DCC questions answered

Through detection: current-sensing detectors on gapped track sections, RailCom, or sensors such as infrared and Hall-effect types. The detectors report to the command station or computer over a feedback bus.

Yes. DCC-EX’s EXRAIL scripts run on the command station itself — for example waiting for a sensor, then stopping, reversing or throwing a point. See building a DCC-EX command station.

JMRI, Rocrail Basic and Traintastic are free, and DCC-EX’s EXRAIL is part of its free command station. iTrain is paid. See the software comparison for details checked with each maker.

Look at the socket under the body: 6-pin (NEM 651), 8-pin (NEM 652), 21MTC, PluX, Next18 or the newer E24. Each needs a matching decoder plug. Our decoder plugs page lists them.

Usually not. Setting CV29 bit 0 (adding 1 to CV29) reverses the direction in DCC mode, and the directional lights follow.

Many makers use a write to CV8 as a reset; ESU, for example, says write 8 to CV8. Check your decoder’s manual, because the reset value varies. See resetting a decoder.

It sets acceleration (momentum). By the NMRA formula, seconds per speed step = CV3 × 0.896 ÷ the number of speed steps in use; 0 means no momentum. CV4 does the same for braking. Some makers use their own scale, so check the manual.

The track is powered all the time and carries digital commands as well; each loco’s decoder obeys only commands sent to its own address, so you can run several trains independently on the same track. See What is DCC.

A short address is 1–127 and lives in CV1. A long address lives in CV17 and CV18 and goes up to 10239; CV29 bit 5 chooses which one the decoder answers to — it only answers to one at a time. Our CV calculator works out CV17 and CV18 for you.

For reading CVs back and for some settings, yes: service-mode programming reaches every decoder on that track, so it must be a separate, isolated section with only one loco on it. Programming on the main (POM) works on the running layout, but some decoders limit what you can change that way — ESU, for example, says long addresses can’t be set by POM on its decoders. See setting up a programming track.

The track side is low voltage, but everything is powered from the mains: use power supplies designed for the job and protected by a fuse or circuit breaker, do all wiring with the power off, and don’t leave a layout running unattended. When soldering, ventilate well — rosin flux fume can cause asthma.

Digitrax suggests feeders roughly every 6–10 feet, to both rails, with at least two sets per power district — then check with the coin test: short the rails with a coin around the layout, and the booster should shut down each time.

On a reverse loop, the rails meet themselves the other way round. The loop needs gaps in both rails at each end and an auto-reverser feeding the section between them; that section should be longer than your longest train. See wiring a reverse loop.

An isolated section with its own booster or circuit breaker, gapped in both rails, so a short in one place doesn’t stop the whole layout and you have more power for more locos. Small layouts often run on one; bigger ones benefit from several.