Bi-directional ABC and asymmetric DCC

[vignale]

Hi everybody,

 

I am fitting an old DC layout for DCC operations. I am using Zimo decoders and implementing ABC to stop the trains in the station's platform.

Some of the platforms are bi-directional, so the trains can move from left to right or vice versa. On both ends of such platforms there is a signal controlling the train movement with a isolated ABC section ahead of it.  The right rail is fitted with a DIY BM1 module. I have tested it in a separate track and it works great.

My issue is that the platform is not long enough to fit 2 ABC sections. Therefore, both rails are controlled by a signal and when the signals are red, they both get connected to the BM1 module. I tried to make a drawing to better explain myself.

 

This is my current wiring situation.

From a "theoretical" point of view it's clear to me that this should not work. As both rails have the DCC wave altered, the DCC signal is again symmetrical (although the amplitude of the wave is smaller) and the decoder can't pick up the ABC signal.

As comparison, this is what the wiring looks like for a platform that is longh enough tof it 2 ABC sections (not to scale with the previous drawing).

 

And this should work with no issues.

Of course if at least one signal is green ABC works as inteded even in the first situation. But, with both signals red, the train can not pick up an asymmetric DCC signal and so it doesn't stop.

 

So my question: is there any way to make ABC working as intended if the situation is the one in the first drawing?

The easiest solution is to set the platform as one-way, but I would prefer not to.

If anyone has solved a similar problem in the past I am happy to learn.

 

 

Thank you,

 

Alessandro

[jpendle]

There are a number of ways that this could be done, depends how much money you want to spend.

One simple way would be to add a double pole double throw switch and connect the wires from the signals to this, wired such that in each position just one of the BM1 modules is active.

You could also use occupancy detectors either side of the platform and use them to control relays, or a simple logic circuit, to turn on the correct ABC module when its signal is red.

 

Regards,

 

John P

[vignale]

I have considered turning on a single ABC module, however it would not be a perfect solution. For clarity, this is my current layout. Signals are placed on the left side of the track.

 

 

If a train enters platform 3 from the right, I can turn on the ABC controlled by the signal S3L. However, S3R would not have ABC and so the locomotive could reverse out of the section, possibly against a turnout that is closed and not thrown (look at the one near S2R).

Current sensing occupancy detectors are fitted in each section and IR sensors on each turnout.

 

I know I am probably hoping to achieve something that is not possible to do. As you suggested, I can choose to only activate the ABC module needed to stop the train in the direction it's travelling, even if it means that reversing out of the block is allowed. It's a pretty edge case scenario and I think I could live with it.

Only thing is, the BM1 module is wired directly to the red lamp of each signal so it would mean that I have to show a green aspect (not stop) even is the path is not clear (for example, S3R must show green even if the double slip turnout is turned against the signal). Again, I can live with it, but it would be better if it can be avoided.

 

 

Regards,

 

Alessandro

[Ray H]

Presumably your loco(s) are set to obey the BM1 in both directions of their travel i.e. it doesn't matter which rail the BM1 module is "fixed" to. If that is the case then I think you only need one BM1 module (provided that the braking section is in the middle of the platform track). Clearing the starting signal in the relevant direction will over-ride the BM1 and the train should resume in the direction it was travelling.

[vignale]

This is also a good suggestion, however I see three issues:

1. If the loco is reversed while waiting at the signal, turning the left signal to green will allow it to reverse out of the platform (and it can be an issue, as I explained before);
2. It only checks for a signal to be green (as BM1 is hard-wired to the red aspect), so if the loco is waiting for the left signal to be clear and the right signal turns green, the ABC will be disabled and the loco will leave the station against a red signal;
3. It only works if there is a single braking section in the middle of the platform. In longer platforms there are 2 ABC sections (one at each end). If this is the case, having the decoder set to obey the ABC signal in both directions of travel will prevent the train from entring the platform.
   
   
   
   For example, a train coming from S2 into platform 2 would be stopped by the red signal S2R.

Again, I am absolutely nitpicking and I will be happy with a compromise (probably completely removing the BM1 module connected to S3R).


Regards,

 

Alessandro

[Nigelcliffe]

Yes, the original analysis is correct, if the BM1 is active on both rails, there is no asymmetry, and thus no braking. 

 

A possible fairly simple solution.

Place an optical spot detector (or a track occupancy detector) on the entry to the tracks (at each end) fitted to a relay.  The relay determines the direction that the ABC will work (though signal by-pass still operational).   
An arriving train sets its ABC correctly.  A departing train will reset the ABC for a train in the other direction, but this doesn't matter as another train following will set things to the correct direction.  

This will require a latching relay (simple) and two detectors (optical light detectors would do it).  

 

 

- Nigel