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Thread: Digitrax Brake Section

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    Default Digitrax Brake Section

    Hi,

    With a Digitrax command station and a booster, I wired a brake section on my layout according to the attached diagram. The brake section works fine in stopping a loco while keeping its sounds and lights, though every time when a loco enters the brake section the ballast bulbs glow. It appears that there is always a short when a loco is getting across the 2 power districts. Is this normally or did I wire the brake section wrongly? Many Thanks for your advice.

    Kent
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    I'm not fully aware how the Digitrax Brake Booster is supposed to work, so it may be normal to see the bulbs glow due to different voltage levels or changes in the DCC signal (they need to be there, otherwise to loco wouldn't be stopping, so I guess some glow is perfectly normal, but I can't say how much)

    Do you also see them glow when the switch is set to "RUN" and a loco is entering/leaving the brake district? If so, you should switch the wires between the switch and the brake district track. If not, that part of the wiring is fine.

    However, what you can try is to switch the RAIL A/RAIL B connection from the brake district booster to the switch. See if that leads to less intensely glowing bulbs.

    Hope this helps,
    Heiko

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    Thanks Heiko, I found the lamp just glows when the first set of wheels enters the brake section and turns off when the second set of wheels enters the brake section. I'm just wondering if such "short" short circuits has to be there when a loco gets across different power districts. Thanks again.

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    Yes, that's the timing I expected, anything else would be real strange (that is, unless there are lit passenger cars or similar).

    But I was wondering more about the intensity than the timing.

    Also, when the switch is set to RUN you shouldn't see any glow, right?

    Heiko

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    I still see the bulbs glow briefly when the switch is set to RUN, though they turn off right away when both sets of wheels of the same loco are on the same power district. I guess this might be normal and is why there are ballast bulbs in the design. Am I right???

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    When the switch is set to RUN, the bulbs should NOT glow. Not even when the loco bridges the gap. Try reversing the wires between the switch and the brake district.

    When the switch is set to BRAKE, a little glow should be expected.

    Heiko

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    Hi Heiko, I thought you meant whether or not the Loconet Connection was set to RUN in the previous message. If RUN refers to normal connectivity, then there is no shortage at the gaps. Only when the switch is set to BRAKE, there is a little glow. I guess that should be considered normal. Many Thanks Heiko.

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    Yes, that I would consider normal and yes, I was talking about that switch in the Digitrax schematic.

    Now everything properly muddied up?

    Best,
    Heiko

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    This is correct behavior.

    To understand this, you need to understand a bit about how DCC works under the hood. The DCC messages are turned into a series of 1's and 0's, put on the track in a square wave signal that alternates polarity, so it looks like AC. So for instance, loco 10 forward 25 might end up being:

    0011001001 (This is not a real translation, just an example)

    This pattern goes out on the loco net, and all boosters repeat it simultaneously. When a loco bridges a gap between boosters, they both go high at the same time, and both go low at the same time, which means there is no desire for the electricity to flow between boosters.

    With a brake district the booster running that district takes the "loco 10 forward 25" message and turns it into "loco 10 forward 0". This way it's still getting a DCC signal, lights and sound stay on, and it knows to stop. But that translates into a slightly different message:

    0011000000 (Again, not a real translation, just an example)

    Now, while the loco is bridging the district one booster is putting out a 1 (power) and the other a zero (no power), and so current tries to flow from one booster to another. If left unchecked it would quickly spike to multiple amps and damage the booster. The light bulbs serve as resistors that limit this current, in fact a plain old resistor would work. When this current flows it causes the bulbs to light up. (It turns out 1's and 0's are both power on, but for different lengths of time which still leads to a one on, one off situation. A detail that the pedantic may bring up but doesn't matter for the discussion here.)

    More info at http://www.digitrax.com/tsd/KB193/brake-sections/

    I'm personally not a huge fan of this feature, but then I also don't have a use case for it. I am curious about your use case.
    Last edited by bicknell; 23rd Jan 2019 at 08:50 AM.
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    Quote Originally Posted by bicknell View Post
    This is correct behavior. <snip>
    Points for an excellent layman's description of what's going on, and bonus points for heading off the pedantic 1/0 correction at the pass. Well done, Leo.
    Never mistake a guy who talks a lot for a guy who has something to say...

