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Problem using Lenz BM1s to work a bidirectional branch - NOW WORKING


richard w
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Hi Guys

 

I've setup an end to end branch on my layout.  Plan is to have a unit shuttling between the station and some sidings off scene automatically for scenic interest if nothings running on the mainlines.  

 

I've installed a BM1 at the station (Point A) and the sidings (point B).  I've been testing as I go but have come across a strange problem when trying to operate the whole thing.

 

Problem is the train ignores the BM1 at point A and hits the buffers.  If I press the emergency stop before this happens the train stops as you would expect, when released the train waits and reverses back to point B as per the settings in CV 51 and 54.  Point B works fine every time and the train then returns to A where it fails to stop.

 

Both BM1s are wired correctly taking a track feed from the bus to a braking section in point A and B.  No bypass switches are fitted as both points are dead ends.  The braking sections are on the right hand rails according to direction of travel.  So at point B DCC+ is being interrupted by the BM1 and at point A DCC - is being interrupted.

 

The manual makes no mention of polarity for these things.  Just says to create a braking section on the right hand rail.

 

I'm confused by two things.

 

The section being ignored is partially working...

 

90% of the time the train does not stop but does correctly wait and reverse when the emergency stop is released

9% of the time the train slows down but fails to stop (appears to be going into slow approach mode) 

1% of the time the train stops and reverses out correctly (this has occurred about 3 times after hours of messing about)

 

I can flip the problem from point A to B...

 

By reversing the polarity of the DCC bus wires on the back of the command station the issue moves entirely to point B and point A now works faultlessly.

 

Any ideas?  It would appear that the BM1 (or bunch of diodes if you've made one) has a stronger signal depending on the polarity of the right hand rail.  If true then am I correct in saying shuttle mode with these is not possible despite the manual and internet saying otherwise?

 

Track is fresh peco code 75 & bullhead

All wiring is soldered except the connections to the BM1 terminals

Locos are a pair of Dapol 121s (both exhibit the same behavior)

Decoders are new 21 pin Lenz Silver

Both decoders have been reset at least once

Lenz ABC and push pull mode is activated in CV 51

Constant braking distance is turned off

Controller is a Lenz LH100 command station and handset. (purchased in 2009)

 

Appreciate the help!

Edited by richard w
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Does the Lenz decoder have a setting for the sensitivity of the decoder to the Asymmetric signal ?   (I can't remember. I do know that Zimo and CT have a setting for this.).

 

If trying home-built BM1's, try adding another diode into the ring, so five or six on one side of the drop.  And the single diode could be a Schottky type, which doesn't have a voltage drop over it.  That will help make the asymmetry more pronounced, so the decoder is more likely to spot it.   

 

 

 

Yes, a pair of BM1's will work an auto-shuttle.  I've had a railbus running up/down a branch line, stopping at each end before reversing.  Two home-brew diode sets, and a loco with Lenz decoder with the "shuttle mode" option in CV settings.

 

 

- Nigel

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I don’t have a Lenz decoder to play with here but the Hornby Sapphire ADCC allows you to set up for stopping in one direction or both. It also talks to an intermediate stop but this is apparently an error in the paperwork as the decoder does not actually support it.

 

Again the right hand rail gap is in the direction of travel so you will get a psuedo positive and negative DCC polarity gap created. It shouldn’t make bit of difference as the basic DCC signal should be symmetrical. It may be you have something else on the rails affecting the offset.

 

The single direction stopping allows you to drive into a controlled stop dead end then reverse out. The both ways stop allows for a stop in either direction on a single track and with push-pull mode also giving an intermediate stop.

 

These options set by CV52 bits 0 (enable), 1 (define one/both directions), & 2 (push-pull mode) in Hornby’s case.

Rob

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Thanks for the help chaps.

 

 

I've had another test this morning but haven't got any further.  The ABC braking at point A is ignored every time and I have to hit emergency stop.  Push pull is working correctly as train always reverses out after going into emergency stop.  I've swapped the BM1s over and it made no difference (point B continues to work fine).  I wired both BM1s in series at point A out of desperation but again no luck.

 

 

As said previously if I reverse the polarity of the bus wires the problem moves to point B.

 

 

The layout is very much bare bones at the moment with only the bus wires and a few droppers connected.  I've checked the manual for the decoder again and there doesn't appear to be a sensitivity setting for the ABC signal.  Push pull and ABC is done from CV51 on the lenz chips.  You can choose with or without intermediate stop.  There is no option for single direction stopping as far as I can see.

 

 

Current thoughts are to use something like a reverse loop module to switch the polarity of the branch at the half way point.  Should bodge the system into working as both sides want the right hand rail to be positive.

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What is the power supply to your system? - is it grounded on one side or symmetrical ? - is it from a source recommended by the manufacturer ?

Are you mixing any other power supplies eg 5V rails and sharing any common wiring ???. is there any other wiring connected to the track not involved in the plain dcc and BM modules??  .the 'commoning' might occur in other areas of your layout - out of sight and out of mind.... just trying to think of any 'external' factors which might cancel out the diodes...

