DCC works fine with individual locos but fails when two are placed on track together.

[whiskers]

I have a small N-gauge loop (new to the hobby) that I'm playing with. I bought a loco + dummy pair (https://railsofsheffield.com/blogs/news/n-gauge-lner-nrm-40-hst) and installed DCC decoders in both (the dummy car allows lighting control).

 

If I place either on the track on their own then the DCC signal looks fine (as per a logic analyser and a scope) but if I place them both on the track at the same time then the DCC signal seems to be killed completely and I can hear a faint whining sound from the DCC decoder PCBs. I've attached images below of the good and bad states in case that helps. The top trace is a digital representation of the DCC packet and the bottom trace is the analog voltage.

 

Any idea what could be happening here? Any other information I need to provide? I'm well versed in electronics and have pored over the DCC spec to see if there are any clues to no avail...

 

This is the DCC signal looking great with a single loco...

 

 

And here it is when the second device is placed on the track... It looks almost like it is shorted out with brief pulses which I guess is where the whining noise comes from.

Edited January 10, 2022 by whiskers

[grriff]

Not sure if this helps but do both decoders have the same address? 

[whiskers]

Originally they did - which I believe should be fine according to the spec. Subsequently I programmed them each to have different addresses and the same issue still occurs.

[jpendle]

So the addresses don't matter. 

 

What decoders have you fitted, what DCC system do you use?

 

Does the motorised car work as it should when it is the only vehicle on the track? Have you tried both decoders in the motorised unit?

Have you any other decoders that you could try?

 

Regards,

 

John P

[WIMorrison]

Those traces look like two completely different command stations, as the waveforms are very different.

[whiskers]

The decoders are these: https://railsofsheffield.com/products/rails-connect-ros-n18-rails-connect-decoder-next-18-4-function-decoder which I assume are just rebadged products made by someone else.

 

The DCC system is my own code based on the DCC specification which works fine with a single loco on the track but when a second loco is added there is some sort of electrical fault that occurs (see the traces I posted) - this fault isn't coming from my side of the system (it has no way of knowing a second loco was just placed on the track...).

 

"Does the motorised car work as it should when it is the only vehicle on the track? "

 

Yes!

 

"Have you tried both decoders in the motorised unit?"

 

Yes! And I've just done it again to make sure. I ran the motorised unit with the decoder it had in it (address 4 after reprogramming) and it is fine, I swapped the decoders and ran it again (address 5 this time) and it works fine.

 

Have you any other decoders that you could try?

 

Sadly not. If you could recommend a Next18 decoder that would be compatible (I'm new to this..) then I'll buy a couple to try out!

Edited January 10, 2022 by whiskers

[whiskers]

9 minutes ago, WIMorrison said:

Those traces look like two completely different command stations, as the waveforms are very different.

 

They are both taken from the same setup within a minute or so of each other, nothing was changed apart from placing the non motorised portion of the twin set on the track alongside the motorised part.

[SamThomas]

26 minutes ago, whiskers said:

Sadly not. If you could recommend a Next18 decoder that would be compatible (I'm new to this..) then I'll buy a couple to try out!

Try a couple of branded decoders such as Lenz, ESU or Zimo.

 

I would avoid any rebranded decoders.

 

If you still have the same issue with branded decoders do you know anyone local to you who could bring & try out their syste ?

Edited January 10, 2022 by SamThomas

[Dungrange]

What happens when you remove the motorised power car and just leave the non-motorised power car on the track?  Does the trace revert to the original or remain as per the trace with both items on the track?  I'm just wondering if your problem has nothing to do with having two locos but that there is some sort of fault with the non-powered car.

[whiskers]

2 minutes ago, Dungrange said:

What happens when you remove the motorised power car and just leave the non-motorised power car on the track?  Does the trace revert to the original or remain as per the trace with both items on the track?  I'm just wondering if your problem has nothing to do with having two locos but that there is some sort of fault with the non-powered car.

 

That is fine, I did say so in my original post. :-)

[whiskers]

4 minutes ago, SamThomas said:

Try a couple of branded decoders such as Lenz, ESU or Zimo.

 

I've heard the name Lenz from some videos but the other two mean nothing to me. I'll grab a couple of Lenz ones to have a go with, thanks!

[SamThomas]

21 minutes ago, whiskers said:

 

I've heard the name Lenz from some videos but the other two mean nothing to me. I'll grab a couple of Lenz ones to have a go with, thanks!

Don't forget to let us know how it pans out.

[Lemmy282]

If each one works correctly independently, could it be that your power supply is not up to driving two decoders? I know it is N gauge and unlikely, but worth trying.

