Wiring Your Layout with LocoNet

There are no real restrictions on LocoNet wiring with respect to wire pairs. Most Digitrax customers choose to use 6 wire Telco type flat ribbon cables because they are cost effective, simple to wire and give superior network performance. We engineered LocoNet to use 6 wires because of several advantages outlined below. LocoNet can actually run on just 2 or 3 wires. For general information about LocoNet click here.

1) In a 6 wire flat configuration, as crimped onto a RJ12 6 pin style plug, the left 3 wires are effectively a "mirror" image of the right 3 wires. This allows you to "daisy-chain" outlets without worrying about whether the cables are "reversing" or "non-reversing."

2) There are 2 ground and 2 LocoNet data connections, so the effective "loop resistance" is lower due to paralleled wires. This makes it possible to run LocoNet over greater distances than any other command control system.

3) If a ground or signal connection is broken or intermittent the network can still maintain a reliable connection. These types of faults are the greatest nightmare to locate and fix!

4) The two outside wires, typically Blue and White in a 6 conductor Telco ribbon, actually carry opposite phase copies of the master system rail packets, this is called RAIL SYNC. Because these are broadcast differentially in a single cable, we can accurately and reliably tap a remote Track booster anywhere along a LocoNet cable run. We can do this many thousands of feet from the Master Packet generator (Command station) with very good signal fidelity, even in the presence of a lot of noise and interference! Also, this signal is capable of supporting a number of low current draw modules that can tap on anywhere in the network. An example of this would be LocoNet input sensors and UT-1 Utility throttles.

5) The balanced nature of the cable and the way the signal currents propagate in this "RF Quad" configuration allow the lowest possible RFI radiation outwards, and EMC susceptibility or inward interference pickup. This is a good thing. This is part of the reason Digitrax LocoNet handily passed the FCC Class B radiation Certification requirements.

6) The LocoNet philosophy and architecture were carefully crafted to allow "free-form" wiring with no termination or "linear-bus" restrictions. You can "star", "tee" into, branch or expand the network any way that is convenient for you.

It is important to note that the LocoNet network must not loop back upon itself by making a return connection to the command station (like a snake biting its tail).

These are the reasons we would recommend the loop around the layout be a 6 conductor ribbon type wire. The wire gauge in the range of 22AWG to 28AWG is OK. Telco uses typically 26AWG. If you don't mind the extra work, you could use round 3 pair cables. It is best to stay with a fixed color to pin number in the jacks throughout the layout to prevent later problems debugging!

We find it best to break up this "backbone" wiring into sections. Each section will be a run of cable connected by male-male 6 conductor cords with RJ12 plugs on each end. This allows the network to be quickly disconnected and isolated for fault-finding or expansion. Digitrax UP5 (and other previous models of Universal Panels) are very convenient for quick layout hook up. All UP5s have two RJ12 jacks in the back for easy hook up of LocoNet, two RJ12's in the front for throttles to be plugged in and one additional RJ12 jack on the side of the panel for hooking up additional throttles or LocoNet devices that won’t be daisy chained to other devices. You just plug in your cables and you are ready to play. It may be a little less expensive to use 6 conductor dual wall plates and wire them in parallel around the layout. This takes more time, may not be as reliable but will save a little money. The main down-side to this is that if any of the cables are disturbed, it is very time-consuming to try to repair a "bird's nest" of small wires under the layout! The choice is up to you!

Your LocoNet wiring scheme is very flexible and easy to wire. It was designed to be "plug & play" because we know you would rather spend your time running your trains instead of troubleshooting the wiring. The primary concern really boils down to having a physically secure and maintainable wiring strategy and discipline. The "glow" of low price wire and fixtures quickly fades, as you become the poor individual who has to trouble-shoot a maze of "spaghetti" that was disturbed by someone who tripped over "some wires" under the layout!!

The Digitrax Big Book of DCC

This is a wonderful wiring resource. It explains wiring for Digitrax Command Control in simple, easy to understand terms. It covers basic and advanced wiring topics.

Wiring for DCC Home Page

Digitrax also recommends that you visit the Wiring for DCC Home Page by Allan Gartner. This is an excellent resource for answers to your basic and advanced wiring questions.

Layout Wiring: Bus Feeders and Track Feeders

We are often asked, "What gauge wire do you recommend for a) bus feeders from block to block underneath the layout and b) or track feeders from the bus trunk to the tracks in each block?" This question can be complicated by the actual layout and dimensions. Basically, you need a good feeder system that can safely support the continuous full current rating of any attached booster anywhere on the layout, i.e. 5 amps for the DB150. The concepts are similar to normal DC throttle practices, but we are dealing with significantly higher current loads- otherwise why use DCC if you cannot have multiple locos in the same block/power district!

