Twisted Pair Bus Wiring

Below is a discussion about using "twisted pair: wiring technology to address potential DCC control and reliability problems people can experience on a large layout.  This section is written in Layman's terms as much as possible.

IN THiS SECTION
1) What are the layout conditions that I need to be concerned about that suggest twisting wiring is beneficial?
2) What are the DCC problems that twisting wires solve?
3) DCC Problem Analogy
4) What is Twisted Pair Wiring?
5) Why Physically Twisting?
6) What are the benefits of Twisted Pair Wiring?
7) Where can you use Twisted Pair Wiring?
8) Where to use Twisted Pair Wiring in DCC?
9) Where NOT to use Twisted Pair Wiring in DCC?


1) What are the layout conditions that I need to be concerned about that suggest twisting wiring is beneficial?

Simplistically it comes down to a layout that is larger than a small 2 car garaged.

Specifically twisting is not necessary if the track bus wiring path length starting at the booster to the farthest track power connection point of track is at or UNDER 30 Feet or 10 meters.  That said, there is no harm in twisting even if you under 30Ft.

With large layouts, the DCC problems appear typically at the location farthest from the booster in terms of track bus wires path length.  As often as the case, Murphy's law, the problems occur on a part of the layout where the trains just are on the main line running and no throttle input is required. So you may have a problem but you cannot see it because you not attempting to control the train when it is happening.  Ignorance of the problem is bliss.



2) What are the DCC problems that twisting wires solve?

The following is a list of DCC problems that can be solved.

The list of problems is from foggy to very bad.  Coincidentally they are also in terms of the least visible to the most visible.

1) Decoder less responsive to DCC commands. Decoder seems slow and inconsistent in getting throttle commands.
2) Decoder missing DCC commands. Engine does not respond to commands on certain parts of the layout.
3) Decoder running away after a short/derailment event.
4) Decoder lossing program settings (CV values).
5) Decoder blowing up. Someone let the smoke out.


3) DCC Problem Analogy

With respect to the list above, the analogy problem is like walking down into a unlit tunnel (Track Bus cables) attempting to read a book (Book of DCC commands). When your in the opening of the tunnel (near the booster) the light is very bright and reading the book is clear and easy (Decoder decodes 100% of the DCC commands). But the deeper you go into the tunnel (the longer the track bus cables get) the harder it gets to read the pages (Decoder is getting less and less successful decoding the DCC commands).

Problem 1:

It getting darker and your skipping sentences but it easy to get back on track,.

Problem 2:

It is now dark enough you cannot read words consistently

Problems 3

Your losing you place on the page. Your getting lost. Making reading decision mistakes.

Problems 4

Your lost and blind now hitting your head on the support beams of the tunnel but still holding the book. Worse case you sit down waiting to be rescued (Reprogrammed)

Problem 5:

Your fallen down a shaft. The book is gone. Your dead.


4) 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.
 




5) Why Physically Twisting?

1) The primary goal of the twisting wires is make sure the two LOOSE wires that are carrying the same signal/power out and back stay in close proximity over the ENTIRE length of the bus (cable) run as much as possible.

2) The secondary goal is keep the two wires of the cable the same distance from all the other parallel wires not related to bus.


6) 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:How Twisted Pair Wiring Works


7) 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.


8) 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.


9) 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.