Trains.com

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

DCC and reversing loops

13974 views
15 replies
1 rating 2 rating 3 rating 4 rating 5 rating
  • Member since
    April 2003
  • 305,205 posts
DCC and reversing loops
Posted by Anonymous on Sunday, July 27, 2003 9:29 PM
I am just reentering the hobby and have decided that the quickest way to have a working layout is to build the Woodland Scenics, Grand Valley, "layout in a box" (or several boxes). With an eye to the future, I have decided to make several modifications to the track plan, one of which is a crossover that connects the outer loop to the inner loop thus creating a reversing loop. The Atlas customer service office says I should just buy a reversing module and install it by following directions. I guess I can take this step but I still don't understand how this will work. This isn't a simple reverse loop as explained in all of the literature. When my locomotive crosses the track gaps, the whole layout will change polarity if I understand how reversing modules are supposed to work. What will happen to any other locomotive operating elsewhere on the layout?

I'm sure I am not the first person to have this problem. Can someone help me? I would love to hear from you.
  • Member since
    April 2003
  • 305,205 posts
DCC and reversing loops
Posted by Anonymous on Sunday, July 27, 2003 9:29 PM
I am just reentering the hobby and have decided that the quickest way to have a working layout is to build the Woodland Scenics, Grand Valley, "layout in a box" (or several boxes). With an eye to the future, I have decided to make several modifications to the track plan, one of which is a crossover that connects the outer loop to the inner loop thus creating a reversing loop. The Atlas customer service office says I should just buy a reversing module and install it by following directions. I guess I can take this step but I still don't understand how this will work. This isn't a simple reverse loop as explained in all of the literature. When my locomotive crosses the track gaps, the whole layout will change polarity if I understand how reversing modules are supposed to work. What will happen to any other locomotive operating elsewhere on the layout?

I'm sure I am not the first person to have this problem. Can someone help me? I would love to hear from you.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Monday, July 28, 2003 6:57 AM
If you are using DCC the rest of the railroad does not reverse polarity. Only the isolated reversing track changes polarity, but unlike DC the locomotive keeps going in the same direction (controlled by its decoder, not track polarity).

The DC method requires a polarity change on the block that the locomotive will be entering AFTER leaving the isolated reversing track. This could be the entire railroad, but if broken up into several blocks only the following block need be reversed, and of course successive blocks as the loco reaches them. Other locos on other blocks will not be reversed.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Monday, July 28, 2003 6:57 AM
If you are using DCC the rest of the railroad does not reverse polarity. Only the isolated reversing track changes polarity, but unlike DC the locomotive keeps going in the same direction (controlled by its decoder, not track polarity).

The DC method requires a polarity change on the block that the locomotive will be entering AFTER leaving the isolated reversing track. This could be the entire railroad, but if broken up into several blocks only the following block need be reversed, and of course successive blocks as the loco reaches them. Other locos on other blocks will not be reversed.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Monday, July 28, 2003 1:31 PM
The man at Atlas is correct, buy the module and hook it up as directed. They are much to complicated to try and homebuild and unless you have a degree in digital electronics the way they work can be assumed to be magic.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Monday, July 28, 2003 1:31 PM
The man at Atlas is correct, buy the module and hook it up as directed. They are much to complicated to try and homebuild and unless you have a degree in digital electronics the way they work can be assumed to be magic.
  • Member since
    January 2001
  • From: Guelph, Ont.
  • 1,476 posts
Posted by BR60103 on Monday, July 28, 2003 11:15 PM
DCC decoder equipped locos do not care which way the AC in the track supply is headed. They have rectifiers in the decoders.
Intersting things do happen if you run a non-decoder loco around the reverse loop!
Make sure your reversing section is long enough. You don't want the cars at the back of the train flipping the reverse loop back to its original state. (I think there's a thread somewhere that talks about this...)

