[Richard Johnson]

QUOTE (Brian @ 13 Feb 2008, 23:00) <{POST_SNAPBACK}>Hi all
I have been reading with interest on another forum the topic of whether of not the power bus (which also carries the DCC data signals) should/can be wired under the layout as a ring or radial circuit. Searching the internet I can't find any reference to ring circuits being used anywhere, as they all refer to radial power buses.

Will any short circuit currents be better and fast returning via a ring or a radial bus
Will the data on the power bus be best served by a radial or ring circuit?

So, is a DCC ring bus advisable or is it to be the radial or perhaps it just doesn't matter?

***Brian, on a small layout its not critical, but the generally recommended configuration on many fora from many wise men over many years is radial - command station at the centre and roughly balanced left/right length.

There is not benefit power wise from a ring and there are potential negatives for data as the layout size increases with a ring... so if a simple answer is wanted "radial" is best.

At this point someone usually chimes in "but the track is a ring" - the answer is no, it is not - unless it is a simple oval , otherwise its a series of short sections, as every item of pointwork is effectively creating a break in the "track ring"

Richard
DCCconcepts

 

[Richard Johnson]

I think both the attempts to quantify it and their qualifications are well meaning guesses or "illustrative", and these will always fail as they presume something in isolation.. which related problems never are..

Trying to express the cause and effect in simple terms will always fail. The point is that the bus on a model railway is always imperfect and will always have more or less of other influences on it.... Inductance, resistance, accidental shortcomings of various kinds.... careless connections, bad wiring layout etc etc.... all accidental and really largely unnoticed and unseen.

DCC is rubust & reasonably tolerant but often, irrespective of layout size, circumstances can combine to make things work less than perfectly.

All we can do is take aspects of best practice and apply them where we can - trying to create a real world set of recommendations that will give the "average but careless with wiring" modeller a best possible chance of a low fault or frustration rate:

Therefore we who advise consistently try to recommend certain ways of creating the wiring configuration such as separated or twisted bus, certain sizes of cable and certain other practices. Each may indeed seem to be slightly overkill if taken in isolation, but all taken together and applied as recommended, will result in an almost 100% success rate.

So - forget rationalising one part of it: It will always fail as a debate. We are trying to give pragmatic real world advice that will always work for the average railway modeller who considers most things connected to wires as akin to witchcraft. A tee or radial bus is part of this.

In the real world too: layout size isn't always needed for ring Vs raadial to have a real world difference

I have modified busses in layouts large and small to resolve problems. In at least two real world cases, both smallish layouts by world standards, removing the ring and then adding termination/suppression to each of the bus ends resolved several problems that had bedevviled the layouts and frustrated the owners for months.

Technically why - it des NOT matter - it did the job... removing the ring worked. Adding the 150 ohm resistor+ 10microfarad apacitors as resistors worked too. Which was most responsible for the improvement, I have NO idea - I don't take a digital storage scope with me to look before I act.

The point is, BOTH are issues which are agued over ad nauseum for the sake of it by those who just like to argue.... but the truth is, pennies in parts and ten minutes did the trick...less time than I've had to spend on the last wo replies. Why waste time in questioning a pragmatic reality when there is no benefit to the argument, and no real cost in time or money to the adoption of the "conventional wisdom"

Sometimes, ignoring the debate and "just doing it" is the right answer .

Richard

 

[Richard Johnson]

QUOTE (Gordon H @ 15 Feb 2008, 22:15) <{POST_SNAPBACK}>But it does matter.
Apparently solving a problem without knowing how or why the cure works does not really constitute a solution, certainly not a repeatable one. Without understanding its mechanism, you cannot know for sure that the problem won't re-occur when presented with a slightly different, but equally valid set of conditions.
A DSO is precisely the sort of thing that SHOULD be taken along when investigating this kind of matter, so that the effects can be understood and quantified for future reference and explanation. Otherwise, we go round the whole thing time and again when others inevitably ask the same question.

