Reverse Loops Digtrax AR1 or PM42 which one do I get?

[Spaceman Spiff]

Hi, I am in the process of getting my layout going now. I have two reverse loops so I was wondering do I get 2 Digitrax AR1's or 1 PM42? What is the difference between the 2?

Spiff

[The_Ghan]

Hi Spiff,

The PM42 manages 4 sub-districts.  Each sub-district can be configured to manage short circuit OR an AR.  The AR1 manages a single AR loop only.  You might say that a PM42 is also an AR4, in that it can manage up to 4 AR loops.  Depending on your layout, you could use either ... or both!

A layout in its simplest form has a single power district.  I would still use a PM42, or a basic automotive fuse, to protect your booster and power supply.  My choice would be the PM42.

If you have a small layout with just a few sub-districts, a single PM42 managing AR on 2 loops and 2 other districts would do the job.  If you have a larger layout you might have more than one PM42.  You can also install an AR1 trackside of the PM42.  You can even locate the AR1 physically right at the reversing loop.

I use a BDL168 with each of 2 sub-districts for occupancy detection on my subway.  I have two simple loops with sidings at the rear.  Each BDL168 is detecting 12 sections across 3 zones.  Conveniently, I have the 4th zone free for future expansion.  The outer loop and inner loop - running in opposite directions - on separate sub-districts.  I've got trains arriving and departing every 30-40 seconds.  If an east-bound train derails the PM42 cuts power to the whole east bound loop.  The west bound trains still run.

I don't have a reversing loop.  But if I did, I would probably dedicate a thrid sub-district to it unless it specifically belonged to one of the other two sections.  If I wanted occupancy detection on the AR loop I would route sub-district 3 of my PM42, configured for AR, though zone 4 of one of my BDL168s. 

However, if for example, the AR loop belonged to one of my two existing subway loops, for example, it was used to turn trains around on the east-bound track only, then I would wire in an AR1 trackside of the PM42 (which would remain set to short circuit management, which would THEN route through zone 4 of one of my BDL168s.

You can't run two AR loops on one sub-district.

To sumarise: Consider how your layout is broken down into power districts and sub-districts.  Evaluate where the reversing loop fits into all of this.  Is it a separate district or does it belong to a district?  A single PM42 should cope with an 8A power supply and around 20 trains on the move.  By default, the trip threshold of a sub-district is 3A. 

I don't know if I've made your decision any easier.  Perhaps a track plan indicating your sub-districts might help.

Cheers

The_Ghan

[Spaceman Spiff]

Thanks for the thorough reply The_Ghan.

My layout plan is is going to be based on the HO Atlas plan called the Oregon Pass Lines. I have attached an image its not the best quality though. My tables are  "L" 32 inches wide by 13 feet on side and 8 feet on the other side. My yard will be different than planned. The one in the plan is a hump yard and I going for a regular yard with entrances on both end. My yard will added on the bottom part of the longerst side the layout as an expansion so the yard area will be 6-7 wider at that point of the layout.  The loop area towards the bottom of the layout is where I'll put the passenger terminal (probably Kato station kits).

I am not up to speed in regards to "districts" and what not. I will have to do some reading up on it. It will be a digitrax DCC system. The guy helping me told me to get some autoreversing hardware hence the question.


Thanks

Spiff

[The_Ghan]

Hi Spiff,

Your layout reminds me of the Penn Dixie plan, one of the most interesting layouts around.  It has about 6 reversing loops and is difficult to wire, for me anyway.  Looking at the Atlas layout it is difficult for me to judge the overs and unders but there are two reversing loops coming off the figure 8: one on the left side heading up north and one at the top heading off west.  When I track my way around it appears that it is only the short section up the left side of the 8 needs isolating, but I can't be sure without unravelling the spagetti.

That said, the layout needs a PM42.  I'd configure it as follows:
    1.  Short circuit management: main loops
    2.  Short circuit management: sidings
    3.  Auto reverse management: figure 8 bypass
    4.  ......... free, or AR management for something I've missed.

Hope this helps.

Cheers

The_Ghan