To achieve this on any normal size layout the main feed bus from the two "Rail A/B" terminals of the booster should be at least 16AWG or better. Many people use 12AWG house type wire for low cost and convenience. You can run it from the booster directly to the "power district" you are supplying. This is often more convenient than running a "common rail" bus around the whole layout, especially when you feed many separate power districts from different boosters.

If you use a single conductor wire of an AWG that does not conveniently fit into a DB150's terminals, simply splice in a couple of inches of similar AWG size stranded wire that will compress to a better cross-section.

From this main "feed bus" we recommend dropping feeders (20AWG) approximately every 6 to 10 feet of nickel-silver track up to both rails in a power district. This ensures that there is minimum "voltage droop" or voltage loss when we draw normal operating currents in just that district. There should be at least 2 sets of feeders per district from the feed bus to allow the short circuit maximum current to be carried if a short occurs during a derail. This wiring method does not rely on the nickel-silver rails to carry current more than 3-5 feet from a "good quality" power feed.

The actual wire gauges (AWG) can be increased or decreased, depending on the layout dimensions and operating power/current loads. Note that wire is reasonably cheap compared to your valuable hobby time. It usually doesn't hurt cost or installation wise to have fair sized main feed bus wires or around 12AWG or so. If you wire the layout from the boosters "directly home" for both rail feed sides you won’t need to have really heavy wire gauges (for example 8 AWG "welding cables") strung out in a serpentine "common rail" type bus. Note that "common rail" here is a misnomer, since we should not solely rely on the nickel-silver track to carry the load current. In this way a "common rail" strategy is simply an extreme form of "feed bus". Note that it is a good idea on larger layouts (more than 25' or 30' in dimensions) to place the booster as close to a power district as possible so the "feed bus" from the booster to the first track feeder is less than about 25' to 30' from the Booster. The Digitrax LocoNet interface to Boosters was carefully setup to simply allow for this "distributed Booster" approach.

The final test to tell if your layout wiring is adequate is to go around every track section with a metal coin and short out the rails every foot or two. You should see a booster safely and quickly react to this intentional short circuit and "shut down" the power properly. If this is not consistently the case at all points of the layout, your wiring busses and feeds may be undersized with so much loss/resistance that the Booster cannot safely distinguish a short circuit condition. This MUST be corrected before you begin operations! We put the audible short-circuit alarm in the DB150's to make this task quick and easy when trying to work out what is happening on the layout.

You might also consider adding indicator lamps at strategic points to indicate the power state of some adjacent track(s) to aid in diagnosing short conditions later. You can disguise these "track status" lights as building lights or even trackside lamp posts. If you use a Bi-color LED with a proper dropping resistor, instead of a 12V filament lamp, you’ll have the added advantage of the LED "color" showing how much Analog stretching is being used. It is also useful to place a lamp across the gaps at an auto-reversing section to show if the gaps are matched in polarity or not.

In operation, since power sections of many tracks are common to a booster, it is useful to help isolate live track with these "track status" lamps. This becomes even more useful when you have many operators handling lots of trains simultaneously such as a club or large operating session!

Wiring DCC and Block Occupancy Detectors

If you wish to incorporate occupancy detectors in a DCC layout, you’ll need to consider wiring capacitance when you plan the layout. If you do not plan to have block detection or signaling you can disregard these comments.

Digitrax incorporates block occupancy detection into the total layout control plan by using the BDL162 to provide detection for 16 detection sections.

Basically, the high transition speeds of the DCC signal cause "capacitive charging currents" to flow in the track and feeds, even when no locomotive or resistor wheel set rolling stock is present! This is basic physics, and is independent of the actual DCC system you purchased. Check the information from the detector manufacturer to find the maximum "un-occupied" feed capacitance a detector can tolerate. For correct system operation you need to ensure your feed and track capacitance in a detected section of track is less than that specified by a manufacturer. In practice, most short wiring sections won’t have a problem and there is no need to measure the actual capacitance. For large sections of track that need to be detected, it is wise to plan for the rail that is being detected to have minimum capacitance. To minimize the capacitance of the wiring attached to the rail you wish to detect, it is usually enough to ensure the wire bus feed for just this rail section and the attached feeders are physically located as far from other wires as possible. A physical isolation of just an inch or so will dramatically lower capacitance. It is OK to come close at many locations to other wires, we are simply avoiding consistent close proximity, such as a "twisted pair" or shared cable sheath. This precaution is ONLY needed on large track sections that are to be reported, and the balance of the layout wiring can be wired without regard to these issues. This issue first came to our attention several years ago on a layout using "inductive block sensors" that was converted to DCC. Some of the longest and most distant blocks falsely reported occupied when they were empty. These particular detectors were being tripped by the capacitive charging current of the feed wiring for these particular "worst-case" blocks. This does not show up in conventional DC operation!

