LocoNet Control Bus

LocoNet is a bus developed by Digitrax, but also licensed to other manufacturers so there is some third-party hardware available. Also, it can be used even if you don’t have a Digitrax command station, although it provides additional value when used with one of those.

LocoNet Overview

LocoNet wiring is simple three-pair wire made for telephone use, with the same modular connectors. The pins carry the following signals:

1: Rail Sync (-)
2: Signal Ground
3: LocoNet
4: LocoNet
5: Signal Ground
6: Rail Sync (+)

Note: most “telephone” wires use six-pin connectors, but only have four wires between them. These won’t work for LocoNet. Six-wire cables are sometimes sold, but mostly it’s easier to make your own with a simple crimping tool, a box of connectors, and a spool of cable. You also need to buy the crimp-on modular plus with six conductors, not four (these are called 6P6C connectors on packaging, but it’s easy to see if there are four gold pins or six).

The LocoNet wires (pins 3 & 4) carry the actual LocoNet signal. This is not a mirrored signal like the two wires of a DCC bus, but rather identical copies of the signal. In fact, it’s really only pin 3 that carries it from the command station; but any throttle plugged in will normally bridge the two wires together so that the signal is present on both pins. If the LocoNet wire is reversed at some point and there is no bridging device, the signal will only be present on the “wrong” pin, and some devices (apparently including the DT400 throttle, which doesn’t bridge the pins) will not operate properly. Both the LNRP and UP92 bridge these pins, however the UP5 panel does not. Digitrax’s LT1 tester (included with their Command Stations, and available separately) is a handy tool for checking for this and other LocoNet wiring problems.

The Rail Sync wires carry the DCC control information (which is also present on the track) so that devices that need to receive commands do not need to connect to the track. This is used by boosters and some other devices, and is compatible with Lenz and some other third-party systems.

LocoNet wiring does not need to be arranged in a line, but can go in multiple directions from one device. The maximum length limitation is around 1,200 feet (early documentation said 2,000), but this depends on many factors, and real-world distances may be more constrained. Additionally, the distance between two devices should not exceed 600 feet. Neither of those limits are likely to be a problem in most model railroads.

At any given time, one device (typically the command station) is the “master” device, and generates DCC commands to the track. Devices on a LocoNet can communicate with each other even in the absence of the command station, and communication is direct and does not need to involve the master device. The master device maintains a list of active locomotives, and keeps track of the state of stationary decoders (it polls the bus when turned on to learn their initial state), but doesn’t perform any coordination or arbitration between devices on the LocoNet.

Throttles (and computers acting as throttles) communicate with the master device to update speed, direction and function settings, and to learn information such as fast clock time (which doesn’t seem to work on my throttles, see the Fast Clock page) and cab signal status. Each active throttle is associated with one of up to 120 “slots” on the master that keep track of known locomotives. Some command stations, like the Zephyr, have fewer active slots (the Zephyr has 10, the new Zephyr Xtra has 22).

The command station also provides “termination” of the LocoNet bus. When operating a LocoNet without a command station (useful if you want to add signaling and/or computerized switch control to some other DCC system) you’ll need to provide that separately. Some people believe that termination is also needed on longer LocoNet runs (and have evidence to back up that belief), although Digitrax says it is not.

Railsync, Boosters and Throttles

RailSync is mainly needed if you connect a booster to the layout, as that is how it gets a clean copy of the commands to send to the track. Digitrax added a bit of complication to this (see Allan Gartner’s page below for details), and RailSync doesn’t go through the front or side connectors on the fascia panels (e.g., the UP5), but only through the two rear connectors. Thus, if you use the side connector to “tee” the LocoNet in two directions, you can’t connect a booster on the part of the bus connected via the side connector.

Because RailSync is essentially a DCC signal, it carries power, although not a lot (the wiring is limited to one amp). It is enough however to power handheld throttles, and thus it’s also used as the power supply for these. This can lead to problems on large layouts, and it may be necessary to separate the throttle section of the LocoNet from that between boosters, and replace RailSync on the throttle section with a simple DC supply.

