Basic DCC reversing loop question

[GaryHinshaw]

Still new to DCC and I have a basic question about auto-reversers, especially about a specific situation that I haven't seen covered very clearly.  First, let me make sure I have the basics of DCC correct, then I'll pose my question.  As I understand the literature, the voltage applied to the rails with DCC is a square wave with a peak voltage of ±Vo, where Vo is scale-dependent, but is typically 12 Volts in N scale.  Each cycle of the square wave represents one bit of digital information: 1 of it's a 58 µsec pulse and 0 if it's a 100 µsec pulse:



The role of the decoder is then to:

• Decode the bit stream for commands specific to its address,
• Rectify the track voltage to DC at +Vo,
• Pass the appropriate DC voltage (including polarity) to the motor, as per the throttle setting for that address.

Also, if I understand correctly, the sign of this applied square wave does not matter, because the decoder always rectifies the voltage to +Vo after decoding the bit stream, and it sends the proper polarity to the motor, based on the throttle setting.  Thus, for example, it does not matter which power lead you hook up to Rail A on the command station, the loco will still go forward when commanded (for the default CV setting).

Now for the reversing loop question.  Suppose I have a schematic like this:



The track controlled by the reverser is shown in red.  Suppose a train that is longer than the red section enters that section from the top track and exits from the bottom track.  If the polarity of the rails at the first gap is opposite, a wheel-set or loco spanning the gap will cause a short and the reverser will switch the polarity of the voltage applied to the reversing section.  The decoder doesn't care because it rectifies the voltage (though it seems like there must be at least one bit of information that gets corrupted by the switch: is there automatic error correction in the packet protocol?)

What happens when the train is crossing both gaps?  Does the auto-reverser go crazy and switch the polarity every time a wheel set bridges one or the other of the gaps?  Is this configuration frowned upon?

Thanks in advance,
Gary

P.S. If I misstated any of the basics, please correct me.

[C855B]

What happens when the train is crossing both gaps?  Does the auto-reverser go crazy and switch the polarity every time a wheel set bridges one or the other of the gaps?  Is this configuration frowned upon?

Yes. And yes.

This is the singular advantage of plastic wheels, where the train becomes a "don't care" once the power is fully in the reversing block. If you use metal wheelsets, you need to be sure that your longest train is within the reversing section. However, in the case you illustrated you only have a reverse loop when you use the crossover, so you would gap the reversing block as close to the crossover as you can. Since you won't be able to cross-over a train into itself, it becomes a self-solving problem.

[peteski]

Usually modelers try to make the reversing section longer than the longest train traversing it (especially if they use metal wheels).

[GaryHinshaw]

Got it - thanks for the quick response.  I'm glad to know I was at least thinking about it correctly - and in advance!  In addition to metal wheel sets, I was thinking about trains with rear-end power: if a lead loco crosses the first gap while a trailing unit is crossing the second, you'd have a bigger problem.

But this raises a more pernicious issue: the situation where this configuration arises on Tehachapi BC is the main staging yard, which is currently planned to be loop staging, as in this schematic:



The main yard ("SY1" in the diagram) is a reversing loop that I was planning to place under a single auto-reverser with gaps just inside the entrance/exit crossover (lower right corner of the diagram).  But this means that two locos cannot be crossing those two gaps at the same time, a situation that would be very difficult to enforce as an operating restriction in a busy lead.  I think a better option is to place each individual yard track under its own reverser with gaps as shown in red in the diagram.  The restriction then is that trains cannot enter and/or exit both ends of a single track simultaneously.  If the tracks are long enough to accommodate their trains, this would be a very natural condition that would only rarely be violated.  But I think I'd be best off with a separate reverser for each track ($$).  Any comments and/or suggestions for a specific implementation?  PSX-AR, other?

The mainline run is itself is also one big reversing section (though "T Loop" itself is not).  I was thinking I'd put "SY3" under an auto-reverser; it's plenty long enough between the red gaps to accommodate the longest trains,  but I still have to be careful not to have separate trains crossing the gaps at both ends of that staging simultaneously.  That seems problematic...  Is there a better way to block this that I'm not seeing now?  (Note also that there will be multiple power districts in the mainline run.)

Thanks again,
Gary

[jagged ben]

Gary,

Reversers are expensive.  Relays are cheaper.   Since this is a staging yard, the cheaper method would probably be to power only the track(s) that have the turnouts lined for them.  You can do this through the tortoises on the the cheap, like we do for our staging yard at the club, which also involves a reverser.    Or you can do it through relays, which in your case might also be necessary because of your configuration.

I haven't fully thought it through, and I have to go to work now, but I think there's a better way than buying all those reversers.

It also might be helpful to remind us exactly how you intend to divide the staging yard between UP and BNSF. 



 

[seusscaboose]

PS-X from Tony's

(Just Ask Eric W.  )


EP

P.s Make sure you wire them right lol... I'm dealing with that currently. 

[bdennis]

I have used the OG-CB from Tony's trains on my N scale layout and they are great!.
They are the little and cheaper brother of the PSX.
The OG-CB's are about $25 each. I would not bother with realys etc when the OG-CB is a simple 2 wires in 2 wires out installation.

[GaryHinshaw]

Interesting, thanks for the follow-up.  Here is a comparison of PSX vs. the lower-cost On Gaurd (OG):

http://www.tonystrains.com/technews/psx_vs_onguard.htm

which states:

Small Scales

The adjustable lower current setting of the PSX series is important when used with smaller scales with small rail sizes. With the high setting of the OnGuard there can be enough resistance in the small rails to look like a heavy load and not a short circuit. This can lead to damaging heat in delicate parts.

