Distributed vs Central

There are two type of wiring topologies: Distributed And Point to Point (AKA "star")

The following topics are discussed here:

1) DISTRIBUTED WIRING
2) POINT TO POINT (STAR) WIRING
3) WHICH WIRING SYSTEM IS BETTER?
4) MIXING THE TWO WIRING SYSTEMS
5) WHICH WIRING SYSTEM DO DCC PRODUCTS USE? (NCE vs Digitrax example)


1) DISTRIBUTED WIRING

Distributed works on the concept of a long linear main high power bus.  Your telephone poles use this type of wiring topology.  A main bus of wires carries all the power from the booster (power Plant) flows through the area (on poles) where there are many track points (houses) that need to pick off power from the bus.  This system uses wire efficiently (hence cost effective) and is native to how the REAL track itself is physically run.  The idea is to put the devices near the location where they are used as opposed to some central location.  The down fall is that it the bus electrically breaks, everyone on the bus losses power. Trouble shooting means you must go to the location of the fault and check.

Below is a diagram of a distributed bus system. (DET = Occupancy Detector, TRK = TRACK, BLK = BLOCK)

                          Track bus-->
Booster ---------+--------------+-------------+
                       |                 |                 |
                     DET             DET             DET
                       |                 |                 |
                TRK BLK1      TRK BLK2      TRK BLK3


2) POINT TO POINT (STAR) WIRING

Point to Point or "star" is just what it says.  It would be like every house ran it own separate power lines directly back to the power plant.  The advantages are that regardless of what happens to another house power lines, you are not effected.  Another advantage is you can concentrate all your DCC equipment (Block detectors, circuit breakers, signaling system, switch motor drivers) all in one location.  Trouble shooting is made easy since you only have to go to one point to check every thing.  The down side is you will consume a TONs of wire and have a huge cable management problem and spend a lot of money on the wire.  There is no main bus.  Only a bunch of local bus's belonging to each track block.

Below is a diagram of point to point or star wiring system. (DET = Occupancy Detector, TRK = TRACK, BLK = BLOCK)

               --DET-------------------------TRK BLK1
             /
Booster +---DET-------------------------TRK BLK2
             \
               --DET-------------------------TRK BLK3

The term "Star" come from looking at the origin of all the wire.  They all fan out in different directions in a star formation from a central location which, in this example, is the booster output.


3) WHICH WIRING SYSTEM IS BETTER?

Most layouts, PRE-DCC, are often a mixture of the two systems but lean towards using a star.  Star wiring is best used for small layouts were wire runs are not long for best economical results. Distributed wiring is best for large layouts.   WHY?  Depends on your priorites:

DISTRIBUTED ADVANTAGES (What the prototype uses!)
 
1) Cost a lot less in terms of wire for it uses far less wire than a Point to Point system.
2) Faster Installation.  Less total time involved in wiring things up.
3) Minimizes electrical problems that happen with lots of multiple long wire running side by side especially when dealing with DCC.  (Inductance/Capacitance/Noise/Crosstalk)
4) Easily Scalable or modular.  Adding on to a distributed system localizes the new work required to the just the area it is needed.  No new long wire runs.
5) Cleaner Installation given the fewer wires.  Cleaner wiring is much easier to trace.
6) Easier to color code and hence create standard and wiring documentation.

POINT TO POINT ADVANTAGES.

1) Simple to understand in terms of installation.  For this function, just run wires from here to there.
2) Centralized all your command and control functions in one location for easier visual monitoring.
3) Less complicated connections involved.
4) Centralized command/control functions offer cost advantages in that the cost per function is less because they put so many of them on one board..

There is a perception that centralize "monitoring" will help you debug problems.  Yes and No.  

Yes Reasons:

a) You can KNOW of SOME types of faults faster than a distributed system would let you know.  But in practice is does not help you solve the problem any faster and there solutions for remote notification.
b) The person feels more secure because they do not trust the system enough to distributed it out along the track where it needs to be.  The Watching Eye!

No Reasons:

a) The extra wiring involved actual increases the chances of having more faults or intermittent performance issues.   In other words the solution can be worse than the problem its trying to prevent from happening!  

4) MIXING THE TWO WIRING SYSTEMS

Distributed DCC products will work just fine in a Point to Point (Star) configuration.  However it will not be as cost effective since you will need to potentially buy more products to cover the same amount of functions.  But this will be the most expensive installation option for you combining the worse parts of each wiring system. 

Centralized DCC products MAY not work well in a distributed system.


5) WHICH WIRING SYSTEM DO DCC PRODUCTS USE? (NCE vs Digitrax example)

NCE and DIGITRAX has OPPOSITE philosophies in terms of how they design their respective DCC accessory products.  

a) NCE accessory products are meant to be used in a distributed system. 

b) Digitrax accessory products "tend" to oriented to centralized installation.