# Track power: P-pole vs N-connection



## Andrew_au (Jul 27, 2021)

The Zimo brochures (and MX-10 illustrations) make reference to P-pole and N-connection (e.g. Zimo system brochure, bottom of first page). My Roco Z21 also labels the track pins P and N. However, I can't find any documentation on what this labelling means or when I should care about it.

Can someone point me to some info?

Further notes:

Zimo StEin documentation refers to "positive rail" and "negative rail" (pg 6), shows diagram where "positive rail" is cut into sections and "negative rail" is common (pg 5).
This seems to be for their HLU train detection, which sections the "P" rail and tracks current draw from the sections.

In contrast, the Z21 occupancy detector sections the N rail and connects it to the detectors.
The labelling of P and N seems to be arbitrary. Is this a hang-over from analogue days?
DCC wiki goes to some length to point out that DCC has no "negative" polarity. Instead, at any given point, one rail is powered (HIGH) and the other unpowered (LOW).
Confusingly, it also says that the NMRA standard defines the "positive rail" as the right-hand rail when the loco is facing forward. This is obviously a different concept than DCC track wiring.

My guess - they needed some way to name the different tracks for when it matters for wiring, and chose P & N.


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## Greg Elmassian (Jan 3, 2008)

The DCC wiki needs a lot of work. DCC is an AC signal, shape is square wave.

positive rail in nmra refers to DC only

Greg


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## phils2um (Jan 11, 2020)

Some naming/rail convention for the two DCC wires is necessary, in particular when multiple boosters are involved. The DCC signal needs to be in phase in all the booster zones. Otherwise the loco decoders will short out when crossing zones. It doesn't matter which rails are "positive" or "negative" just as long as you are consistent throughout.

For example, I'm using a Märklin CS3 with additional boosters. Märklin labels their outputs to the track as "O" and "B". I'm using black and white wires as my track feeds. I connect the *b*lack feed wires to the "B" outputs and white wires to the "O" outputs of my CS3 and boosters for convenience. Schematically, my layout is a big loop even though there are four separate power feed zones. The black wires always go to the "outside" rail of the loop and white wires to the "inside" rail.

I do have a situation on my RR where the outside and inside rails are ambiguous - a wye junction. In this case the rail "polarity" (inside vs. outside) is handled by a reverse-loop module. But, the reverse-loop module is still wired based on the "inside" rail "outside" rail principle.


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## Greg Elmassian (Jan 3, 2008)

Likewise:

You can see the white and black wires on the stack of double banana connectors.


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