Santa Susana Railroad Historical Society

Welcome The Santa Susana Railroad Historical Society

What's a LocoNet

by Don Crano

Don takes the mystery out of LocoNet. We appreciate his allowing us to edit and reproduce the article. Be sure to visit Don's Model Railroading with Digital Train Control web site for lots more great stuff. The article covers the following topics:

bullet Technology Overview
bullet How Is It Wired?
bullet How Do I Test It?

Technology Overview

The easy answer would be a communications bus used with Digtrax systems. This would not quite be the whole story though. In reality the LocoNet is a peer-to-peer LAN [Local Area Network] using a CSMA/CD [Carrier Sense Multiple Access with Collision Detect] protocol. This has been optimized for use with Digitrax systems to allow 100% traffic capacity with less than 1 in 300 collision rate.

Definition of Terms

Peer-to-peer Peer-to-peer simply means there is no one do-it-all master. Not that there is no master; there is. Called the command station, this is the glue that holds the system together. It doesn't control devices such as throttles, etc, but actually takes commands from them. It also allows devices such as DT100 throttles, which have the fast clock built in, to act as slave/masters to the system.

Example: the Digitrax Big Boy master DT200 command station knows nothing about fast clock protocol. But add a DT100 to the system, and you now have a fast clock. Add another DT100 and you have two master/slave fast clocks; either will control or set the other. In other words, as long as there is another device to master a code, the message will pass across the LocoNet. Any other device that can use this code will, but other devices do not have to know anything about this code.

LAN LAN is a typical networking of local area computers, such as in a small office, interconnected home computers, etc.
CSMA/CD CSMA/CD is a popular Ethernet protocol used in LANs around the world. In a CSMA/CD network, devices are free to communicate at any time. If a collision does happen (essentially, two or more messages hit the line at the same time), the protocol detects it and tells the sending devices to send their messages again after a given backoff time.

Simply put, when you buy a computer with a network interface card (NIC) or buy a NIC for an existing computer, you are probably getting ready to set up a LAN system for your computers. That LAN will very likely use the CSMA/CD protocol. Digitrax LocoNet protocol is also a CSMA/CD protocol. If you are familiar with terms like bridges and routers, you know that this means the capacity and capability of LocoNet are almost limitless.

 

How Is It Wired?

Standard wiring for the LocoNet is a six-conductor Telco cable using RJ-6p6c jacks and plugs. Basically, that's the same stuff that six-wire phone systems use. Cabling can be in several forms, such as modular with crimp-on connectors or the type of cable used to install the jacks for your phone system.

The actual wiring of the LocoNet is a balanced RF Quad configuration. This allows for a free-form non-terminated architecture. This basically means there are very few restrictions on how you can run the LocoNet cables. Daisy chain, branch, star, even loop, are all possible. There is a basic 2000-foot limit; but that's about 1/3 of a mile. Note: If the LocoNet is looped, then the polarity of pins 1 and 6 needs to be matched to prevent crossing lines.

Specifications of the LocoNet Cable Conductors

Note: Colors may vary with different cable manufacturers.
 

Pin Color Signal Voltage
1 White Rail-Sync [B] 7vdc
2 Black Common ---
3 Red Data 14.5vdc
4 Green Data 14.5vdc
5 Yellow Common ---
6 Blue Rail-Sync [A] 7vdc

Voltages are relative to the LocoNet common (pins 2 and 5) and are approximate. Rail-Sync lines are mirror images of the track voltage. They are used to synchronize additional boosters to the command station and to supply LocoNet power to devices that require low-current power.

The LocoNet is redundant by design. Some people believe that the intentional separation of pins 3 and 4 is a problem. You can fix it by either plugging in any LocoNet device (except the UR91 currently) or by making a small pigtail with an RJ plug that shorts pins 3 and 4 together.

