What is DCC? A plain-English guide to Digital Command Control

Step by step

1. 01

   Step 1: Picture DC first

   On a traditional DC layout the controller changes the voltage on the track, and every loco on that piece of track does the same thing. To run two trains independently you split the track into electrically separate blocks and switch them.

   Next: step 2
2. 02

   Step 2: Now picture DCC

   With DCC the track is kept live at a steady level all the time. The command station (through a booster, often built into the same box) puts a fast-switching square-wave signal on the rails. Each locomotive carries a decoder that rectifies that signal for power and reads the digital messages in it (NMRA S-9.1). Because every message is addressed, you can drive several trains on the same track independently.

   Next: step 3
3. 03

   Step 3: Learn the parts

   • Command station – builds the messages (packets) from your throttle commands.
   • Booster – amplifies the signal to track power. Starter sets combine the two.
   • Throttle / cab – the handset, app or software you drive with.
   • Decoder – the small circuit board in each loco (or in a point motor or signal, called an accessory decoder).
   • Programming track – an isolated piece of track for setting up decoders.

   Next: step 4
4. 04

   Step 4: Understand addresses

   Each loco decoder listens for its own address. A short address is 1–127 (stored in CV1; new decoders usually come set to 3). A long or extended address goes up to 10239 and is stored in CV17 and CV18 (NMRA S-9.2.1 and S-9.2.2). Accessory decoders use a separate address range, so loco 5 and point 5 do not clash.

   Next: step 5
5. 05

   Step 5: Meet CVs

   Decoders are configured with numbered Configuration Variables (CVs). Some are standard everywhere – CV1 short address, CV2 start voltage, CV3 and CV4 momentum, CV5 top speed, CV29 configuration – and many higher numbers are manufacturer-specific (NMRA S-9.2.2). You change them from your command station, usually on the programming track.

   Next: step 6
6. 06

   Step 6: Know the standards

   DCC is standardised by the NMRA in America (the S-9 series) and MOROP in Europe (NEM standards such as NEM 651 and NEM 652 for 6- and 8-pin decoder sockets). Kit from different makers that follows these standards should work together for basic running, although extra features vary.

   Next: step 7
7. 07

   Step 7: See where automation fits

   Once trains are addressable you can add detection (so the system knows where trains are), accessory decoders for points and signals, and software such as JMRI, iTrain, Rocrail or DCC-EX's EXRAIL scripts to run trains automatically. The rest of this site covers each of those steps.