--David

  • Member since
    January 2001
  • From: Guelph, Ont.
  • 1,476 posts
Posted by BR60103 on Monday, July 28, 2003 11:15 PM
DCC decoder equipped locos do not care which way the AC in the track supply is headed. They have rectifiers in the decoders.
Intersting things do happen if you run a non-decoder loco around the reverse loop!
Make sure your reversing section is long enough. You don't want the cars at the back of the train flipping the reverse loop back to its original state. (I think there's a thread somewhere that talks about this...)

--David

  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Tuesday, July 29, 2003 10:53 AM
Thanks for your responses!

I wish I were able to draw a picture of the Grand Valley track plan. Following a review of the above responses I still have concerns.

I suspect there is a way to verbally describe the plan...a folded dog bone or an expanded figure eight, whatever...the plan as provided by Atlas suggests only one block. When I connect two sections of the plan (and introduce the reversing loop) there must be a reversal of polarity...a reversal of polarity of the entire block. I think I understand thus far. But what happens if another locomotive is active in the block at the time of reversal? Does it reverse direction?, or does it ignore the polarity reversal and continue in the original direction?

I was planning on track gaps in only one location, that being between the two turnouts that create the reversing loop.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Tuesday, July 29, 2003 10:53 AM
Thanks for your responses!

I wish I were able to draw a picture of the Grand Valley track plan. Following a review of the above responses I still have concerns.

I suspect there is a way to verbally describe the plan...a folded dog bone or an expanded figure eight, whatever...the plan as provided by Atlas suggests only one block. When I connect two sections of the plan (and introduce the reversing loop) there must be a reversal of polarity...a reversal of polarity of the entire block. I think I understand thus far. But what happens if another locomotive is active in the block at the time of reversal? Does it reverse direction?, or does it ignore the polarity reversal and continue in the original direction?

I was planning on track gaps in only one location, that being between the two turnouts that create the reversing loop.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Tuesday, July 29, 2003 1:43 PM
I've been studying the same problem with the layout I'm designing. The key is to isolate and size the reversing block so that only one train would be in it at a time. You can have multiple blocks with DCC but I think you will only need two for what you are planning. Take the section of track which causes the reverse loop and separate it from the rest of the layout. Hook up the reversing module to that section only. The rest of the layout can be one block. When the train enters the reversing section, the reversing module will flip the reversing block to match the polarity of the track the engine is entering from. It will do the same thing on the exit side. The decoders keep the engines moving in the forward direction regardless of polarity.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Tuesday, July 29, 2003 1:43 PM
I've been studying the same problem with the layout I'm designing. The key is to isolate and size the reversing block so that only one train would be in it at a time. You can have multiple blocks with DCC but I think you will only need two for what you are planning. Take the section of track which causes the reverse loop and separate it from the rest of the layout. Hook up the reversing module to that section only. The rest of the layout can be one block. When the train enters the reversing section, the reversing module will flip the reversing block to match the polarity of the track the engine is entering from. It will do the same thing on the exit side. The decoders keep the engines moving in the forward direction regardless of polarity.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Tuesday, July 29, 2003 2:09 PM
I would gap some more track creating more blocks, so that when you have an electrical problem, you will be able to find the area the problem is. If the engine will work in all areas but one, then you know you've found the "rough" area of your problem.

Run feeders to your blocks. Wiring for dcc is easier than dc. Run two power bus wires down the centre of your layout - underneath - and solder all your feeders to the two power buses (wires)

If you have a friend with a scanner, draw a rough plan of your layout and post it here.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Tuesday, July 29, 2003 2:09 PM
I would gap some more track creating more blocks, so that when you have an electrical problem, you will be able to find the area the problem is. If the engine will work in all areas but one, then you know you've found the "rough" area of your problem.