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Is it something as simple as changing the running rail side where the BM1 is fitted at one end of the track?

 

I only suggest this in case the system always assumes the train is moving forward when it enters the "brake zone" and that, when travelling in reverse it needs to see a different voltage in the same/pother running rail.

 

The fact that you can swap the links to the track bus over to get the opposite end of the layout to work could be the reason why.

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The power supply I'm using is a Lenz TR150.  Everything I'm using is Lenz right down to the decoders.

 

I've tried some other locos with Lenz silvers of various ages and all produce the same behaviour.  Currently the layout only has the command station and the BM1s connected to it.  I've made up some snubbers/rc filters from bits lying around and fitted them at each end of the bus which runs in a U shape.  Unfortunately the problem persists despite the filters.

 

Something I have noticed more recently is even on the end that works there is sometimes a considerable delay before the loco actually starts ABC breaking.  In some cases it runs for about 2- 3 feet into the braking section before it actually starts slowing down.  This happens more often now the filters are installed so perhaps one of the components I've used for those is faulty.

 

I'm going to setup a short length of track on the floor and plug everything into that and see what happens.

 

Edit - I have tried using the left hand rail just to see what happens and the loco ignores it.  Once its reversing out the left rail becomes the right and the loco then stops and reverses back into the siding.

Edited by richard w
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I've removed the layout from the equation and setup both BM1s on a short stretch of track.  It still doesn't work.

 

As before the train stops and reverses out correctly at point B only.  Swapping the track feeds over on the command station moves the issue from point A to point B.  You can see the setup below...

 

post-23223-0-52863800-1530476180_thumb.jpgpost-23223-0-11097400-1530476399_thumb.jpg

post-23223-0-74708600-1530476390_thumb.jpgpost-23223-0-71674200-1530476417_thumb.jpg

 

I can only think the command station is somehow at fault?  Its about 9 years old and spend all of them in the loft.

Edited by richard w
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I know this might sound silly, but have you tried swapping the wires around on one of the BM1s?

 

AFAIK they are just diodes and if you are getting the negative part of the signal when the decoder is expecting the positive then that should be corrected by swapping the wires around - your suggestion that swapping the feeds wires around swaps the ends of the problem is what is making me think it might need this silly a solution.

Edited by WIMorrison
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I've tried that a few times but the results are the same (loco doesn't stop).  It also stops the loco reversing out after releasing the emergency stop button, manual control doesn't work either.  

 

I've read quite a bit on the subject but looking deeper into the below suggests negative asymmetry is ignored unless I'm misinterpreting it?

 

https://dccwiki.com/Asymmetric_DCC

 

I might be able to chop the left hand rail instead and turn off the direction dependency setting in CV51.  Although that may get trains stopping at both ends it will probably mess up the push pull.

 

I really hope it’s something silly but so far I'm at a loss as to what it might be...

Edited by richard w
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If the BM1's can be swapped from (rail) end-A to end-B, and the fault stays at end-A, then that suggests both BM1's are working properly. (and there is next to nothing to go wrong with them).   Correct rails cut.  I assume the DCC has been reduced to just feeding the short test track, nothing else connected.   

 

The BM1's two terminals are "handed", so it is important to use the right way round, but that appears to be the case.

 

Which does seem to point at the command station.   I can just about see a scenario where that might be a factor.   Is there any way of trying a different command station, just to rule that out?  Even a Sprog in "Sprog Command Station" mode would do for testing.

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I've tried that a few times but the results are the same (loco doesn't stop).  It also stops the loco reversing out after releasing the emergency stop button, manual control doesn't work either.  [/size]

Would you not get this if the BM1 is connected the wrong way round because it is endeavouring to drive a train in the direction that the BM1 is designed to make the train stop?

 

Is there any mileage in noting which way everything is connected at Point A and then removing that BM1 so that you only have the BM1 at point B to try various combinations with? Make sure that the railcar remains the same way round all the time though.

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Swapping the BM1 terminals over is the same as gapping the left hand rail as opposed to the required right hand rail, i.e. changing the direction of travel.

 

The rail side gap is according to the direction of travel. The ‘manual’ makes that very clear.

 

It may be that you need to have two isolated sections in between the BM1s, i.e. main track, isolated track, second isolated track, main track. Are both BM1s attached to the same isolated section.

 

Rob

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  • 2 weeks later...

I've got it working!  However the underlying issue is still there.

 

I've set the BM1s up on the layout again and created an isolating section between them.  On the end that doesn't work I've reversed the polarity of the track feeds so it does work.  The isolating section between them prevents a short circuit and on it I've installed a reverse loop module.  

 

The unit now stops and reverses back down the branch at both ends.  The reverse loop module provides a transition between the two sections.

 

post-23223-0-29052500-1531654126_thumb.jpg

 

I suspect the real issue might be with the command station but not having another unit to test with prevents further troubleshooting.  A reverse loop module was substantially cheaper than an new command station so I'll stick with that.  I'm just happy its working..

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  • 2 years later...

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