I’ve had my DCC system installed and functioning flawlessly for a number of years now. Unknowingly at the time, I ran my bus all the way around the perimeter of the layout ( about 90 feet ) and tied the end back in from the starting point, thus creating a loop.
Now I’m reading that the bus SHOULD NOT be connected to form a loop !!! What is the potential harm or problems ??? I’ve had absolutely no issues what-so-ever using a base throttle and three wireless throttles. Have I just been lucky or what’s the deal ???
Since the DCC bus is also technically a serial network, making a loop like that can cause ‘echoing’ (I dont remember if thats the technical term of not) The serial signal will travel around both sides of the loop and can either pass each other or collide and reflect. This phenomenon really only happens on systems that have very little resistive loads on it and have lots and LOTS of wire to travel on.
Also, they may have been talking about the potential to wire the system out of phase when you bring it full loop, but you would know that the instant you powered your booster on
Either way, your best bet for signal integrity, current distribution and phase relationship is to split your loop at the point farthest from the booster so you have to legs of equal length.
Yup, that’s the answer. But, remember that you probably need a terminating resistor at the end of each of the new branches. Consult the documentation that came with your DCC system. My Lenz system came with one resistor for this purpose, but I added a second branch so I had to make one of those extravagant Radio $hack purchases.
It’s OK to loop the POWER bus. Even if you cut the bus - the track it connects to forms a complete loop most of the time.
What’s NOT ok with most systems is to loop the throttle or command bus.
As for termination on the power bus - the only time I’ve even heard of this being required is on some VERY large layouts. Running a few runs out from an existing bus in the form of branches from a tree trunk isn’t going to make a huge overall length of run. If a 10 foot long branch feeds off 20 feet up the bus from the booster, it’s just a 30 foot run. In fact, because the 90 foot bus loops around and connects back to itself, the longest run on it currently is 45 feet. You can eliminate most logn bus issues by not runnign the two wires perfectly parallel (ie don’t use 2-conductor speaker wire). I don’t twist my purposely, but I do keep them from running parallel to each other, end result is there are a few twists, just not a specific quantity (ie 3 twists per foot).
Which is exacly why you need power districts or gaps in the rails.
Yuppers, is a serial network based on the rs-422/485 system and shouldnt be looped. But the Power Bus is merely an amplified version of the control bus and is subject to the same limitations, including looping. But as said above, this is really only applicable where long runs of the wire are concerned. When you check your documentation, remember that the lengths are ‘as the cable runs.’
The DCC system as a whole is pretty resilient, so you can pretty much do whatever you want, especially on a smaller layout like a 4x8.
When I first installed my bus, I tried running without the terminating resistor. My layout is only 5x12 feet, so the longest run on the control bus was only about 12 feet. Still, it did not work reliably until I put in the resistor at the furthest point. This is a Lenz System 100, but the control bus was done with “generic” jacks, not the high-priced store-bought “official” control bus jacks. Do those have any terminating resistors built-in?
I disagree that the power bus is just an amplified control bus. But I agree that ideally it should not be looped, but that in practice, on most layouts, it isn’t going to matter in the least.
I was thinking, I guess it depends on which bus was being referred to, so I might back off ion that a little. In essence, there are three buses, the power bus that connects to the tracks, the non-amplified power bus (which is what I now realize you were referring to) which goes to the boosters, and what I would have called the control bus (which I guess might of might not be a bus, depending on the implementation) to connect the control station, throttles, and whatever other hardware there might be together (loconet in the Digitrax case, what ever in other cases). So I was thinking in terms of not looping the loconet, in the digitrax case. Of course, depending on the bus a loop might be fine, if it as designed to be a ring.
I wasn’t even really thinking of what you referred to as the control bus as a bus, though it could certainly be wired as such.
Hmmmm, seems as though I’ve created some confusion. In my original post, I should have maybe clarified I was referring to the TRACK POWER BUS … and if there were any issues about looping it.
