# DCC and Reversing



## jackpresley (Dec 19, 2017)

So why aren't reverse loops handled in the locomotive decoder?

I'm not an EE. I'm an RE (red neck engineer) so go easy on me. (Geez. Red+Neck = *****?!)
If each section is insulated, why does it matter in the DCC world which rail is A or B?

Why can't the locomotive do this:
My black box has one terminal for each pickup wheel, only allows one way flow (no shorts) and sends out consistent DC to the motor?









This "Magic Bus" would be an integral part of the decoder (or whatever picks up the track power).


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## Dennis461 (Jan 5, 2018)

any metal wheel touching two tracks with opposite polarity is a short. Don't even need a decoder.


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## jackpresley (Dec 19, 2017)

Dennis461 said:


> any metal wheel touching two tracks with opposite polarity is a short. Don't even need a decoder.


Excuse my crude drawing, but no single wheel is touching rail A and B simultaneously.


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## Steve Rothstein (Jan 1, 2021)

jackpresley said:


> Excuse my crude drawing, but no single wheel is touching rail A and B simultaneously.


I believe the single wheel would touch both at the gaps. It might work if there were insulated joiners forcing a big enough gap and the joiner came up to the surface of the rail to prevent the wheel from dropping down into the gap and shorting.


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## MichaelE (Mar 7, 2018)

A wide gap is not needed. Only a short insulated section on the rail top and side. 3/16" would probably be fine. Maybe less.

Think of insulating the wheel from those tight converging rails at the frog on some turnouts.


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## Lemonhawk (Sep 24, 2013)

However if you have just an insulated gap once you cross the gap you will have one truck on one phase and the other truck on the opposite phase and you will still short out.


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## jackpresley (Dec 19, 2017)

Lemonhawk said:


> However if you have just an insulated gap once you cross the gap you will have one truck on one phase and the other truck on the opposite phase and you will still short out.


That's the point of my original message. Why not have the "Magic Bus" or decoder keep things in phase? Yes, the "Magic Bus" exists only in my imagination. But how does auto-reversing electronics work? Why isn't that built into the locomotive's decoder?


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## Lemonhawk (Sep 24, 2013)

The decoder can't do the switch because the pickups are not at a point they are scattered along the wheelbase of the locomotive, so if the decoder attempted a switch it would still show as a short to the controller. The controller would shut down because it has detected a short and it all comes to a halt. The reverser works because it switches faster than the breaker on the controller and gets the phase set correctly all along the wheelbase so there is no longer a short as the locomotive continues across the gap.


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## mesenteria (Oct 29, 2015)

A decoder will not detect the quick change of phase under it. That's what the auto reverser does...in milliseconds. The decoder doesn't care which wheels are getting which part of the square wave, as long as it can't get both parts of the wave, meaning a conflict, meaning a dead short, concurrently, along the same wire from the same input. So, what the auto reverser does is to swap phases in a blink between the rails, and the decoder shrugs and keeps interpreting rail signal packets when it gets them and doing what you want it to do.

I don't know if you have diagrammed exactly where the conflict takes place, but it is at the frog and beyond when the train returns, having gone through a 180 loop, and its first metal pickup tire meets the first bit of rail that conflicts with the phase being picked up rearward along the frame on that same side of the frame. As soon as this happens, the auto reverser switches phase under the rest of the locomotive, but the decoder doesn't notice...it's simply too fast and it is 'alternating current' anyway...who cares which rail has the zero and which has the one.


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## gregc (Apr 25, 2015)

jackpresley said:


> Why not have the "Magic Bus" or decoder keep things in phase?


since there is usually more than one wheel picking up power on a rail, there is a short when a wheel on each side of the gap. when there is a short, there is no track voltage, no power to the decoder.



jackpresley said:


> But how does auto-reversing electronics work?


with the reversing section fully isolated from the mainline, the auto-reversert detects the short and very quickly (mille seconds) reverses the phase to the rails in the reversing section.

this same techniques is used in frog juicers and the short detection circuit is used in circuit breakers


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## Severn (May 13, 2016)

I think maybe enough electronic things could be added to a decoder to handle it but it must not be cost or space effective.

Here's the way I've thought about it.

If we assume there's a gap of no current between the different phased track...

