What is the significance of polarity in DCC?

[Tim Brien]

Being an analogue devotee with positive current to the right rail when driving forward (must be largescale standard and not NMRA standard - opposite polarity), I find the polarity issue confusing. One looks at the Massoth 'L' manual illustrations and finds positive polarity on one rail and yet for the same orientation of drive and decoder positioning in the Massoth 'LS' manual, it shows the positive to the opposite rail and states positive to left rail driving forward. I assume this is the NMRA standard.

Obviously, I want to configure all my locomotives to 'standard' direction and thus need to forget 'largescale' polarity standards and follow NMRA standards.

 

[Tony Walsham]

Tim,
I think you will find the NMRA standard is + (positive) to the right rail when traveling forward.
When LGB first came out they had it the other way around and it has been that way ever since for Large Scale.
It is also the reason why Bachmann fit a polarity switch on some locos to select either the NMRA or LS standard wiring.

 

[trammayo]

Tim,
I think you will find the NMRA standard is + (positive) to the right rail when traveling forward.
When LGB first came out they had it the other way around and it has been that way ever since for Large Scale.
It is also the reason why Bachmann fit a polarity switch on some locos to select either the NMRA or LS standard wiring.

Its a pity Bachmann didn't do it with the Thomas range. When I started out (in relative isolation - internet access difficulties) I just remembered that trolleybus overhead had the positive to the centre of the road and, by a happy coincidence alone, my adoption of the same proved to be the NMRA standard!.

 

[shropshire lad]

Being an analogue devotee with positive current to the right rail when driving forward (must be largescale standard and not NMRA standard - opposite polarity), I find the polarity issue confusing. One looks at the Massoth 'L' manual illustrations and finds positive polarity on one rail and yet for the same orientation of drive and decoder positioning in the Massoth 'LS' manual, it shows the positive to the opposite rail and states positive to left rail driving forward. I assume this is the NMRA standard.

Obviously, I want to configure all my locomotives to 'standard' direction and thus need to forget 'largescale' polarity standards and follow NMRA standards.

Adding a value of "1" to CV 29 will reverse the motor if it runs the wrong way anyway.
Always worth bearing in mind if you use POM and want to double head locos smokebox to smokebox too

 

[steve parberry]

It dosent really matter as long as you set the the motor to the same pole.
Lets not forget you can put your loco on either way and it will always go forward if forward is selected due to the chip have a bridge rectifer built in.

So as the lad said, you can either reprograme the once or swop the wire but once done it will always be correct.

 

[Dtsteam]

I think you will find the NMRA standard is + (positive) to the right rail when traveling forward.
When LGB first came out they had it the other way around and it has been that way ever since for Large Scale.
It is also the reason why Bachmann fit a polarity switch on some locos to select either the NMRA or LS standard wiring.

Thanks for that Tony - I've been wondering why some of my trams (which are chipped with HO scale decoders) always ran backwards at first

 

[ntpntpntp]

Must admit, I've standardised on the LGB polarity rather than NMRA, as it tends to be the other brands of loco that get physically re-wired when I fit decoders. The LGB locos are either plug and play or factory-chipped.

Yes you can flip a bit in CV29 to reverse the motor direction, but I have noticed in a couple of cases that the headlight direction doesn't flip and you get the rear light on going forward! I forget whether this happened when running the chipped loco on DC or when running DCC as well, but anyway I found swapping motor wires or headlight wires cured the problem.

My main objective is always to make sure the chipped loco continues to run in the same direction as everything else under DC and runs "forwards" when instructed under DCC, with the correct lights on. With a bit of experimentation the corect combination of CV29 and wire swapping sorts it out.

 

[stockers]

I agree with Nick - just swap the motor wires over. I too have noticed the lights problem using CV29.