Twisted Pair Bus Technical Discussion

This section covers the techinical aspects of twisted pair wiring.


1) Benefits of Twisted Pair Wiring

1) Minimize wire inductance.
2) Maximize digital signal waveform.
3) Reduce radiated noise from the twisted pair into adjacent wires.
4) Reduce inject noise from adjacent wires into the twisted pair.

Items #1 & #2 are about high speed communication.
Items #3 & #4 are about noise prevention.


2) How does Twisted Pair cable wiring work to maintain high speed communication?

It reduces the natural self inductance property of wire.  Inductance restricts the speed of the signal you can send down the cable. 

Mechanically it involves placing the two loose wires right next to each together that carry SAME signal/power out and back together, we can use the laws of wire physics to solve the inductance problem.  Twisting action allows us to take two loose wires and build a tightly spaced pair of wires.  The closer the two wires are to each other, the better the inductance reduction in the wire pair.

Physics wise it is all about using the magnetic field that appears around a wire when current is flowing in the wire against itself.  The polarity of the magnetic field is tied to the polarity/direction the current is flowing in the wire. If we place 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 two magnetic fields from each wire will then fight each other and cancel each other out.  The cancellation of the magnetic field in wires in turn cancels out the natural inductance in the wires.


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. 


3) How does this apply to DCC?

A track bus buy definition two wires tha form a closed loop current path (complete circuit) of the DCC power/signal current going from the booster out to the track and back to the booster.  Properly installed, that means the track bus current going down one its two wire comes back on the other wire. So the corresponding magnetic fields within each wire of the bus are:

1) same strength.
2) opposite polarity to each other.

This satisfy the requirement needed for wire inductance cancellation.


4) How well does the inductance cancellation work?

Mechanically the closer the two wires get to each other without shorting each other out, the better.  We are talking in terms of millimeters of distance between the two wires.   But no matter how close you get the inductance is only minimized and not zero.  Why not zero?  The magnetic field cancellation is NOT PERFECT.  Why not perfect? The wires in the bus cannot occupy the same geometric or mechanical center center line of each other.  The two wires bump into each other but cannot occupy the same space at the same time.  That means the magnetic fields do not share the exact same dead center alignment of origin or start point.   Stated another way, the magnetic fields are offset from one another by the spacing between the wires needed to allow space for wire insulation.  The end result is the magnetic field cancellation is NOT PERFECT.


5) How close to the wires need to be to each other to cause inductance cancellation.

What is also true is that the noise rejections and/or emissions get for the cable/bus. This is why we say Twist the Track Bus wiring together. Given it carries the highest current levels and all the electrical noise that trains create running down the track, we need to minimize the magnetic field emissions from the Track Bus wiring. The primary goal of the twisting wires is make sure the two wires that are carrying the same signal/power out and back stay in close proximity over the entire length of the bus run as much as possible. The secondary is the fact the spinning makes sure the two wire on average are the same distance from other parallel wires not related to bus. This further helps deal with noise rejection and emissions.


What makes a bus or wire a potential emission noise source?

When it carries high current! That is why we say keep the high current track bus away from all the other buses that carry low
current (Cab Bus and Control Bus). The less the perfect cancellation allows the noisy bus to transmit noise in the from of a magnetic field to the other bus nearby. Specifically it can attack nearby weaker lower current buses that also have less than perfect cancellation to pick up the noise.


1) How does Twisted Pair cable 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.



More specifically uses the magnetic field of one wire to counter act or cancel the magnetic field of the other wire!  This works because polarity of the current flow in the wire and the polarity of magnetic field outside the wire are tied to each other.  Since the current running in one wire is equal but opposite of the other wire, the two magnetic fields cancel each other out!




IF everything was perfect, no magnetic field equals no magnetic radiation in the air (the "M" in EMI) to coupled to adjacent wires (no crosstalk).  No radios will pickup the energy.  Since the magnetic field is tied to inductance, no inductance in the wire!  With no inductance in the wire, we can send down higher speed signals since there is nothing to oppose changes in current flow!  The opposite is true.  IF an adjacent wire sends out a strong magnetic field that cuts across our two twisted wires (cable), both wires in the cable pick up the same magnetic field and convert it to current in the wire going the SAME direction.  But they are not affected by it.  How?  When the two current in each wire meet at the end of the cable loop at the load, they collide into each other and cancel themselves out! This has the same result a shielding a cable from outside interference.



