The latest Scale Rails (NMRA magazine) includes an article about DCC
signal forms under various conditions. Three of five systems tested
earned a Conformance Warrant because their signals maintained a smooth
wave form at the transition from negative to positive (as measured at
the zero volt point), which is crucial for correct interpretation by the
decoder chip. Hornby Elite was one of the three. The two systems that
failed showed multiple, closely spaced +/- voltage variations instead of
a nice clean rise.
Thought you'd like to know.
Erm, on re-reading the article, I realise I may have erred. I inferred
that the Conformance Warrant (CW) was granted after the tests reported
in the article. Although the article refers to passing CW tests, it is
not clear whether the specific problems discussed were the reason for
granting or refusing CWs. I don't know why the two systems without CWs
don't have them. IOW, "passed" just means that the systems have a CW.
So with that caveat in mind, read on. The following represents what I
understood the article to say.
The article actually reports on the effect of the layout itself on the
DCC signal, and specifically the occurrence of high-frequency "bounce"
or "ringing" around the transition from -15V to +15V. This transition
should be as sharp and smooth as possible, since the quality of the
signals received by the decoder chip depends on it. If the transition is
bad, the the information packet may not be read correctly. The HF bounce
or ringing is an effect of the impedance of the power bus/track.
So, first of all, all systems showed this ringing or bounce at the far
end of the layout when tested without a "terminator" or "snubber." IOW,
high frequency filters are essential for trouble free DCC. The two
systems without CWs showed bad transitions at the command station, too.
NB that all five systems listed below showed acceptable waveforms when a
terminator (150 Ohm resistor plus 0.1mFd capacitor in series) was used.
IOW, if a high-frequency filter type of terminator is used, all systems
are acceptable. However, none of the DCC systems supply a terminator,
which means that a plug'n'play test simulates what people actually do:
plug in the system and go. The implication is that the two failed
systems have unacceptable range and duration of the high frequency
bounce during the transition.
BTW, all five systems showed a degradation of the square wave towards a
sine wave form when a 1.A load was placed across the track. This implies
that boosters must be used.
The terminator was placed at the end of the 66ft "single track line",
which was simulated electronically by using two "white boxes", each of
which had the same impedance characteristics as 33ft of track.
I take the results to mean that three systems passed a "plug'n'play"
test, while the failed systems would require a terminator, ie, an
additional component supplied by the purchaser. NB that several
manufacturers indicate that such terminators or snubbers should be used.
NCE recommends one every 20 ft or so.
The article also gives circuit diagrams for the white box and the
I infer from the technical discussion included with the test results
that a terminator should be placed at the most distant point from the
booster or command station. The degradation of the signal at the end of
the 66ft track indicates to me that boosters should be used when the
total signal path reaches roughly this figure. Ie, keep power districts
at 20-30ft or thereabouts. I am about to convert a 16ft shelf layout to
DCC, so this consideration won't apply.
With CW: Hornby Elite, Lenz LZV100, NCE Power Pro.
Without CW: MRC Prodigy Wireless, Digitrax Chief.
I do not recall the article mentioning that any of them received a
conformance warrant because of the testing done for the article. The
Lenz system does have a Conformance Warrant.
It was merely designed to show how bad the electrical environment of the
average layout is and what can be done to improve it.
If that's what the article said, then it sounds like a bias towards a
particular manufacturer whose decoders rely on edge recognition to
decode the packets. Packets can be decoded by regular sampling of the
DCC signal and this is much more immune to non-monotonic edges.
What were the stated output limits of the systems being tested?
So nothing like a typical layout! On 66ft of track I would expect more
than one loco to be loading the signal, *and moving about* and
continually changing the load characteristics. All they've done is
look at a model of an empty layout. The above comment about waveforms
degrading towards sine waves shows why this doen't really tell us
anything useful. Did they account for multiple stub droppers feeding
the track from a bus in their impedance modelling?
Was there any mention of the importance of the wire sized used for
power distribution on large layouts.
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