Run feeders to your blocks. Wiring for dcc is easier than dc. Run two power bus wires down the centre of your layout - underneath - and solder all your feeders to the two power buses (wires)

If you have a friend with a scanner, draw a rough plan of your layout and post it here.
  • Member since
    September 2002
  • From: Nova Scotia, Northumberland Shore
  • 2,479 posts
Posted by der5997 on Tuesday, July 29, 2003 3:03 PM
The book Digital Command Control, by Ames,Friberg and Loizeaux was published in 1998. In it (Section 4.3) on Reverse loops, having said that manual polarity reversing DPDT switches and auxillary contacts on switch machines work fine, as do relays, and detectors, they write
" However, with DCC it is also possible to install simple automatic polarity reversing modules. These modules can be stand alone products, or incorporated into DCC power stations." (Which is what you guys are saying)
They continue "When using DCC power stations with automatic reverse loop capability, the reverse loop itself needs to be defined as a complete power district. (They refer to a diagram showing a reverse loop powered by a separate Power Station [PS#2]linked through the main Power Station [PS#1] to the Command Station and the hand held Cab.)
They say" In this example PS#2 is an automatic reversing Power Station, while PS#1 does not need to perform automatic reversing.

They go on "Some layouts have more than one reverse loop. Under certain conditions, several reverse loops could be considered as one power district, and be powered by only one Power Station.. Otherwise , a separate Power Station is required for each reverse loop when using the automatic reversing feature. Multiple reverse loops may be defined as one power district connected to one Power Station IF AND ONLY IF no more than one train at a time crosses any of the double gaps in any of the multiple reverse loops."

They paint a gloomy picture of the result of two trains entering such loops simultaneously!

My question is:- Is this old technology? The book knows nothing of Programming on the mainline, for instance. Do we have to buy a separate power station for each reverse loop? (my layout has two loops, and a wye ) I'd like to limit my expense to the reversing modules, not have to add power stations just to get automatic reversing. Switch machine contact operated reversing begins to look good again!

Puzzled on the Eastern Shore...[;)]

"There are always alternatives, Captain" - Spock.

  • Member since
    September 2002
  • From: Nova Scotia, Northumberland Shore
  • 2,479 posts
Posted by der5997 on Tuesday, July 29, 2003 3:03 PM
The book Digital Command Control, by Ames,Friberg and Loizeaux was published in 1998. In it (Section 4.3) on Reverse loops, having said that manual polarity reversing DPDT switches and auxillary contacts on switch machines work fine, as do relays, and detectors, they write
" However, with DCC it is also possible to install simple automatic polarity reversing modules. These modules can be stand alone products, or incorporated into DCC power stations." (Which is what you guys are saying)
They continue "When using DCC power stations with automatic reverse loop capability, the reverse loop itself needs to be defined as a complete power district. (They refer to a diagram showing a reverse loop powered by a separate Power Station [PS#2]linked through the main Power Station [PS#1] to the Command Station and the hand held Cab.)
They say" In this example PS#2 is an automatic reversing Power Station, while PS#1 does not need to perform automatic reversing.

They go on "Some layouts have more than one reverse loop. Under certain conditions, several reverse loops could be considered as one power district, and be powered by only one Power Station.. Otherwise , a separate Power Station is required for each reverse loop when using the automatic reversing feature. Multiple reverse loops may be defined as one power district connected to one Power Station IF AND ONLY IF no more than one train at a time crosses any of the double gaps in any of the multiple reverse loops."

They paint a gloomy picture of the result of two trains entering such loops simultaneously!

My question is:- Is this old technology? The book knows nothing of Programming on the mainline, for instance. Do we have to buy a separate power station for each reverse loop? (my layout has two loops, and a wye ) I'd like to limit my expense to the reversing modules, not have to add power stations just to get automatic reversing. Switch machine contact operated reversing begins to look good again!

Puzzled on the Eastern Shore...[;)]

"There are always alternatives, Captain" - Spock.