You can also reduce LocoNet to a three-wire system by connecting pin 1 to 6, 2 to 5, and 3 to 4. This lets you use stereo-type phone jacks in place of or along with the RJ plugs and jacks. It's even possible to reduce the throttle portion of the LocoNet down to two wires by connecting pin 2 to 5 and 3 to 4 and not using the Rail-Sync lines. This only works with devices that don't get their power from the LocoNet, such as battery-powered throttles.

All this allows for very simple, forgiving cabling.

How Do I Test It?

This section covers three topics:

bullet Testing Devices
bullet Debugging
bullet Good-looking Crimps that Don't Work

Testing Devices

There are several devices available for testing your LocoNet installation:

bullet Digitrax LT1 Tester
bullet Ohmmeter
bullet LAN Cable Tester
bullet Do-it-yourself Tester

Digitrax LT1 Tester

The easiest way to test the LocoNet is with the Digitrax LT1 LocoNet/decoder tester. This 4-LED device can be purchased and is now supplied with all Digitrax starter sets. Each of the 4 LED's checks for the proper voltages as described above.

LED1 = pins 1 and 2
LED2 = pins 2 and 3
LED3 = pins 4 and 5
LED4 = pins 5 and 6
 

With this list and the voltage table, it's pretty easy to see how it all comes together. The tester is designed to find open circuits, or simply broken wires, or missed crimps on the RJ connectors. It requires the use of a suitable power source, such as a typical booster. Off the layout, use batteries or any other source that can supply the approximate voltage to the proper pins.

An LED that doesn't come on or is dim could be the result of a dead short or high resistance. In any case, this would tend to indicate a bad cable and/or connector.

Ohmmeter

You can use an Ohmmeter or continuity tester to check LocoNet cable and connectors. Using the grouping for a three-wire LocoNet, check for shorts against each of the groups. This is a good way to check a LocoNet cable off the layout.

LAN Cable Tester

You can purchase LAN cable testers commercially. It must be able to check RJ 6-conductor cables. These testers send a known data signal down the cable and read the data at the other end. It's much more then required, but is very efficient at testing cables.

Do-it-yourself Tester

If you would like to build a tester for your LocoNet cables, go to the FREMO website. (Achtung! Some of the articles on this site are in German.).

Debugging

Regardless of which method you use, always test your cable as you install it. Whenever a strange problem occurs on an existing layout, simply plug the LT1 into the Booster and area jacks for a quick check of what the LocoNet wiring looks like.

Note: By its nature and design, the LocoNet can work even with only 1/3 of it connected, and battery throttles can hide a lot of LocoNet wiring problems. But once batteryless throttles and/or the UR91 are connected, these problems can suddenly show up. The LT1 will find them all for you.

How Can I Have a Perfectly Good-looking Crimp that Doesn't Work?

Ah, the easiest for last! <g>

There are several causes of bad crimp connections:

bullet The wrong crimping tool
bullet Using the crimping tool incorrectly
bullet Inferior or incorrect plugs
bullet Something other than the connectors is causing the problem

Use the Right Tool

There is a variety of crimpers, some for 4 wire, some for 6, 8, etc. Some are multi-purpose. Others have interchangeable dies for different wires and connectors. Make sure you have the right crimper and dies for six-wire RJ plugs.

Know How To Use It

In other words, follow the instructions. Some crimpers have built-in cutter and sheath stripper; this saves a lot of headaches. If yours doesn't, makes sure you cut the end of the cable square and cut the sheath back the proper distance.

Inferior or Incorrect Plugs

Not all RJ plugs are created equal. Some are better quality than others. There are also different styles of RJ plugs for various types of cable (solid wire, stranded, shielded, round, and flat/oval). Make sure you buy the proper plug for the cable you are using.

Something Besides the Connectors

Finally, it's always possible the connections aren't bad--not usual, but it does happen. This is not an easy one to spot, but there is one trick for the standard flat oval modular type cable. Visually examine it, then use your fingers to feel for strange humps in the cable. The cable should generally be smooth and flat. A hump means either the wires got twisted inside the sheath or the machine erred when making the cable.

 

 

 

 

 

 

 

| Site Map | Privacy Policy | Contact Us | © 2004-2008 SSRHS