Has anyone ever fried a component with an OG in N?  Also, it looks like I'd need OG-AR's for reversing sections.

Cheers,
Gary

[GaryHinshaw]

Trying to think of other ways to block the layout that have smaller component counts and fewer restrictions.  The topology of the layout is a simple dog-bone with crossovers:



The traditional way to handle this is with reversers in each loop.  However, in my case, the left loop is almost the entirety of the visible layout, and the right loop is an 8-track staging yard.  It would be much more efficient to put the reverser in the short-ish double-track connector, something like this:



In the simplest configuration, the red block would be under a reverser and the black and green would be fixed.  In the plan, the overall length of the red section could be made longer than the longest train, but the cross-overs present a problem because a train using them could span gaps across two ends of the red section.  (I want trains to be able to enter or leave the right loop - the yard - either clockwise or counter-clockwise, and to enter and leave the mainline run either way as well, so the cross-overs are critical, and there is not enough space to place them a train length away from the ends of the red section.)

Another way is to make the green block a 2nd reverser, but I think it must be problematic to adjoin two reversing sections, because they would both try to flip simultaneously.  Is there a way to set up a hierarchy so that one reverser trips at a lower current than the 2nd?  Is that anything like a recommended practice for situations like this?

-gfh

[bdennis]

Has anyone ever fried a component with an OG in N?  Also, it looks like I'd need OG-AR's for reversing sections.

Gary,
On my N scale layout, I have used OG-CB's (3) and OG-AR's (2) on my layout.
I know of 2 other larger N scale layouts that have also used the OG-CB's.
All of the above layouts have used them with great sucess and no issues at all.

Having said that, I have also used the PSX-ARSC device but this was a special purpose where I wanted the auto reverser to throw a snap coil turnout once the train was in the reversing loop so that the train can enter the reversing loop, and exit the loop all by it self with no user intervention needed to throw the turnout.
I did not use the PSX-AR for its enhanced power rating I only used it due to the extra function that it provides...

I recommend the OG-CB for N scale..

[bdennis]

Re the issue of the train being longer than the reversing section as discussed earlier..
I have cars with metal wheels (with resistors installed for detection).
One of my reversing blocks is definatly shorter than the longest train and in fact it is shorter than all trains (2 x loco's 12 x 50ft cars and a caboose).
I do not have coaches with lights in them.
I found that, as long as the reversing block was longer than the loco's on the train (ie 3 or 4 or 5 loco lash-ups) then the Auto reverser was happy and worked perfectly.
The only issue would be if there are more loco's or cars with lights in them that may span the gaps while the loco's are on the other gaps at the other end..

[GaryHinshaw]

Thanks for the follow-up, on both counts, Brendan.  It's especially good to hear of your experience with metal wheels.

The only issue would be if there are more loco's or cars with lights in them that may span the gaps while the loco's are on the other gaps at the other end..

This is the issue I'm concerned about since I'll be running rear helpers.   In thinking about this a little more this evening, it occurred to me that if I swap the crossovers in the middle section, I can get a solution with only one reverser:



As long as the distance from gaps 1 or 2 to 4 (and 3 to 5) is longer than my longest train, I'm good.  I think I can arrange that, and I don't think I suffer any loss of routing options with this swap.  This almost seems too simple: am I missing something obvious?

-Gary

P.S. I still need to do a more careful analysis of the traffic levels through this stretch to make sure I have enough capacity with two tracks, but that's another topic.

[jagged ben]

As long as the distance from gaps 1 or 2 to 4 (and 3 to 5) is longer than my longest train, I'm good.  I think I can arrange that, and I don't think I suffer any loss of routing options with this swap.  This almost seems too simple: am I missing something obvious?

Swapping the crossovers is .... brilliant!

Another tool to consider is X-blocks.  An X-block is a section of track (usually facing point) whose source of power is determined by the position of a turnout.  (This is done with a DPDT relay, and is quite simple to wire.)    For example, in the pic above, suppose you are having trouble making the distance between 3 and 5 long enough.   Well, you can take a section of track running from 3 towards the main part of the layout, as long as the extra length you need, and make it an X-block.  If the crossover at 3 is thrown, then the X-block is part of the red reversing section, making it long enough.  If the crossover is closed, the X-block remains part of black, causing no issues for trains going through that way.  You could do the same for a section of track from 4 into staging, if necessary.

I was going to e-mail you a picture showing an X-block solution to your previous track arrangement before you swapped the crossovers.  If you decide for other reasons that the previous 'unswapped' crossovers are better, I can still share that with you.

I'm absolutely convinced you can do this with only one reverser, as long as you will be disciplined about train length.   

[GaryHinshaw]

Swapping the crossovers is .... brilliant!

...lucky.

I was going to e-mail you a picture showing an X-block solution to your previous track arrangement before you swapped the crossovers.  If you decide for other reasons that the previous 'unswapped' crossovers are better, I can still share that with you.

This is a nice idea jb - I'd love to see it sketched up.  Feel free to post it here or pop me an email.

I'm absolutely convinced you can do this with only one reverser, as long as you will be disciplined about train length.   

Never!  Actually Mark Dance is talking me into keeping the old live helper pocket at Bena for added ops interest.  (This is located well into the mainline loop on the schematic.)  This could be a way to reduce the number of DPU-equipped trains through the terminal area, easing some concerns.

-gfh

[rsn48]

Looking at Gary's diagram, lets label the top turn out A and the bottom on B, the longest length of train assuming you'd start the reversing loop, auto-reverser at A, would be not just the red area but from just inside the loop from A to B.