  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Wednesday, July 30, 2003 3:01 PM
You add 1 module to each reversing area. DPDT switchs can be used, but if they are in the wrong position as a locomotive enters or leaves the locomotive becomes a short circuit and everything instantly stops. See, the thing is the loco is set with the module to supply + power to the top brush and - to the bottom (as an example). The track is basically AC and the decoder is a rectifier. When it enters another area the ac is still ac on the track and the decoder just keeps putting power to the motor as it was. But as the wheels cross the insulated area they can short the track as the inside track is now the outside. So the trucks and train frame become a dead short between the lead wheel(s) and the rest. The reverse module detects this short and almost instantly reverses the two track in the reverse area to match what the train's other wheels are still in. So the train keeps moving. Then as it leave the reverse track is again flipped when the lead wheels cause another short. If you run two trains in at the reverse area at the same time from different ends you could have a problem as the module would jump from train to train trying to flip off the short. But if it didn't they would head-on anyway. I have never tested this crash scenario as reverse modules are $50 and I don't do destuctive tests on high priced parts unless somone else pays. It's possibe though no harm would come because dcc uses overload protectors and shuts down if you short the track with tools, derailments, or even dragging kadee couplers. And don't ever try running non dcc locos through dcc reverse loops. One last thing is lighted cars can cause problems if the train length was just right as to cause trucks on 2 different units to bridge both ends of the reverse area at once which would be like the two trains and cause short circuit city.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Wednesday, July 30, 2003 3:01 PM
You add 1 module to each reversing area. DPDT switchs can be used, but if they are in the wrong position as a locomotive enters or leaves the locomotive becomes a short circuit and everything instantly stops. See, the thing is the loco is set with the module to supply + power to the top brush and - to the bottom (as an example). The track is basically AC and the decoder is a rectifier. When it enters another area the ac is still ac on the track and the decoder just keeps putting power to the motor as it was. But as the wheels cross the insulated area they can short the track as the inside track is now the outside. So the trucks and train frame become a dead short between the lead wheel(s) and the rest. The reverse module detects this short and almost instantly reverses the two track in the reverse area to match what the train's other wheels are still in. So the train keeps moving. Then as it leave the reverse track is again flipped when the lead wheels cause another short. If you run two trains in at the reverse area at the same time from different ends you could have a problem as the module would jump from train to train trying to flip off the short. But if it didn't they would head-on anyway. I have never tested this crash scenario as reverse modules are $50 and I don't do destuctive tests on high priced parts unless somone else pays. It's possibe though no harm would come because dcc uses overload protectors and shuts down if you short the track with tools, derailments, or even dragging kadee couplers. And don't ever try running non dcc locos through dcc reverse loops. One last thing is lighted cars can cause problems if the train length was just right as to cause trucks on 2 different units to bridge both ends of the reverse area at once which would be like the two trains and cause short circuit city.
  • Member since
    September 2002
  • From: Nova Scotia, Northumberland Shore
  • 2,479 posts
Posted by der5997 on Wednesday, July 30, 2003 8:36 PM
Flea307 So, if I go DCC, but don't install auto-reverseing modules on my reverse sections, I can use DPDT switches to revese the polarity. If I also use a polarity indicator light system, I may be able to avoid the situation you detail. I must keep my lighted passenger trains to a length shorter than any of my reverse sections, or employ an onboard lighting system such as rechageable battery circuits. (I've built those in HO, but not N )
Am I reading you correctly?

"There are always alternatives, Captain" - Spock.

  • Member since
    September 2002
  • From: Nova Scotia, Northumberland Shore
  • 2,479 posts
Posted by der5997 on Wednesday, July 30, 2003 8:36 PM
Flea307 So, if I go DCC, but don't install auto-reverseing modules on my reverse sections, I can use DPDT switches to revese the polarity. If I also use a polarity indicator light system, I may be able to avoid the situation you detail. I must keep my lighted passenger trains to a length shorter than any of my reverse sections, or employ an onboard lighting system such as rechageable battery circuits. (I've built those in HO, but not N )
Am I reading you correctly?

"There are always alternatives, Captain" - Spock.