To fix these blocks with problems we can:

a) Move the detector as close as possible to the affected reporting block to minimize feed bus capacitance.

b) Rewire just the rail in this block that is being reported with a separate single conductor feeder and separate rail feeds. You can leave all the other wiring in place, we are simply isolating the wiring to this single detected rail section.

Since most occupancy detectors operate on the current draw of a block they may be affected to some degree. Generally as the detector sensitivity is increased to reliably detect an e.g. typical 5K ohm resistor wheel set it will be more sensitive to capacitive charging currents.

Are common rail track detectors compatible with DCC?

We have a number of customers who are running Digitrax alongside their existing block detection circuitry. There are numerous block detection schemes and circuits, and we cannot cover all the variations here. Generally, it is necessary to isolate the block detector power supply and ground system of your existing block detector system from that of the Digital boosters. The most common detectors use "back to back" high current diodes to sense block current-draw. Detectors with "opto-isolated" or relay outputs generally have no problem sensing current draw of a loco, etc., on the DCC current, and will signal the occupancy without any ground or power supply conflicts. Many DCC boosters, including Digitrax, work with both rails live and a separate system ground wire. This ensures no possibility of booster output voltages being "added" accidentally across adjoining booster power districts, and operation on catenary is possible.

If your layout is common rail, all you need to do is be sure to double-gap the layout between power sections supplied by different boosters.

Within the "power district" of a single booster you CAN have a common rail return between detection blocks as long as all the detectors are within the same booster and the detector power supply is isolated from other detectors and booster grounds. If you are not sure, simply disconnect the detector power supplies and reconnect them in stages while running the layout. If there is a conflict it will generally short-out one of the booster outputs and shut down. This allows you to isolate the problem ground or power supply.

Bi-Color LED's as Track Indicators

Each UP5 has a bicolor LED that indicates the power status of the local track section when it is hooked up to local track power.  If you are not using UP5s the following explains how you can add this feature.

Hooking up Bicolor LED indicators around the layout is a convenient way to see the power status of sections of the layout at a glance. The LED indicates whether a track section is powered up, if "Zero-stretching" Analog mode is being used and its local direction, and can even be used to indicate if the GAPS in a reverse section are matched or not. We use these indicators on our demonstration layouts and customers always want to know what they are for.

You will need:

One 2 lead bi-color LED (Radio Shack #276-012)

One "ballast" or "current setting" resistor. We recommend a 1K 1/4 watt resistor (Radio Shack #271-1321) for reasonable brightness and current levels.

Connect the 1K resistor in series with either one of the LED leads to make a "ballasted" LED.

With the 2 leaded bi-color LED there is no strict polarity to observe, the emitted color depends on the direction the LED leads are connected to the track.

Simply connect the "ballasted" led across the track to indicate the track is powered. If you connect a "ballasted" led across one of the double gaps of a reverse section the LED will be OFF (not lit) when the gap polarity is matched.

Keep Alive Power Jack (For DT200)

When the layout is powered down we recommend that you change the DT200 that is running as the Master command station to Advanced Throttle mode so it can power down and conserve its battery. Do this by pressing Run/Stop &Left Arrow keys together.

All other Advanced Throttles on LocoNet will power down and show "idLE" when the LocoNet bus wires (pins 3 and 4 of the RJ12 plug) fall below +5Volts for more than 1/4 of a second. If the RailSync lines on LocoNet (pins 1 and 6 of the RJ12 plug) remain above +12Volts, the minimal keep alive power needed by the DT200's will be drawn from LocoNet and not the internal battery.

To build a "Keep-Alive" power jack for your Big Boy Set:

1. Wire pins 2,3,4 and 5 of an RJ12 socket to the negative lead of a 12 volt power supply.

2. Connect the positive (+) lead of the 12 volt supply through a 1k 1/4Watt resistor to pins 1 and 6. This keeps a DT200 Throttle from using the internal battery when it is hooked into this socket, thus extending battery life.

If you don't want to build your own battery saver, the UP-5 offers this capability when you add an external power supply to it and Loy's Toys (310) 944-1069 offers a Battery Saver Station for DT200 & BT2 for $19.95.

Don't be put off by the issues since most of the time the wiring is very straightforward and common-sense usually prevails. If you have any doubts or concerns, it is sensible to FAX your layout diagram and questions to your DCC manufacturer for comments and suggestions. They should have a technical person on staff who can rapidly answer your questions in full.