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Twisted Pair Bus Wiring

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What is Twisted Pair Wiring?
It is the use of two wires that carry the exact same signal or power out to a far distance and back.  One wire carries the current going out (+) and the other carries it back (-) to form a complete circuit path.  These two wires form a "Wire Pair".  Unlike lose wires, the "Wire Pair" are mechanically tightly twisted together with multiple twist per foot to form a "twisted pair" cable.  The picture at the left shows a cable with 3 twisted pair wires inside.
 
 

 
Why twist?
Twisted wiring is the name of a electromechanical solution to an electrical problem.

Wires have inductance.  To learn more go here: Track & Wire Inductance
 
The electrical issue that effect the electrical performance of AC signal on long wire comes down to address both the inductance and its corresponding magnetic field.  The two are directly tied to each other.   The way you get ride of wire inductance is to get the two magnetic fields of each wire to overlap each other and cancel each other out.  No magnetic field, no wire inductance.


What are the benefits of Twisted Pair Wiring?

Twisted pair wiring offers the unique combination of :

1) Reduced noise pickup (low level signals) Think shielded cable.
2)
Reduced noise transmission (high power) Think shielded cable.
3)
Higher speed communications  (Higher usable signal frequency range or bandwidth)
4) Very low manufacturing cost benefits over all other alternative wire cable construction techniques such as Coax Cable.  

Stated another way
, twisted pair wiring offers almost all the benefit is a shielded coax cable without the expense.
 
To learn more about the technical details of twisted pair wiring, go here:Twisted Pair Bus Technical Discussion
 

Where can you use Twisted Pair Wiring?
 
Any where a pair of wires need to carry power or information in the form of an AC current that needs to run long distances with minimal loss of signal integrity and maintain high speed.  In other words, where reliable communication and control must be established over long wire runs.  A common example of twisted pair wiring is found in ethernet cables used to connect computers together.
 

Where to use Twisted Pair Wiring in DCC?
 
For DCC, there are three busses that can potentially use Twisted Pair wiring.
 
1) Cab/Throttle Bus:  The bus that connects cabs/throttles to the command station.
Often the DCC system has identified a cab bus wiring system that has been predetermined by the DCC system to work in most layouts as is.  However on large layouts, upgrading the cable system to use cheap CAT-5 cable will improve the bus performance.
 
2) Control/Booster Bus:  The bus that the command station uses to control boosters.
Same as the Cab bus situation.
 
3) Track Bus:  The bus that carries the DCC signal from the booster to the engines on the track.
The track bus is something you install and can be greatly improve electrically performance wise with the use of twisted pair construction on long cables runs on large layouts.  For more information, go here:
 
NOTE: Unless you having reliability problem with you Cab/Throttle Bus or Control/Booster bus, it is recommended you follow the DCC system manufactures wiring method.   Only on the largest of layouts would one consider substituting the type of wiring used.

Where NOT to use Twisted Pair Wiring in DCC?
 
Block Detection: Twisted pair wiring should not be used in any track bus wiring that runs AFTER any "current based" block detector.  Current based occupancy detectors use diodes or a transformer and are wired in series with the track block they are to monitor.
 
Why? One of the side effects of twisting the wires together is the increase in "leakage current" between the two wires caused by the large increase in capacitance between them.  (To learn more about wire capacitance go here:  Track & Wire Capacitance)  In other words wire capacitance can contribute to false occupancy detection issues.  This forces one to turn down the block detectors' sensitivity which can reduce the reliability in detecting a true block occupancy condition.
 
If block detection was not in use, this capacitance effect is a non issue.  But with block detection high sensitivity to ANY level of current flow on the track can see this wiring capacitance based leakage current as a potential train sitting in the block.  
 

Benefits of Twisted Pair Wiring
 
The goal using a "Twisted Pair" is to keep the mechanical spacing of the two wires as small and tight as physically possible over the entire length of the cable.  The tighter the better.  The tight spacing between the wires allows one to take advantage of physical properties of wire and environment they work in to improve communication and control over long cable runs. 
 
1) How does Twisted Pair Wiring work to reject noise?  
 
Simple Answer: In "lose wiring" often found in typical layout wiring, a noise source (usually a different wire carrying an unrelated signal) can be physically closer to one member of a wire pair than to the other over an entire wiring-run length.   In such cases, more noise (capacitively or inductively) couples to the closer wire than to the more distant one, producing a different noise voltage in one wire relative to the other.  This difference in noise level between the wire pair can be large enough to corrupt the data.  When you use the twisted pair, the noise source is equally close to each of the wires.  Therefore, the two wires pick up roughly equal noise voltages with the same polarity.  The "receiver" located at the far end of the cable is looking for a difference signal between the two two wires.  Instead it finds the identical noise voltage in both wires and is able to reject this noise voltage because they appear to be the same (no difference) or "common" to both wires.  Hence the noise voltage is unable to influence the important "differential" signal being sent down the twisted pair cable.
 
2) How does Twisted Pair Wiring work to maintain high speed communication?
 
It eliminated the natural self inductance property of wire which restricts the speed of the signal you can send down the cable.  With the two wires right next to each other, the opposite current flow in each wire will in turn create equal but opposite magnetic fields.  The magnetic fields from each wire will then cancel each other out.  The cancelation of the magnetic field in turn cancels out the natural inductance in the wire that all wires have.  Inductance is an electrical property of all wires that only effects AC signals.  Its existence in the wire opposes fast changing AC signals which means it will oppose high speed communication rates.  Inductance literally attacks digitals signal waveforms and distorts them and limits the upper frequency in which you can use to carry the signal.  With the cancelation of the inductance, high speed signals will be able to travel long distances before being slowed down again.  Why?  There is no such thing as perfect inductance cancelation which means some amount of inductance remains. 
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