All of Digitrax’s fascia panels will replace the front jack’s Rail Sync with DC power, as the throttles will rectify the Rail Sync signal into DC anyway, and the panels will use a locally-connected supply if there is one. If you’re wiring your own jacks using telephone hardware, you can also do this by simply splicing a DC supply into the lines (i.e., cut the wires for the Rail Sync pins between the throttle section and the command station, tie both to + on a DC supply, and tie - on the supply to the ground wire (pin 5) as shown here).

Rail Sync is reportedly used by some DCC accessories (Digitrax notes the BDL16x, DS64, SE8 and LNRP) and so a real Rail Sync signal, or the DCC signal itself when using a non-Digitrax Command Station, needs to be present where such devices are connected. That’s an important consideration, and one reason to use an LNRP to create a separate LocoNet string for the signaling and detection hardware and the turnout controls from that used for fascia panels.

The Rail Sync signal appears to be identical to the signal defined by the NMRA for Command Station to Booster communications (the Power Station Interface defined in RP-9.1.2), although I can’t be certain it conforms in all aspects, and Digitrax doesn’t appear to claim it is conformant to the NMRA RP. Rail Sync is also apparently identical to the signal used for the same purpose by Lenz Command Stations and Boosters, as the two can be intermixed.

LocoNet Hardware

While LocoNet wiring can be entirely passive, connected via ordinary telephone jack wall plates and boxes, there are some specialized hardware components that are useful:


The LocoBuffer series of interface, available from RR-CirKits, is one of several systems to connect a LocoNet network to a computer. I’ve always preferred this (or it’s precursors) to other systems due to the design: it is small, simple, and incorporates optical isolation to protect the computer from any stray voltages that might be introduced to a LocoNet network by miswiring or induced surges (both a risk in the complex wiring environment found under a layout).

The LocoBuffer allows programs such as JMRI to interact with LocoNet devices. If a LocoNet-connected command station (such as any of Digitrax’s) is used, computer-based throttles can also interact with decoders on trains (i.e., can control trains). This is often used to enable Wi-Fi based throttles using JMRI’s support for this capability.

The most recent version of this (from around late 2015 on) is the LocoBuffer-USB rev. N (I’m not sure what the “N” stands for, as they seem to have skipped the preceding letters of the alphabet and the manual is presently labelled “Rev. a” even though the title is “LocoBuffer-USB Rev-n”).

Because these are isolated the LocoNet is not powered from the LocoBuffer. As a result, power draw is very low (between 10 and 20 mA on both the older LocoBuffer-USB and the new rev-N model when I measured mine). This is handy to know if you are connecting these to a smaller computer, such as a Raspberry Pi, where power use often needs to be considered. Power requirements this low are negligible and will not affect the Pi.

On at least OS X and Linux (I don’t have Windows experience) this can be somewhat confusing to install, as the operating systems dynamically create serial interfaces when a serial device of this kind is connected. On Linux (well, the Raspberry Pi version I use anyway) USB0 is normally created (regardless of which USB connector is used) if this is the first such device. On OS X a similar logical interface name will be created, however this goes away if the device is disconnected and can change depending on the USB connector. If you’re familiar with the Arduino you’ll recognize this behavior, as both are using FTDI-based USB serial interfaces.

Although the manual claims Windows support only up to Windows 8, mine (bought early 2016) came with a driver CD supporting up to Windows 10, along with OS X support up to “10.9 and later” (back to 10.3). OS X 10.9 introduced signed drivers so it should be good for later versions where these were more strongly enforced, and it installs on 10.11 although I haven’t tested it fully yet. Linux is also supported, without any need to install a driver (at least on a Raspberry Pi, other Linux distributions might require something, but there’s no Linux driver on the CD).

Note: from 10.9 Apple actually created their own FTDI driver, but it won’t work for most FTDI-based devices (like this) so the driver from the CD needs to be used. On newer Macs without a CD drive, you can download the drivers from the Manuals page on the RR-CirKits website.

