The recently installed turntable requires the reversal of polarity of its bridge track. The Digitrax PM4 module allows for automatic control of a reversing section of track.

The unit is shown here installed on the Bear Creek & Eastern layout. There are two mounting screws that allow for this module to be installed on a layout.

I removed the blue, 40-pin connector from the module to do all the soldering on the workbench. If you are interested in how I wired the unit, I will explain this next.

The PM4 has four independent sections that can each work as a short circuit breaker or an auto-reverse unit. What I wanted was an auto-reverse section, coupled with a short circuit breaker. The PM4 allows you to do that. This prevents the following scenario. If a short occurred on the bridge, the whole whole system would to shut down, and I wouldn't be able to move the bridge anymore. This is because the bridge's motor is controlled by a mobile decoder.

I set section 1 up as an auto-reverse unit, and section 4 up as a short-circuit breaker. The following table shows all the connections I made to get this work.

The last two connections make section 4 act as the short-circuit breaker for the auto-reversing section 1. I had to use a 12-volt 1-amp dedicated wall power supply to get the unit to turn on reliably. I had to temporarily remove the PM4 unit from the layout to hook up my throttle to set section 1 to be an auto-reverse unit (set the OPSW "06" to "c"). By default all the sections act as circuit breakers. The instruction manual that comes with the unit was pretty straight-forward about how to do this. I tested the circuit breaker by temporarily shorting out the track, and then I placed a locomotive on the turntable bridge, and turned the bridge all the way around. The polarity changed correctly, with only a momentarily loss of power to the locomotive. I had made sure to put the turntable pit rails such that their gaps were not near an expected track location. What this means is that the locomotive temporarily looses power while the bridge is in motion, so the engine shouldn't be moving anyway.