# Help with my PM42 and reverse sections



## Walman (Dec 18, 2011)

Details: HO scale, DCC power via a Digitrax DCS100 Superchief, DT402 throttles, and managing power including “auto-reversing” via a PM42 controller.

The problem is understanding the auto-reversing. I am about to finalize the wiring on my track which entails wiring up the inner “local” track of my layout which are “reversing loops” as compared to the outer mainlines. I plan on isolating the inner sections through the use of double gaps where they connect with the mainline. As I said the power for all the layout will be controlled by a Digitrax PM42. I purchased the PM42 and had an electrical engineer friend wire it up so that all 4 sections are reversing controllers (instead of short circuit control). It appears this is working fine.

Test: So to test out my logic before wiring up my layout I created a temporary teardrop loop and switch and wired it via my PM42. I double-gapped the loop and controlled each resulting section through one controller on the PM42 card. See the attached drawing.

Result: My test (single 4-wheel) engine runs fine when completely inside either isolated section of the test loop. When the engine crosses the gap between the two sections VERY FAST I hear the click but the train makes it over the gap and it will continue running. When the engine crosses the gap between the two sections at SLOW speed I hear the PM42 “click” then I get a short circuit either at the PM42 or the DCS100. This latter event is the issue I want to understand and fix.

Questions:
1-	At face value it would seem this is more of a sensitivity or timing issue versus wrong logic but I am not sure so I hoped someone could help or comment.
2-	Also, the manual mentioned using an “OpSwch” command to “program” the PM42 to be in reverse mode (versus short circuit protection). I’m not sure I did that correctly can anyone comment and more importantly lead me through the steps via a DT402 throttle? 
3-	Very basic question but to make sure I did it correctly: If the “Rail A” wire from the PM42 is wired to the outside rail of the mainline track then the “Rail A” wire should be wired to the inside rail of the inner (reverse loop) track, correct?
What else do you need to know? Thank you for your help.

Walman


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## DonR (Oct 18, 2012)

Not familiar with that specific device, however a
reverse loop controller does have a sensitivity
control that if set wrong would cause what you
have experienced. Sometimes setting it can
be tricky. Reread your manual for precise
directions on that.

The way it works is that when the loco spans the
insulator gap it causes a short circuit. A very fast
acting circuit trips a relay (the click you hear)
that matches the
polarity iso section to the main track and the loco can continue
without pause. It does the same at the other end
of the isolated section. 

You should not have to worry about the 'polarity'
(phase) of the wires going to the isolated track.
The controller will correct any mismatch of
polarity. The input to the controller should be
from the buss near the loop.

There is one other thing you should know about
a reverse loop. If there is a loco, lighted car,
or even metal wheels spanning the insulated
joint at the 'other' end of the isolated section from where the loco
is you can get a short circuit. This is one reason some
say the iso section should be as long as your longest
train. There are ways to over ride this, however.

Don


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## Brakeman Jake (Mar 8, 2009)

I have to go for now but will be back later.In the meanwhile,questions...
-How many reverse loops do you have on the layout?
-How many "sectors" you'd like the layout divided into?
-How large the layout?
Your drawing has an error as it is,but we need to know more to help....


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## Brakeman Jake (Mar 8, 2009)

Posting a drawing of the layout would be great.


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## DonR (Oct 18, 2012)

Jake jolted my thinking.

When I went back and re-read your post I saw that I
may have missed the meaning of what you wrote.

Are you saying you hooked up your one reverse controller to two
different reverse loops?

That would change the thinking dramatically.

That unit has the capability of having 2 reverse controllers one
for each reverse loop.

As Jake requested, a full drawing of the layout would be helpful.

Don


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## feldon30 (Dec 30, 2012)

A PM42 is a 4 district circuit breaker. Any or all of the outputs on it can be set instead to behave as reversing loop controllers (normally sold separate as an AR1). Digitrax reversing loop controllers simply detect a short circuit and switch the polarity.

As per the PM42 instruction manual, in addition to using a different wiring arrangement for Autoreversing, you must also set some OpSwitches using your DT402 throttle. This is a matter of dialing an address and sending a message.

http://www.digitrax.com/static/apps/products/power-management/pm42/documents/PM42.pdf


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## Brakeman Jake (Mar 8, 2009)

The PM42 is a fine device that can be used as adjustable circuit breaker (up to four) or as polarity reversing unit (up to four also) or in any combination of the two functions.As fine as it may be,it is stupid and won't compensate for improperly planned electrical schematics.

