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Digitrax: PM42

Summary: The Digitrax PM42 is a power management device which allows the creation of multiple power districts with one booster. It can also function as a automatic phase inverter for reverse loops.

See more Misc DCC Items
General information
DCC Category
Manufacturer Digitrax
MSRP 85 USD86.53 EUR <br />75.71 GBP <br />128.46 AUD <br />114.67 CAD <br />
Compare Prices Latest Prices
Manufacturer Part Number
Common Name
UPC Number 5266703004
Release information

Product URL External Product Link
Product Manual URL External Product Link
Year Released 8/2002
Has computer interface
Firmware Upgrade Possible
Predecessor PM4
Successor PM74
Misc Details
Height 0.8 in2.032 cm <br />20.32 mm <br />0.0203 m <br />0.0667 ft <br />
Length 4.5 in11.43 cm <br />114.3 mm <br />0.114 m <br />0.375 ft <br />
Width 4.5 in11.43 cm <br />114.3 mm <br />0.114 m <br />0.375 ft <br />


Upgrade to the PM4 Quad Power Manager. This device was replaced by the PM74.


PM4 Power Manager

The Digitrax PM42 is used to segment the power from a DCC booster into four separate power districts. This is useful if you have a large single booster and want to limit the short-circuit current in case of a derailment. Dividing your layout into power districts prevents the entire layout from stopping when a train causes a short in one of the 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.

A layout divided into four power districts.


Think of your booster as the main breaker in a home's breaker box (a.k.a. fuse box, or its 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 requires 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.

The PM42 can provide up to 4 circuit breakers, or be configured as four automatic phase inverters. Combinations of the two modes are also possible.

Read the installation instructions thoroughly first, and draw a diagram of the necessary connections.

Do not use an autoreverser and a breaker on the same card together, as an endless loop can occur when the phase inversion is too slow to react before 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.

For proper operation a heavy gauge wire must be connected between the Ground connections on the booster and PM42.

PM4 Upgrade

It was possible to upgrade the older PM4 to a PM42 by replacing an IC, but that part is no longer availalbe.


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 be used on a standalone LocoNet, 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).

See LocoNet Termination for more information

See Also

Digitrax Knowledge Base


Search for more PM42 FAQs

See Also

Related Digitrax Other DCC Equipment Items