I can’t see how there could, and have operated mine for years that way with no problems … it’s just that a few sites I’ve been on state the TRACK POWER BUS should NOT be looped !?! [%-)]
Right, theoretically, technically, in a perfect world, the track power bus would not be looped. The reason is that a loop means that there are two different paths that the signal takes to get to any point on the loop(more if you consider multiple times around, but never mind that) and there will be some degree of interference due to this. In the real world, with a DCC track power bus the tolerances on the signal quality are such that in most cases (and that is most as in very, very nearly all) cases this interference is not going to cause any observable trouble, as you have seen.
Well, the power bus is the power bus, and the control bus is the control bus.[:D]
Seriously, the command bus, besides being lower power, uses a proprietory format to transmit information, such as commands and feedback data.
The NMRA specs do not cover the command bus, which is why each manufacturer is able to develop their own command bus format.
For example, NCE and MRC use a polled bus, while Digitrax (LocoNet) and Lenz (ExpressNet) are more Ethernet-like.
But even busses of the same general type have different specs, which is why you can’t plug let’s say a Lenz throttle into a Digitrax system and have it work.
It’s the command station’s job to translate the command information from that proprietory format into NMRA-spec packets and put them out on the higher-power track bus.
Ahh, just re-read the thread and did a little looking and now I see the problem.
It’s not so much incorrect terminology on your part as on mine.
Randy referred to “the throttle or command bus”, and I read that as being the same bus with either name because the terms are often used interchangeably. But now I see it could be taken as two different busses.
What I wrote in my previous post, if we consider “throttle” and “command” to be two different busses, would apply to the throttle bus.
In that same context, the bus between the CS and the booster could be considered the command bus, and it is in fact a low-power version of the track signal.
We need to clear up some thinking here. If we consider a single source of power to a track and we “loop” the track in a circle, the electrical current on the track does not actually flow in a loop. It will always flow from the source through the path of least resistance back to the sink, ground, neutral, phase, return path or whatever you want to call it. So as a locomotive moves around the “loop” the actual direction back to the feeders will follow the path of least resistance. So if it is traveling in one direction and then passes the half way point (assuming the resistance of the track and the contact resistance of the wheels on the track are identical all the way around) then current will travel in one direction (with respect to the direction from the source) and then once it sees a lower resistance (past the half way point) it will come the other direction from the source.
The concept carries forward if you have multiple feeders to a track bus, regardless of whether it is in a straight line or a loop bus feeder system, along with the track. The only addition here is that there is an additional loop created between the feeders and the track between each pair of feeder sets. If we were really concerned about a loop, then this would be worse because there are really a set of loops chained together.
It’s not even close. The track power bus has the full DCC signal on it, providing whatever voltage set byt e command station and deliving however many amps your boosteris cabale of. NONE fo the control busses from the majr DCC systems duplicates this signal at any point. They also aren;ty all RS-485 busses, either. Digitrax loconet includes a pair of wires that is a low power mirror of the track bus, but that is to provide timing to slave booster and accessories that need to knwo the track waveform such as their trnasponding detectors, as well as providie a power source for throttles. The actual Loconet wires the pass the signals from the throttles to the command station does NOT use a DCC square wave to do so.
Since you strike me as knowledgeable on this subject so let me toss this at ya:
I worked with Siemens Building Technologies for a bit and they use a flavor of RS-485 for networking all of their automation equipment together. Now RS-485 is a differential DC network, to my knowledge damn near identical to DCC in operation, and we took detailed training on why not to T junction the network nor not to create a loop. For both of these reasons, even with using terminating resistors, signal reflectivity WOULD occur and severely degrade network performance.
If it was so bad with RS-485, why is it NOT bad with DCC?
(This is an honest question, not an attempt to pick a fight)
Also, the DCC track power is actually floating relative to ground in most systems. The booster output is typically am H bridge and the two rail lines are each one hald of the H bridge. The DCC voltage is in reference to an internal common - which is why on a lot of systems there is a (technically mislabelled) GND terminal that should be connected betwene all boosters in a multiple booster environment.
In the specific case of Digitrax, the PM42 circuit breaker/reverser and any booster configured as an auto-reverse booster tdetermines the need to reverse based ont he signal relative to the common. This common is also contained in the Loconet cable, but that’s two conductors of some #26 phone wire, and it may be required to handle the full short circuit current of the booster. Thus the recommendation to connect the GND terminals of all booster together with wire of the same thickness as used for the actual track power lines.