To work properly,the reversing PM42 (and the AR1) have to connect a polarity reversing section of track to two constant polarity sections of track.Just imagine a loco on reversing section A jumps the gap onto also reversing section B,resulting in both sections simultaneously reversing their polarity....get the picture?

From the original post,I understand the gentleman wants to wire the whole layout as a reversing trackplan...sorry...this can't work right.


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## feldon30 (Dec 30, 2012)

Thanks Jake for sussing out the issue!

An autoreversing loop, at least on Digitrax, simply detects for a short circuit and if so, reverses the polarity. If I understand what's happening, then with one autoreversing loop directly proximate to another (Even with insulators), anything that straddles the two sections would cause a short, which causes both loops to switch polarity, which causes a short, which causes both loops to switch polarity, which causes a short... You get the idea.

I'd have to see a photo of the layout and/or drawing of track plan to understand why an entire layout would be setup as nothing but reversing loops. On my 21 x 23 layout, so far I have one.


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## DonR (Oct 18, 2012)

I just reread this man's original post.

He said he had an electrical engineer set all
4 sections of the controller to be reversing units. 

Then I interpret what he wrote to mean that each
reversing unit is connected to an isolated section
of track making the entire layout a series of
reverse loops. As Jake noted, this can't be done.

That is why all of the clicking being heard when
the loco spans the joiners between to reverse loop
controllers.

Again, Jake is right, there must be a constant polarity section between
any 2 reverse loops.

Why have reverse controller control of non reverse loops?

Don


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## Brakeman Jake (Mar 8, 2009)

I just wish Walman comes back to us and tells about his goal so that we could help him.It would be better than discussing what should be done,may be done and can't work.


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## DonR (Oct 18, 2012)

Agree Jake

A drawing of the layout, as has been requested, would
be very helpful to get it right.

Don


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## Walman (Dec 18, 2011)

Hello everyone,

Sorry for my delayed response I am traveling. 
Thank you for the comments. I can see a couple of errors I made. 1) It was not my intention to set the layout as multiple reversing sections but my test essentially did that since I did not have a “regular” section of power. In the test I had 2 power districts both connected to auto-reversing (although different modules on the PM42) sections so that is a problem . 2) As Feldon30 points out I probably did not get the Opswitches configured correctly so when I fix the wiring issue I’ll try that again.

So attached is my layout. Blue is the mainline with Magenta and Green representing local districts. My original plan was to run my “regular” power through the blue mainline. I was going to isolate both local districts where they connect with the mainline as well as from each other. The mainline>local connections are easy to spot. So now that I understand that I am going to need a run of “normal” polarity like the mainline the question becomes where to isolate the two local districts from each other? I’m thinking the Magenta, parallel tracks to the right side of the horizontal run up to the slip switch. Open to your thoughts and suggestions. 

Lastly I wonder about the switch itself. Isolated and powered? The same polarity as one of the districts or mainline?

Thanks for your help so far and sorry for the bad drawing.

Walman


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## Brakeman Jake (Mar 8, 2009)

I like to see your drawing as a basic drawing...unless you have a huge space available for your layout,you won't be able to have so many turnouts in so little space.Your drawing shows TO's with 30+ and even 40+ degrees while real TO's are more like 10 to 15 degrees.They eat landscape much faster than we'd like.

According to your drawing,you need only two reversing devices,one each for the magenta and green sections.First,the blue main would be constant polarity,plus the link to the slip then the slip itself.I don't know the brand of your slip,but it likely has a mechanical way to deal with polarity within itself,so you don't want to complicate further more with the PM42...slips are complicated devices.

Then you'll need insulators at both tracks every where the magenta and green sections link to the slip and to the blue main line.Wiring within the sections is straightforward,you have no polarity issues there,and you can connect your supplies from the PM42 anywhere...it doesn't matter within the sections,the whole section switches polarity when needed.


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## Walman (Dec 18, 2011)

Great information. I'll be home over the weekend where I will try to get a better drawing posted and well as try out test2!

Walman


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