And an engine straddles it so the front wheels get the phase one way, and the rear get it the other...

Then what in the middle could sort that out?

In what I've built there's an ac to dc chip that has 4 prongs -- two say outer ac prongs, with dc the middle two If on the engine the blue wires are in one side of the wheels and the red on the other....

And the blue set go to one ac prong, the red to the other...

Then it seems like this would be a problem as the phases on each color would be opposite in this straddling engine scenario.

I don't know enough if the chip can handle that or if the phases cancel or what.

Let's say it's a problem. Because we need voltage to go into something to that can control the motor by dcc command. And we need to derive the track voltage from the dcc signalling.

Then the dcc signal itself needs to be picked off and decoded. Again our two blue and two red wires are out of phase there and this seems bad.

So simplistically if the short is avoidable at the wheel by some kind of gap...

We'd still have to figure out sorting the phases that are mismatched in the front and rear pickup wires ...

It's worth noting I'm also not an ee or even re even.

But I've wondered the same thing overall.. . I've actually wanted to talk to an ee about this kind of thing if they'd take the time and dumb it down for me.

No takers so far...


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## jackpresley (Dec 19, 2017)

First of all, I appreciate everyone giving this their brain-bytes. @Lemonhawk @mesenteria and @gregc -- You are all thinking in terms of what exists, and _how things work now_. @Severn gets it. I'm proposing this "Magic Bus" that CAN NOT short the track. It can sense a short and reverses the polarity in milliseconds, just like an AR1, but it isn't sending the output to the track, it is powering the decoder. Everything is onboard the locomotive.


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## gregc (Apr 25, 2015)

Severn said:


> I think maybe enough electronic things could be added to a decoder to handle it but it must not be cost or space effective.


how do you prevent the short between the wheels on either side of the gap.? this isn't just a short for the loco or car straddling the gaps, but a short of the booster or power district. all the locos are dead.

or if all the pickup wheels do clear the gap (inertia) and the front wheels are on the A phase rail in the reversing section and the back wheels are on the A phase (yes A phase) of the mainline, how does the decoder get voltage between A and B?


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## Severn (May 13, 2016)

Is the answer a battery? Just a joke.

Firstly I do not know how to detect a short.

So I tried to simplify it to non short but out of phase problem ... Can that be handled?

But if I'm getting the original poster also, there's a magic board, or bus. That sorts all this out.

And your decoder ... Any decoder would get it's power and dcc from it.

Even if it's not viable due to cost or space issues.

I'd still like to know if it could work ...


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## vette-kid (May 2, 2020)

We're just taking about a miniaturized reversing unit and changing the point at which the polarity is sensed and reversed. I see no reason this couldn't work, though space is an issue. 

Another issue is that the dead space would have to cover the width of the whole truck, not just a wheel. Since all the wheels of that truck pick up power (unless you want to just use one, but that might have is own issues), you need a gap large enough that the truck only sees one polarity. I think making the unit so that each wheel is independently sensed would be a real challenge due to space. Each wheel would need is own input into the chip, which then provides a constant power output to the motor, lights, etc.

Sent from my SM-G781U using Tapatalk


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## jackpresley (Dec 19, 2017)

Every pickup wheel is independent. Each wheel sends it's "juice" (A or B) to the MagicBus, which sorts it out and powers the decoder. (Each AC ~ represents a pickup wheel on a six axle locomotive.)


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## DonR (Oct 18, 2012)

This a very interesting thread. I have an idea of what Jack is
suggesting...that an on board decoder 'detect' the 'reverse loop'
ahead, then 'make things right' electrically. The constant is that
the 'main track' is out of phase with the 'loop' track. By whatever
means the 'loop' track must be phase matched with the 'main'
so the loco can continue. Insulated joiners or gaps are mandatory.
These truths must be recognized when designing a digital system. 

The way I see it, we would have to forget the 'short' detection circuit design.
Instead, a 'detection' device in the main track would cause the on board decoder
to send a 'wireless' signal or a shunt back to the main track which
would activate a digital relay that would phase match the 'loop'. Another
such detector under the 'loop' track near the exit would again trigger the
relay to match phase.

That would work, but would it be better than the existing loop controller?
I really don't see that much advantage. Costs would likely be about
the same. You'd need to do two detector installations, a bit more work.

