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Digitrax Grounding

This section talks about a design problem with Loconet that can cause a breakdown in loconet's reliability when used on large layouts or large scale layouts with more than one Booster.  

High (good) reliability is defined as loconet's ability to communicate without loss of control or commands at any time for any reason. 

Digitrax DCC system depends on it's Loconet Cable to connect every device that runs on Loconet together.   This makes the system as close to plug and play as possible when it comes to wiring the loconet side of things.

For most layouts with short Loconet cable runs, it works fine.  But when any of the following conditions 

1) Long Loconet Cable Runs to 2 or more boosters (booster properly distributed around the layout)
2) Large Scale Locomotives

are met, an inherent design problem can now become large enough to create problems.  It gets worse as the layout grows in size.  In other words, Loconet's electrical cabling design as documented was not optimally design in its approach to large layouts.  

BACKGROUND OF THE PROBLEM

Digitrax depends on it's Loconet ground wire to carry multiple independent ground currents at the same time.  Below is a breakdown of the wires on the Loconet Cable and the currents they can carry.

Signal NamePurpose
Loconet 
Pin 
Numbers
VoltageCurrent LevelsCurrent Duration
LoconetData Communication between Loconet Devices
3 & 4
Wires are in parallel.
12V
Nominal

8V Minimum
~10mA (0.01 Amp).  Worst case current depends on number of loconet devices connected to the bus.Continuous
RailSyncPower for Throttles and NMRA DCC signal data for Boosters.
1 & 6 
Wires are  independent.
12V Nominal

7V Minimum
~100's mA (0.1Amp).  Typical current depends on the number of Loconet device connected to the bus that do NOT have their own power source. 

Maximum current is set by the 22 ohm resistor pullup to 12V found in the command station.  (12V - 7V)/22 Ohms = 227mA or 0.227Amps.
Continuous
"Ground"

AKA:
Loconet Ground,  Railsync Ground, Booster Common.
Common Ground for all devices.


2 & 5
Wires are in parallel.
0VAmps.  Return current that is equal to the sum of all the above currents PLUS Booster Current.

Booster Current only flows when two booster are involved in exchanging current at a booster district boundary to support:

1) Locomotive crossing a booster district.
2) Short circuit event that cross the booster district.

Worst Case Peak Current:  Short circuit current equal to highest booster current rating involved between the two given boosters involved.   If you have a 8 amp booster, then you can have 8 amps of current.

Typical current: Depends on the given locomotive motor current draw crossing the booster district boundary (double gap or insulated rail joiner location between boosters.  So a G gauge locomotive would draw G gauge motor current.  All wheel pickup locomotive will have very noisy intermittent motor current.   Locomotive that have "offset wheel electrical pickup" will 100% depend on the booster common to allow the locomotive to make it across.
Signal current is continuous PLUS the Booster Current.

Booster Current:

1) Short Circuit Conditions typically involve a locomotive derailment across the booster district.

a) If the resulting current is just below the shutdown threshold of DCC circuit breaker of booster current rating, it will flow continuously in the loconet ground until the device consuming the current is removed.

b) If the current is above the shutdown threshold of the DCC circuit breaker of booster current rating, the current will flow momentarily in the loconet ground until the device shuts down.  If the protection device has a auto-restart, the current will repeatedly cycle on and off in the loconet ground.

2) Locomotive Conditions:  The time it takes for the locomotive to cross OVER the booster district.

DEFINITION:  The concept of a Booster District only exist when you have more than one booster running the layout.  Multiple boosters are require when a given booster cannot provide enough power to run the trains in a given location or the track wiring that carries the booster power to all point on the layout gets to long.   By breaking up the layout into booster districts you divide and conquer by reducing the electrical problem down to a manageable size.  At the track level, this means you must install double insulated rail joiners to electrically isolate the booster from each other.  That way a short on Booster X does not cause Booster Y to shutdown.  This should not be confused with a Power District created by DCC circuit breakers.  DCC circuit breaker break up a given booster district into small functional sections.  They are independent of the number of boosters required.

LOCONET PHYSICAL CABLE DESIGN  

The loconet cable consist of 6 wire with each wire being 24AWG, 26AWG or 28AWG in size.  The size depends on the cable you purchased.  24AWG is not common.  For the Loconet Ground, two of these wires are wired  in electrical parallel (shorted together) by loconet devices in connects to create an equivalent single larger gauge ground wire.  The two ground wires are placed on pin 2 and 5 to separate railsync signals from loconet signals to minimize the crosstalk and reduce inductance.

GROUND WIRE PROPERTIES

Current flow in any wire will create a voltage drop across it length.  The voltage drop follows ohms law.  The voltage drop created in a ground wire will add or subtract from the signal voltages of the RailSync or Loconet signal amplitude.   If the currents are low enough, the voltage drop will NOT be high enough to create any problem.  But when the voltage drop becomes high enough for any reason, it will destroy signals on the Railsync or Loconet lines for the duration of the duration of the high voltage drop.

SIGNAL VOLTAGE CORRUPTION LEVELS

I have not seen a specification for the definition of what a "logic high" voltage is for Railsync or Loconet.  Given the minimum voltage required is between 7 and 8V, the logic high


IF high enough current flow in the ground wire, the voltage drop in the ground wire will add or subtract 


Loconet Cable Wire Resistance

Resistance is for stranded wire consisting of 7 strands.


WIRE
AWG
Ohm 1000 FtOhms per Foot2 wire in Parallel Ohms per Foot
24


26


28



MAXIMUM CABLE LENGTH

Maximum Loconet Cable length for a given current that will create a 4V drop on Loconet Ground wires.
 AWG 1A 2A 3A 5A 8A 10A 
 24       
 26       
 28