*** Gordon, you may enjoy the technical discussions / have a far deeper set of knowledge than most on this sort of subject and I will give you a qualified answer.

"Yes of course it matters that in the background, each recommendation is based on fact - but it is NOT helpful to over-technify issues on a general list where modellers want simple best practice guidelines and fixes".

It is of course possible to quantify anything in isolation - however every layout is a non-repeatable set of "wire knitting", with more or less of the small errors or issues that create problems - so any qualification has little or no merit.

If I was to take an analytical approach to WHICH of the problems was mostly guilty on a layout before I touched it then I'd waste hours when I could already have most of them fixed. Each contributes more or less depending on the layout, each is a real issue - that is all that matters.

To see the scope of such things / if you wish for an example please look at the latest NMRA "Clear Block" article by Didrik Voss, the technical chair of the NMRA - he simulated DCC bus configurations and prolems and analysed them VERY well. ie - he created a simulated typical modellers bus configuration, then measured the results - following this with various approaches and filters to fix it. His analysis was professional and really well presented. You'll need to be an NMRA member to get the article though...

THIS is what you are saying we must do on list??? I respectfully disagree - it is several pages of very technical data that would put modellers who just want to run trains off in droves when the "useful answer" is:
(1) don't make a ring main
and
(2) add a resistor+ capacitor across the ends of the bus to suppress voltage spikes and assist maintenance of good CC signal.

So...

I maintain that the backgound of such issues is actually totally irrelevant as long as they are founded in fact and they work - just as a modeller does NOT need to know how his loco's motor works or how his DCC system knows which loco is which. I can say with absolute confidence that all the things recommended ARE based on fact and DO contribute to better layout performance - so any layout created with the guidelines expounded earlier in the thread WILL work well and reliably..

So - in fact, my general answer is "Nope - it doesn't really matter - and most definately it is of no interest to the modeller whose layout is fixed and not to the one who follows advice and avoids problems.

Surely... If we can give a simple set of instructions that when combined will sort the problem, then that is all that matters.

Kind regards

Richard
DCCconcepts

 

[Richard Johnson]

Hello Doug

Use a 1 watt 150 ohm resistor in series with a 0.1mfd ceramic or "monolithic" capacitor. Neither is polarity conscious. wattage of the resistor isn't critical - anything 1 watt upwards will be fine if 1 watt not available locally. Cost of these parts will be less than 50p

wiring is bus wire 1 > resistor > capacitor > bus wire two.

No need to change values at all withing the normal 11~18volt range of DCC bus voltages.

Richard
DCCconcepts

 

[Richard Johnson]

QUOTE (firefly @ 17 Jun 2008, 15:04) <{POST_SNAPBACK}>Hi Richard,reference to the dcc bus radial wiring due you add stubbers to both ends or one end,thanking you Mike.

***Hi Mike
Rule of thumb is one on eac "branch" of the bust, but with a smallish to mid size layout one end is adequate.

If its bigger than the average spare room then presuming the bus is a Tee shape add one both ends.

If in doubt do it both ends / if there are three branches to the bus one on each - it can't hurt to slightly overkill their applicatiton and the parts are very cheap...

Richard

 

[Richard Johnson]

QUOTE (TonyDaly @ 17 Jun 2008, 17:11) <{POST_SNAPBACK}>Hi Richard.

Can the end of your bus wires be connected directly to the track or do you need to end them in a terminal block ? If connected directly to the track,you wouldn't be able to fit stubbers or does that matter ?

***Tony, by definition a bus goes under the layout and is connected to the track at many places. If it is simply one pair of wires to the track and they are short, its not a bus really - just track connections. In this case I'd connect it across the two rails at the point furthest from the two track power wires.

If yours is a true bus with droppers from the rails and you simply also connected the bus to the track at the very end, then simply add the suppression/termination across the two bus wires close to the end - the benefit will still happen.

regards

Richard
DCCconcepts