must have a common ground reference,
typically provided by a ground wire in the cable. (Source)
What is the reason for this? I thought that the ground wire was only for protection against electrical leakage. What would be the need for a common ground wire in communication networks?
Ground connection or referencing is used when it is used because long experience has shown it to be the best choice in practice. "Reinventing the grounding "wheel"" may have its place in some cases, but usually not. In many cases there are competing aspects, but the overall best result is gained by using ground. Power distribution systems are one such example.
Mains or grid voltage systems would be safer if the system was entirely NON ground referenced, and this is the principle that safety "isolation transformers" use BUT the moment that a fault fully or partially grounds one leg of the system anywhere on a circuit then the whole system becomes lethally dangerous to users.
Note that ONLY ONE tool should be used with an isolation transformer, and the transformer should be located near the tool. Using long cable runs after the transformer and two or more tools risks a fault to ground in one tool or wiring leaving the other unprotected.
The difficulty in keeping a system isolated is in practice (which is what counts) far harder than the issues caused by grounding. Some shipboard power systems do have both conductors floating relative to ship ground (= seawater potential when you are floating in salt water) BUT and fault to ground is dangerous, as above, ad great effort is made to track down and remove any ground faults. In a land based system that was not ground referenced, any fault to ground on the same phase would affect all users on the same phase. So a whole street of houses may be affected by a fault on one circuit in one house.
Once you have a ground referenced system the safety aspects of detection and management for individual circuits are easily handled. Earthed housings provide both protection and detection, fault currents flow to ground and can be either "encouraged" to allow easy fault termination (fuses) or detected at very low level (ELCB / GFI). Ground referencing is an overall positive in domestic power systems.
Few modern systems use ground as an actual conductor.
SWER (Single Wire Earth Return) power systems were much used at one time and are still used in some rural systems. I saw one here (NZ)some months ago but they are rare. They are in fact very useful and cost effective but are generally eliminated for reasons which often do not make technical sense. The cost of providing a good enough ground connection at each end is in most cases low compared to he cost of many km of adding an extra conductor.
19 kV SWER line:
Superb SWER slideshow / tutorial
SWER - Australian experience with application to developing country use
RF signals are often "launched" as imbalanced signals against a phantom image reflected in the ground. A typical quarter wave vertical radiator has an implicit image reflected in the ground plane. The tall towers of AM brodcast stations almost all use this system. There are economies in materials used compared with dipole or other antennas, radiation pattern is omnidirectional and radiation angles are suited to direct wave communications - most audiences are near the transmitter for AM broadcast stations.
In systems that need grounding, techniques have been developed to provide grounds which are adequately good to adequately minimise the effects of local conditions. Ground proper is of essentially zero resistance as it is of sensibly infinite size. Connecting the local ground to the actual ground is the challenge and methods and needs are well understood for each relevant application.
Yes it is possible, but only really as a extension to radio if you don't want to consider systems that use a implied second conductor, like the earth.
Radio works, so you can use the single wire to greatly decrease the loss between transmitter and receiver. With radio propagating thru space, the power per area falls off with the square of the distance to the transmitter. The falloff with a wire is quite different, with modest distances yielding less loss. The loss in a wire is exponential due to ever-increasing resistance, so eventually radio actually wins over wire.
You can also have what appears to be a single wire system by implicitly using some nearby large conductor, like the earth, as the other "wire". Some old commercial power distribution systems worked this way, with the earth carrying the return currents. If you can send power this way, you can also send information this way. However, no such power systems are being built new, and I think all the old ones have been phased out and replaced with full out and back wires.
Best Answer
Voltage is relative. One voltage level only makes sense if you have another voltage level to compare it to. If you would look at the signal wire alone you wouldn't get any wiser, you wouldn't measure a voltage. You need a reference to measure against, so that you can say that the signal is so-and-so-many volts, referenced to ground.
That referenced to ground is implied when we talk about voltage levels, unless otherwise indicated. Balanced signals are often symmetrical about ground, so that when on 1 wire the voltage goes to +1 V on the other wire it will go to -1 V. Nicely symmetrical. But the receiver won't look at one wire and see the +1 V; it will look at the voltage difference between the two wires, and then ground is not relevant. Such signals will most often be transmitted over twisted pair cables, where one wire is the positive signal and the other one the negative signal. There's no ground wire.
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Kortuk wants to see a pretty picture. Again. (sigh)
Et voilà, une jolie image. Don't look at En-Ya, this is about my wall clock. How high does it hang? Most people will say "about 2 meters". That's because we usually will take the floor as reference level, our "ground". But my apartment is on the second floor, and if I take street level as reference the clock will be at 8 m above street level. A geographer may suggest 35 m above sea level. So "height" has no meaning without a reference, and with voltage it's the same thing.