While there are other LocoNet-Computer interfaces, I’ve never used any of them so I can’t say much about them. Some have additional capabilities, such as downloading sound files, which may be of interest.

UP5 - Universal Panel

The UP5 is a small circuit board with a metal panel designed to be mounted to a layout fascia and to provide two front-panel jacks for throttles. It can be used as-is, with the LocoNet daisy-chained through the two rear connectors. Or a power supply can be connected to it (the PS14 is appropriate, and the circuit board has the right jack to connect one), to provide power to attached throttles when the system is not operating (to prevent the throttle’s batteries from running down). The UP5 can also be connected to the track wiring, and a multi-color LED on the front panel will show that the track is powered, and will show different colors for DC vs DCC on the track. The power supply and LED track power display is available on other Digitrax panels (such as the LNRP and UP9x series), so a UP5 isn’t needed if you have one of those.

Note that only throttles or additional UP5s that won’t be used to connect to other devices should be connected to the front or side jacks, as the Rail Sync signal is not present on these. All other devices (command stations, boosters, stationary decoders like the SE8C and DS64, etc.) should connect only to the rear jacks.

For best use, additional UP5 or other panels must only daisy-chain through the rear connectors (no “tees” off the side jack) to ensure a valid Rail Sync is always present in case it’s needed for something.

LNRP - LocoNet Repeater

The LNRP is externally similar to the UP5, except that it has only one front-panel jack, and the PS14 power supply or equivalent is always required. The real difference is internal. It electrically re-creates the LocoNet signal (acts as a signal repeater in technical terms) between two LocoNet busses: the “side” bus and the “rear” bus. Digitrax recommends that networks with more than 20 devices use an LNRP (or more than one) to break the devices up into smaller groups.

The LNRP has two side jacks intended to create a string of LNRPs connected to the command station, called the “protected LocoNet”. No devices other than boosters should connect to this chain. Each LNRP also has two rear jacks, which are used to provide a local LocoNet string that is electrically isolated from the chain of LNRPs, and can be used for other devices. Thus a problem on one local bus will not affect others, yet all devices will still be able to communicate.

Each LNRP requires 250mA at 12-18 V DC, such as is provided by a PS14. On my layout, an accessory power supply is feeding all of my DCC accessories, including the LNRP. Part of this power is used for the Rail Sync lines on the output, and thus to power any devices that use that for power, such as throttles. Some of Digitrax’s documentation suggests a minimum 15 V DC supply, but others, includes the specification of the “out of range” LED note a 12-18 V DC range.

Note: because the LNRP is a repeater, a device on one segment, such as a computer, will still see all commands exchanged between any two devices anywhere else on the network. This means that the computer can be connected somewhere other than the LocoNet used by the command station, and still see all commands to and from it. The LNRP provides electrical isolation between LocoNet strings, but does not create independent networks.

UR90/91/92 - Wireless Transmitter/Receivers

The three UR models look similar to the LNRP (the fascia panel has one jack and multiple lights), but function more like the UP5. However, each is also a wireless receiver, supporting the use of Digitrax’s wireless throttles. The UP92 is also a wireless transmitter for Digitrax’s wireless cabs (note that this does not support Wi-Fi).

The UR90 is an infrared receiver, and one needs to be mounted within line-of-sight of any place a throttle may be used. It can support up to 20 throttles. The UR91 and UR92 are both infrared and radio receivers. The UR91 is a “simplex” radio (meaning the base station cannot send information to the hand-held throttle) with support for up to 10 throttles. The UR92 is a “duplex” radio (meaning it can send information also), and supports up to 20 throttles. All three require a PS14 power supply or equivalent.

Although the UR92 documentation says that a power supply is required, it will apparently run off the Rail Sync lines if those carry power. This probably isn’t a good idea, as it would be easy to overload the supply or the wire due to the power needs of the UR92. But merely unplugging the power supply will not shut down the radio; the LocoNet wire needs to be disconnected also if this is desired.