Digitrax PM42

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The Digitrax PM42 is used to segment the power from a DCC booster into four separate power "sub-districts". This is useful if you have a large single booster and want to limit the short-circuit current in order to protect your trains from over-drawing current in case of a derailment. Also, dividing up your layout prevents the entire layout from stopping when a train causes a short in one of the sub-districts.

The PM42 can also be used as an auto reversing unit for up to four sub-districts. Each of the four segments of the PM42 can be configured to work as either short-circuit protector or auto-reverser, but not both at the same time. So you might for example configure a PM42 to provide three sub-districts, one of which has auto-reversing. This configuration will use all four segments of the PM42 because three are needed for protection and one for auto-reversing.

Keep in mind, a PM42 will not increase the power available for running trains over what your booster provides. It can only sub-divide the available power into smaller sub-districts.

The AutoReverse feature should not be used with a circuit breaker on the same PM42.

PM4 Power Manager


Think of your booster as the main breaker in a home's breaker box (a.k.a. fuse box, or it's correct term, the load center). A PM42 is like 4 individual circuit breakers - the 15 to 20 amp ones that protect everything downstream from the main breaker.

Should an overload happen on one circuit, the circuit breaker will disconnect that circuit, without interrupting power to other circuits. Or, instead of shutting down the entire layout when a short happens (and it will), only the power district with the short is shut down.

If you are using enough power to trip the main breaker constantly, then adding individual circuits won't solve anything. Likewise, a PM42 is always a good plan. Just don't expect it to take the place of additional power from another booster.


The PM42 takes a large amount of work to prepare. Connections must be made to a female edge connector, and the PM42 is inserted into the connector to complete the circuits.

Read the installation instructions thoroughly first, and diagram what you want to do. The PM42 can be used to provide up to 4 circuit breakers, or one breaker can be configured as an autoreverser. Or all four can be arranged as an autoreverser. As a rule, do not use an autoreverser and a breaker on the same card, as an endless loop can occur when the reverser is too slow to react and the breaker trips.

Wires are soldered to the connector in the manner needed to provide the desired operations. The card has fingers on one edge which are then inserted into the connector. The connector can be mounted in a case, or the card can be mounted on standoffs. Use a terminal strip to terminate the connections from the card, which makes it easier to make connections to the layout without stressing the card.

Read the instruction sheet at Digitrax for more info on the capabilities of the card, and how to wire it.


The PM42 is a LocoNet capable device. The PM42 can transmit information such as shorts, when they are cleared, and when the autoreverser operates.

LocoNet Termination

The PM42 has a jumper for LocoNet termination. If the device will not be used on LocoNet with a command station, short the two termination pins together with the provided jumper. If you plan to connect it to your command station via LocoNet, leave the connection open (disabled).

  • LocoNet termination must be active on any device which is not connected to a command station.

See Also