Or, you could simply eliminate the problem by installing a live overhead
catanary system and run only electric locos similar to what is done in Europe and
on Michaels layout here on the Forum. Power would come from
pantegraph and rails. Since both rails wouil be
connected together there would be no phase problem.

Reverse loops are a problem in any 2 rail system. The 3 rail 0 scalers
have no such concerns.

Don


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## cv_acr (Oct 28, 2011)

jackpresley said:


> Excuse my crude drawing, but no single wheel is touching rail A and B simultaneously.


Not one model locomotive out there picks up each wheel on a completely isolated basis. All of the axles in a single truck pick up power together.

And, even if you did only have a single-axle truck, that wheel could bridge the gap and cause an electrical short that no decoder can ever handle, since the short is directly through the wheel.

The amount of complexity to have six separate pairs of inputs into the decoder for track power, instead of just one, plus the electronics to handle all that switching, would probably make for a circuit board that would be difficult to fit in any locomotive under O scale.


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## cv_acr (Oct 28, 2011)

jackpresley said:


> First of all, I appreciate everyone giving this their brain-bytes. @Lemonhawk @mesenteria and @gregc -- You are all thinking in terms of what exists, and _how things work now_. @Severn gets it. I'm proposing this "Magic Bus" that CAN NOT short the track. It can sense a short and reverses the polarity in milliseconds, just like an AR1, but it isn't sending the output to the track, it is powering the decoder. Everything is onboard the locomotive.


But you magic bus can't do that for a metal wheel contacting both rails on either side of a gap. The short goes right through the wheel. Never even makes it to your circuitry.

Far easier to just put a reverser on the track.


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## jackpresley (Dec 19, 2017)

cv_acr said:


> But you magic bus can't do that for a metal wheel contacting both rails on either side of a gap. The short goes right through the wheel. Never even makes it to your circuitry.
> 
> Far easier to just put a reverser on the track.


My metal wheels never short out when I cross an insulator. Not sure what you guys are talking about -- this is done all the time. Edit: [Added] I used manual reversing (Atlas) back in the 70s and Atlas track insulators to separate the two. I guess I was lucky, as you are the second person to suggest this is going to short.

Edit2: And I was using DC. 
Edit3: And you are right... they're plastic. Shiny but plastic. Sorry.


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## jackpresley (Dec 19, 2017)

DonR said:


> This a very interesting thread. I have an idea of what Jack is
> suggesting...that an on board decoder 'detect' the 'reverse loop'
> ahead, then 'make things right' electrically. The constant is that
> the 'main track' is out of phase with the 'loop' track. By whatever
> ...


One would power each block separately -- and each block is isolated from each other -- and never the two would meet. The power supply doesn't care which rail is A or B, as long as A and B never touch. The engine doesn't care because the Magic Bus keeps things in order.


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## CTValleyRR (Jul 26, 2014)

The reason this isn't done is was concisely stated by cv_acr: it's adding complexity, volume, and heat to the circuitry in the locomotive, and thereby adding cost to the unit, to "fix" something that is much more easily addressed by moving the volume and heat to the underside of the layout, where they don't really matter. The additional expense and effort then only needs to be exerted by those who actually need tur capability: those with reversing loops on their layouts. And don't forget that this is "once and done" -- you only have to wire the reversing unit once.

Engineers like to solve problems, but they also like elegant simplicity in a solution. So if the question is COULD it be done, the answer is "most likely". But there is no good engineering reason to do so.


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## Lemonhawk (Sep 24, 2013)

Jack, they only short if the tracks are not in phase, you were manually switching the DC to correct the problem and hence were always crossing gaps that were in "phase". Otherwise the little orange short circuit light on the power pack would light. On another note, if one did isolate the wheels and run them all thru a magic bus sorter, it still would not handle the locomotive actually stopping such that one wheel was bridging a gap causing the controller to detect a short and shutting down -- something rare but will happen, and this occurrence is easily handled with autoreversers (or by a manual switch in Jack's DC case.)


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## gregc (Apr 25, 2015)

if you have more than one wheel in contact with the same rail and a solid condutctor between the wheels (frame), how do you prevent a short?

do you connect each isolated wheel to a pair of diodes?

doesn't a wheel bridging the gap still cause a short?


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