  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Thursday, July 31, 2003 7:35 AM
Correct. The indicator can be small 12 volt bulbs jumpering across the insulator on one track on each end of the reverse area. If the light is on the track polarity is wrong. Be sure to insulate both tracks on both ends. To run a non dcc train through it you have to stop the engine in the loop, flip the switch and reverse the direction of the loco on the controller and then proceed.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Thursday, July 31, 2003 7:35 AM
Correct. The indicator can be small 12 volt bulbs jumpering across the insulator on one track on each end of the reverse area. If the light is on the track polarity is wrong. Be sure to insulate both tracks on both ends. To run a non dcc train through it you have to stop the engine in the loop, flip the switch and reverse the direction of the loco on the controller and then proceed.
  • Member since
    September 2002
  • From: Nova Scotia, Northumberland Shore
  • 2,479 posts
Posted by der5997 on Thursday, July 31, 2003 7:53 PM
Thanks, I've used red and green 12 volt bulbs before, set in a track plan above the layout. I think I'll try signals this time, evenif I have to go fibre optic the do it in N. (I'm doubtful of finding a 12 volt bulb that will look anything like prototypical for N.

I'm not fussy about my signals showing prototypical aspects if therr is a "Higher Purpose" for the signal, such as this polarity indication at a reverse section.

"There are always alternatives, Captain" - Spock.

  • Member since
    September 2002
  • From: Nova Scotia, Northumberland Shore
  • 2,479 posts
Posted by der5997 on Thursday, July 31, 2003 7:53 PM
Thanks, I've used red and green 12 volt bulbs before, set in a track plan above the layout. I think I'll try signals this time, evenif I have to go fibre optic the do it in N. (I'm doubtful of finding a 12 volt bulb that will look anything like prototypical for N.

I'm not fussy about my signals showing prototypical aspects if therr is a "Higher Purpose" for the signal, such as this polarity indication at a reverse section.

"There are always alternatives, Captain" - Spock.

  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Thursday, July 31, 2003 11:23 PM
Seems to me your solution of signals is harder than getting the simple and cheap MRC auto reversing decoder and using it - less hassle, and better operations.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Thursday, July 31, 2003 11:23 PM
Seems to me your solution of signals is harder than getting the simple and cheap MRC auto reversing decoder and using it - less hassle, and better operations.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Friday, August 1, 2003 8:37 AM
"Seems to me your solution of signals is harder than getting the simple and cheap MRC auto reversing decoder and using it - less hassle, and better operations."

You are correct, but a DPDT switch costs a lot less and you can run decoderless locos through it. I personally use a reverse module, but the person who started this wanted advise on not using a reverse module.
  • Member since
    April 2003
  • 305,205 posts
Posted by Anonymous on Friday, August 1, 2003 8:37 AM
"Seems to me your solution of signals is harder than getting the simple and cheap MRC auto reversing decoder and using it - less hassle, and better operations."

You are correct, but a DPDT switch costs a lot less and you can run decoderless locos through it. I personally use a reverse module, but the person who started this wanted advise on not using a reverse module.
  • Member since
    January 2001
  • From: Guelph, Ont.
  • 1,476 posts
Posted by BR60103 on Friday, August 1, 2003 10:28 AM
You can have your reverse loop wired through a switch machine if it has DPDT contacts. You would just have to decide which part of the layout you want to consider as the reverse loop. You would also have to throw the switch at the right time.
If you use switch 1 as the controlling switch, you could have the reverse block be everything beyond switch 1's frog plus the frog of switch 2 (i.e. the straight side)
or
you could have it as the track from the frog of switch 1 to the heel or points of switch 2 -- switch 2 would be out of the reverse block.

--David

  • Member since
    January 2001
  • From: Guelph, Ont.
  • 1,476 posts
Posted by BR60103 on Friday, August 1, 2003 10:28 AM
You can have your reverse loop wired through a switch machine if it has DPDT contacts. You would just have to decide which part of the layout you want to consider as the reverse loop. You would also have to throw the switch at the right time.
If you use switch 1 as the controlling switch, you could have the reverse block be everything beyond switch 1's frog plus the frog of switch 2 (i.e. the straight side)
or
you could have it as the track from the frog of switch 1 to the heel or points of switch 2 -- switch 2 would be out of the reverse block.

--David

Subscriber & Member Login

Login, or register today to interact in our online community, comment on articles, receive our newsletter, manage your account online and more!

Search the Community

ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
Model Railroader Newsletter See all
Sign up for our FREE e-newsletter and get model railroad news in your inbox!