There was a recent article in MR about installing a buffering circuit across the two bus wires, consisting of a capacitor and a resistor, to do something about correcting a problem with DCC signals.
I called Digitrax to get their "take" on this and was told they have no opinion, one way of the other.
They did mention that they thought the idea originated with Loy Spurlock. Given the source, I'll bet the idea is legitimate.
However, I'd like to know what it does and what is the problem with DCC signals that warrants such a circuit?
Also, has anyone done this and what where the results?
It sounds like it's used to cut high frequency distortions (noise and spikes) on the bus. Exact values will theoretically vary with the impedance of the bus, but the values given in the article will most likely work in any "normal" situation.
I have not done this (I am on DC) but this type of use is commonplace in other areas of electronics.
Hopefully someone on DCC has tried it and 'scoped the line to see the actual effects.
Karl
The mind is like a parachute. It works better when it's open. www.stremy.net
The RC circuits across the ends of the bus are often referred to as snubbers or terminators, and are usually only helpful when your bus exceeds about 30 feet or so.
Their purpose is to prevent spikes as Karl mentioned, and also to prevent reflection of the DCC signal back down the bus, a condition known as "ringing".
More info is available here.
Scope traces are shown here.
Digitrax boosters seem to be less affected by spikes/ringing than some other brands (it seems that snubbers are discussed on Brand N's Yahoo! list at regular intervals), which is probably why they have no opinion either way.
HTH, Steve
Buffers aren't needed on Digitrax layouts. There was this little 50,000 square foot convention layout last year with over 700 modules. Digitrax was there to provide technical assistance. At least 2000 feet of loconet cable was used. No snubbers were used.
Our club layout uses Digitrax(3 8 amp boosters/16 PM42's and #12 buss wiring) and when the original article came out, we did some research. We have Power Bus and Loconet runs in excess of 50-60 feet. We 'scoped' the signal on several of the runs and then built one of the termination units and tested. Other than a cleaner looking waveform, there really was no difference in operation. I suspect these termination/buffer/snubbers might help a layout with bad wiring, but in our case - we did not pursue this further....
BTW - turning off 'Address 0'/DC support) on the system and in the decoders gets rid of most of the 'runaway' issues that clubs see....
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
2000 feet of Loconet cable is not related to the snubber question. Did they have the 2 bus wires snubbed is the question.
Marc
UpNorth2000 feet of Loconet cable is not related to the snubber question. Did they have the 2 bus wires snubbed is the question. Marc
Yeah, you're right. The layout and its bus wiring did consist of 700+ 4 foot modules which would include at least 2800 feet of bus cable. You need to add for the extra "tails" on each module so that would bring it up to at least 3500 feet of bus wire. There were no snubbers used on that layout.
Digitrax reps including A. J. provided technical assistance for that layout. They did find that the new LNRP's needed a 14 volt power supply to work reliably, though.