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    Thanks Leo for the detailed info. I’s using this to automate the double crossing on an atlas n18 layout. I installed 16 arduino IR sensors, 2 underneath the roadbed on each of the 8 tracks entering the intersection to detect incoming trains. Using an arduino mega and an arduino solid state relay board to flip flop between normal DCC signals and Brake section signals supplied to the 8 tracks entering the intersection, 2 trains are now running unmonitored on the double 8 tracks without collision... :-)

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    That is a very busy track plan with lots of interesting "problems" including the crossing and a reversing section. Rather than use the brake section feature, my suggestion would be to use the Arduino to connect to a Raspberry Pi running JMRI and issue throttle commands. That way the Arduino can simply tell the locomotive to stop or go. Then you don't need a break section and won't have this problem. That saves needing an extra booster and eliminates the possibility of current flow between two boosters. Alternatively you could get one of the older serial to LocoNet adapters and have the Arduino issue LocoNet commands directly. That would save needing a computer running JMRI.

    Just suggestions though, your way clearly works as well!
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    Your suggestion sounds great, Leo. Doing away with the USB connection with the PC is my next project. I’ve built a LocoNet Shield basing on the blueprint from Scuba.net. That way I can also add some traffic lights and take away the PC connection. If I also use Arduino to send JMRI commands to the trains through the Loconet shield, do I need to install any special electronics on the locos and the freight cars? I have consdered using RFID readers and tag stickers in the past, though this now is becoming a future project. Kent

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    If you have the LocoNet shield you can issue LocoNet commands directly, no need for JMRI / JMRI commands. Basically you would want to implement the same logic as a "throttle", where you acquire a locomotive, and then run it's speed based on your sensors. You would basically be emulating a DigiTrax throttle plugged into the LocoNet. In terms of driving the locomotive nothing special is required.
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    Oh yes, Loconet commands in the above context. I guess I still need to figure out how to identify which loco is on a specific track (I guess this can be handled through the throttle logic though it may be more advanced than I can comprehend), and how many freight cars are there if I use Loconet commands for collision avoidance.

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    It sounds like you should read up on a couple of things:

    Transponding - Allows you to track where a train is on the layout. http://www.digitrax.com/static/apps/...app%20note.pdf

    Occupancy Detection - Is a current-drawing device present in a block. Can be done with a lot of different equipment.

    Resistor wheel sets - Replacement wheels for freight cars that trigger occupancy detection.

    Basically you would divide into blocks, transpond them to figure out which train is in the block, occupancy detect to see when the last car (e.g. caboose) leaves, and use resistance wheel sets so the cars keep the occupancy detector high.
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    You might still want to involve JMRI (possibly on a Pi) if for no other reason than it would be easier to implement the logic. JMRI can monitor the Arduino(s) for sensor messages and use its own internal logic features to decide when and how to issue commands. JMRI has a whole "Dispatcher Pro" section that is designed specifically for train automation based on sensor feedback, schedules, and other inputs.

    You can do this with just an Arduino but you'll be reinventing a lot of wheels and other parts that JMRI has already pretty thoroughly built into a car.

    And if you're worried about size, the Pi Zero is actually a good deal smaller than an Arduino Uno, and a 3B+ is not much bigger.
    Never mistake a guy who talks a lot for a guy who has something to say...

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    I'm with TD on this. Don't reinvent the wheels, when JMRI gives you the opportunity to have it all already. Plus you can always attach a big screen and a keyboard to the Pi and do some proper debugging...

    Because otherwise this is what happens:

    Quote Originally Posted by https://jmri-developers.groups.io/g/jmri/message/947
    Documentation for this is sketchy, and in trying things out on two different implementations, I get two different results.


    MB+S98<;>S97 on the LNWI selects address 98. I don't know what (if anything) it does with the S97 portion of that command


    MC+S97<;>S96 on JMRI (4.14) selects and modifies address 96.
    Heiko

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    I consider the crossing automation as a patch to the layout so 2 trains can run unattended on the double-8 tracks. It is definitely not a full blown solution, which may involve Transponding, resistor wheels on freight cars, and JMRI automation. I'm still learning and I'll bear in mind your advice moving forward.

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