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.
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.
MAXIMUM CABLE LENGTH
Maximum Loconet Cable length for a given current that will create a 4V drop on Loconet Ground wires.