Summary: Auto Reverse is a method of maintaining the correct phase relationships.
Automatically corrects mismatches in phase between track sections such as cross overs and reverse loops.
Auto Reverse or Phase Inversion is an electronic feature found in some boosters. If the phase on the rails between two power districts controlled by different boosters do not match, a short will occur. To prevent that, both rails are gapped to isolate one power district from the other. When a metal wheel bridges the gap, if the phase is mismatched a short occurs. The auto reverse circuit detects that event, and flips the phase so they match.
- Only one booster should have auto reverse enabled, so an endless loop of phase changes between two boosters doesn't happen.
Fleischmann/Roco use the term Auto Inversion which is more accurate in describing what happens.
Important: Digital Command Control does not have signals of different polarity on the rails. Direction of travel is determined by the instructions sent to the multifunction decoder within the locomotive.
Auto reversing devices are available, to implement automated phase matching on a reverse loop. They can be part of a power management device, or a stand-alone unit from suppliers such as Digitrax, DCC Specialties, Lenz, NCE and others.
If a train enters a reverse loop or balloon track, at the turnout's point rails Rail A will connect to Rail B through the turnouts wiring. To prevent this, there must be double gaps on the track loop.
The auto reverser will detect when there is a phase mismatch across a gap, such as that found in a reverse loop or turntable, and instantly correct the situation. Any situation where Rail A connects to Rail B will result in a short circuit. The DCC signal rapidly switches from one rail to the other, while the other rail is held to ground until it switches rails again. This is why connecting Rail A to B results in a short. This also causes a short when two booster districts are out of phase.
Digital Command Control and Polarity
Digital Command Control has no concept of polarity. Despite what many will say, there are no negative voltages present on the rails, nor do the rails possess a specific polarity.
The correct term, as per the NMRA Digital Command Control standards, is Phase. The rails maintain a phase relationship, where one rail is always the opposite state of the other. Each rail has two distinct states, or phases: High (Energized/On) and Low (0/Off). When one rail is high, the other is low, and vice versa.
A short occurs when the two rails are bridged by a conductor, resulting in current flowing from High to Low.
Isn't Digital Command Control using Alternating Current
Alternating current is an analog signal with a constantly changing amplitude and polarity, with respect to a ground reference or the neutral. DCC is digital, it only has two logical states