 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 the digital signal waveform and distorts them and limits the upper frequency in which you can use to carry the signal.  With the cancellation 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 cancellation which means some amount of inductance remains. 
THE WIRING STRATEGY



It's all a bit more complex that this because I made some simplification (ignored the electrical fields or "E" in EMI for radio) for the sake of clarity.  But they are reduced too.  Regardless, I think you get the idea of why it works.


9) Voltage Spikes versus Decoders: Items 1c, 1e and 1f together is the key reason why we can have a problem on our layout with DCC.  The problem varies with the size of the layout.  It ranges from simple destruction of DCC packets creating inconsistent control of trains or loss of decoders programming (CV reset) all the way up to the worse case scenario of decoders blow up!  The loss of programming or blowing decoders always related to some short circuit event on the track such as the common derailment. Low energy voltage spikes occur naturally in the track circuit when there is no short circuit involved.  The intermittent nature of the wheel contact make this happen.  The current in the track is only at the current level the motor needs to run.  But if you get a short circuit, the current flow in the track bus wires goes instantly to the maximum level as determined by the booster you have.  The energy stored in the magnetic field will be at its maximum.  One the short opens up as the derailment continues on, BANG, the strongest voltage spike possible is dumped on the track.  The cycle can be repeated over and over until the derailment stops.  Given that the electronic circuit used has the lowest voltage rating (relative to the wires and track), they easily become the victim of these spikes.  If a strong enough or frequent enough spikes are generated, you typically can lose programming but eventually the circuit on the decoder blows up too.

10) Killing voltage spikes. Two solutions:

10a) Twist the track feeds: Greatly reduces the inductance and thus the energy that is behind problems such as blowing up decoders.  Do this with new layout construction only.

10b) Install a RC Filter/Terminator: Provide a path for that current to flow when the short circuit opens up clamping the spike voltage.  If you have existing wiring, this is often the best solution.

You can do one, or the other or both. I personally do both.

-----------------


Twisted pair cable consists of a pair of insulated wires twisted together. It is a cable type used in telecommunication for very long time. Cable twisting helps to reduce noise pickup from outside sources and crosstalk on multi-pair cables.

Twisted pair cable is good for transferring balanced differential signals. The practice of transmitting signals differentially dates back to the early days of telegraph and radio. The advantages of improved signal-to-noise ratio, crosstalk, and ground bounce that balanced signal transmission bring are particularly valuable in wide bandwidth and high fidelity systems. By transmitting signals along with a 180 degree out-of-phase complement, emissions and ground currents are theoretically canceled. This eases the requirements on the ground and shield compared to single ended transmission and results in improved EMI performance.

The most commonly used form of twisted pair is unshielded twisted pair (UTP). It is just two insulated wires twisted together. any data communication cables and normal telephone cables are this type. Shielded twisted pair(STP) differs from UTP in that it has a foil jacket that helps prevent crosstalk and noise from outside source. In data communications there is a cable type called FTP (foil shielded pairs) which consists of four twisted pair inside one common shield (made of aluminium foil).

When cable twisted at constant twist rate over the lenght of the cable, a cable with wel defined characteristic impedance is formed. Characteristic impedance of twisted pair is determined by the size and spacing of the conductors and the type of dielectric used between them. Balanced pair, or twin lines, have a Zo which depends on the ratio of the wire spacing to wire diameter and the foregoing remarks still apply. For practical lines, Zo at high frequencies is very nearly, but not exactly, a pure resistance. Because the impedance of a cable is actually a function of the spacing of the conductors, so separating the conductors significantly changes the cable impedance at that point.

When many twisted pairs are put together to form a multi-pair calbe, individual conductors are twisted into pairs with varying twists to minimize crosstalk.