2 rail DCC question

Strange. On my test track all of my locomotives work properly in both directions while they are powered with DCC. What kind of “lok” is it? 

Just to be sure we’re on the same page - your loks are two rail? I’m using a generic 2 rail HO lok. But, specifically , an Athearn current production GP38. I bought it without their DCC chip and I installed a Digitrax DH126

Steve

Steve, yes I am 2 rail O. I have the NCE DCC system. I just have a test track for the moment but I have locomotives with MTH decoders, NCE decoders, QSI decoders and I have one locomotive with a Loksound decoder. They all work in both directions. DCC would have never made it this far if it only worked in one direction or needed a certain polarity for the engine to operate properly. I am not familiar with a DH126 decoder but I find it hard to believe it is only supposed to work one direction. 

When you say “it doesn’t run properly” exactly what do you mean? 

There is something strange going on there.   I have 2 Rail DCC and have installed decoders in more than 50 locos.     I have never had one do that.     I have never used a Digitrax 126 decoder, but I have used a few of the DH123 decoders.    They are work fine.

Now your description may have confused us.    When you put the loco on the track the first way, does it go forward?     Then when you turn it 180 degrees, does it still go forward which is now the opposite direction from the first setting?      That is the way it is supposed to behave.   

The programming defines the forward direction to the decoder.    The decoder than puts out DC to the motor to go in that direction, whenever it gets a signal to go forward.

ON straight DC, changing the loco 180 degrees on the track, changes the polarity to the loco and it goes in the same direction as went before.

Gentlemen, thanks for the replies. I’m going to go back and review what I’ve done. I have extensive knowledge in the electronics area, BUT that doesn’t always translate well into my hobby. Again, let me go do some more testing. Basically, you have answered my question and that is, it should  work correctly no matter which way it is facing on the track.

Thanks, 

Steve

i use digitrax decoders on s gauge two rail and o gauge three rail with out issue

A dcc signal is mirrored to both the positive and negative sides of the signal.  This allows the dcc signal to be pulled off the rails no matter which way the loco sits on the track.  Orientation doesnt matter. You can actually  reverse the polarity of the signal as the loco is running and it would be able to read the dcc signal. 

It sounds like a possibility that you have a diode inline on one of your power feeds to the track.  That would block one half of the dcc signal, so that the orientation on the track could prevent the signal itself from being seen by the decoder.  (One direction the signal is seen, flip it around and the signal is blocked by the diode. )

Furthermore, some locos will change from DCC to conventional mode if it does not detect a dcc signal present. This could cause the loco to simply run around the track, ignoring all commands you are sending. 

The short version: it’s working now.

The long version: Marklin MS2 controllers are supposed to be DCC compatible. 🤨 I’ve been doing my testing with an MS2. I decided to hook up my Digitrax Zephyr and the loks with DCC decoders are functioning correctly.  As mentioned in a previous post, I have years of electronic experience. I am always skeptical when something claims to be compatible because they usually ain’t. Over the years I’ve experimented with different digital command protocols with the intent on using just one.

Anyway, again, thanks for the replies.

Steve

We've seen a lot of weird things happen with "compatible" DCC decoder too.    It is a scarey word.