[Kaput]

Get a pair of Zimo MX618N18's, cheaper than the Lenz ones and just as good if not better.

[adc]

What is the current capacity of your system? Since both work independently, could it be that it can cope with one but not two chips connected?

 

The lower waveform in the first picture doesn't look right to me, the DCC voltage should go -ve.

 

Alan

 

[jpendle]

5 hours ago, whiskers said:

The DCC system is my own code based on the DCC specification which works fine with a single loco on the track but when a second loco is added there is some sort of electrical fault that occurs (see the traces I posted) - this fault isn't coming from my side of the system (it has no way of knowing a second loco was just placed on the track...).

That doesn't really answer my question.

Do you mean that you have a piece of home built electronics controlled by a PC?

If that's the case then the likelihood is that's were the problem lies.

 

Regards,

 

John P

[Izzy]

Are these Rails decoders able to work on DC? If so just connect up a 9vPP3 battery to the track instead of your DCC system. This would be a quick way to see if the locos or system are the issue at least cost. Either could be possible. 

[whiskers]

10 hours ago, Lemmy282 said:

If each one works correctly independently, could it be that your power supply is not up to driving two decoders? I know it is N gauge and unlikely, but worth trying.

 

I have it hooked up to a dedicated bench power supply that can output 150W (over 12 amps at 12V), the second decoder is really just turning on a couple of LEDs should be using very little power...

[WIMorrison]

6 hours ago, jpendle said:

That doesn't really answer my question.

Do you mean that you have a piece of home built electronics controlled by a PC?

If that's the case then the likelihood is that's were the problem lies.

 

Regards,

 

John P

This would also be where my vote lies

[whiskers]

10 hours ago, adc said:

What is the current capacity of your system? Since both work independently, could it be that it can cope with one but not two chips connected?

 

The lower waveform in the first picture doesn't look right to me, the DCC voltage should go -ve.

 

The DCC spec doesn't use -ve voltages. The rails are energised alternately (one +ve and one at 0V) this means that the current flow changes direction but there is never a "negative" voltage.

[whiskers]

6 hours ago, jpendle said:

That doesn't really answer my question.

Do you mean that you have a piece of home built electronics controlled by a PC?

If that's the case then the likelihood is that's were the problem lies.

 

It's a microcontroller running code that wrote connected up to a h-bridge that drives the rails. Since the microcontroller can never "know" that there is a second device on the track then how could it change anything to respond to it being added? I'm almost certain the fault is on the track side.

 

Edited January 11, 2022 by whiskers

[WIMorrison]

Given that thousands of people use these decoders in locos and those thousands of people are using commercial DCC command stations and they have no issues, this would suggest to me that the issue lies in the home brew system you have created.

 

take the locos to another layout, or borrow anothe command station and prove that the loco/decoder combination works correctly. You can then fault find the issue as you will know where the fault lies.

 

presently you are blaming the decoders, or anything excepting your homemade device - however you don’t actually know if anything is working properly, therefore any suggestions are purely guesses.

 

proper fault finding techniques will pinpoint the fault, blindly blaming anything will not.

[whiskers]

3 minutes ago, WIMorrison said:

Given that thousands of people use these decoders in locos and those thousands of people are using commercial DCC command stations and they have no issues, this would suggest to me that the issue lies in the home brew system you have created.

 

take the locos to another layout, or borrow anothe command station and prove that the loco/decoder combination works correctly. You can then fault find the issue as you will know where the fault lies.

 

presently you are blaming the decoders, or anything excepting your homemade device - however you don’t actually know if anything is working properly, therefore any suggestions are purely guesses.

 

proper fault finding techniques will pinpoint the fault, blindly blaming anything will not.

 

It's possible, I just can't see how! I'm not blindly blaming anything though, I've done a lot of testing and checking!

 

I think the fault is in the non motorised part of the twin set, that's where the whining sound comes from when the failure occurs, I wonder if it is shorting the rails somehow...

 

I'm going to take it to my supplier today and ask them to check it for me. I can't currently get hold of a controller, they seem to be out of stock everywhere...

 

[BoD]

17 minutes ago, whiskers said:

I think the fault is in the non motorised part of the twin set, that's where the whining sound comes from when the failure occurs, I wonder if it is shorting the rails somehow...

Yet you say that it is fine on the track on its own.  

[whiskers]

3 minutes ago, BoD said:

Yet you say that it is fine on the track on its own.  

 

That's a very valid point - it really is odd. I'm going to try to get hold of a stock DCC controller and see what happens with that, can't understand what's happening here.