DCS coiling extra wire a problem?

Dave,

I've taken the extra wire made it into a loose coil say 3" in diameter ...  should this be avoided?

The only concern I would have is if you're bundling so much wire that you cause a voltage drop. An extra few of bundled wire feet is not an issue for DCS.

On the same train of thought. We were told not to bundle multiple feed wires together.

The only thought I would have is that it's not a good practice to bundle low-voltage wire (12-21 volts) with higher voltage wire, such as 120 volt AC from the wall.

According to the RF engineer who designed the Signal Filters  ( http://www.slsprr.net/technical/filter.htm)  the MTH DCS frequency is not happy with twisted pair wiring or tightly bundled wire.

That's strange.  Barry's book (page 62) recommends twisted pair.

Is it causing a problem, are you having problems?

Old Chinese proverb, "If it ain't broke, don't fix it."

Al

Susan,

the MTH DCS frequency is not happy with twisted pair wiring or tightly bundled wire.

First, "twisted pair" and "tighly bundled wires" are two completely different things.

Second, twisted pair absolutely is good for DCS signal propagation.

Third, tightly bundled wire has never been shown to have a negative effect on DCS signal propagation. Don't confuse "tightly bundled wire" with a magnet or motor coil. Those two items can, indeed, degrade the DCS signal.

In the IT networking world twisted pair is a requirement to prevent signal degradation however loops of wire is a no no. I would assume that DCS, since it is a digital signal, wouldn't be the same. I wold like to see why Susan's source says the twisted pair degrades the signal.

Isn't twisted pair most useful for differential signaling systems, where noise induced in the balanced signal lines is canceled out?

Things like Ethernet or RS-485 are differential systems, but DCS isn't, is it?

Prof,

All I can say on the subject is that DCS, like Ethernet, has commands that consist of groups of data packets.

In fact, DCS signal strength is a measure of how many packets per 100 packets sent, are acknowledged by the engine and received back by the TIU.

Basis from Engineer (he was kind enough to reply after reading posts):

The purpose of twisting wires together is to create a transmission line with a fixed and known characteristic impedance that is matched by the impedance on each end of the transmission line. If:

1.    The two and only two wires twisted together form a balanced pair, AND
2.    The RF (DCS Signal) source is of a balanced construction (not single-ended), AND
3.    The twist rate remains near constant, AND
4.    The spacing of the conductors inside the twist are near constant, AND
5.    The characteristic impedance set by #3 and #4 above is below 300 Ohms

…you might be able to make a case for twisting wires together. Otherwise, twisting wires together can make matters worse.

The reason for not recommending that wires be twisted together is that many times, more than the appropriate two wires get twisted together. This increases the chance of coupling between the wires and RF-frequency signals traveling out and bouncing off of a non-ideal impedance. This signal then returns and can impact the signal strength  across the layout.

If one was to insist on twisted pair wiring in the train layout, I would recommend following this equation with a characteristic impedance (ZL) of between 100 and 200 Ohms. Repeated here (stolen from the link provided):

Where

And 0-(phase symbol) is the pitch angle of the twist: the angle between the twisted pair's center line and the twist.

Given the potential downside of twisting wires together, and the very remote chances of achieving the proper characteristic impedance with a hand-twisted pair, it is probably time saved just to minimize DCS crosstalk between cables by not running them in tight or twisted bundles.

Finally, it appears from very preliminary measurements that the RF signal source inside the DCS RF transmitter is of a non-balanced (coaxial) nature. To properly drive a twisted pair transmission line, a balun (converting the RF signals to a balanced output) would need to be included in the DCS output. That is a non-trivial task because the DCS output terminals also contact 60 Hz power signals. Driving a twisted pair with a single-ended (unbalanced) output defeats the purpose of a twisted pair. You may actually end up creating an antenna - the exact opposite of what a transmission line should be.

If anyone has a schematic for the DCS RF (data signal) transmitter and how it couples to the terminals of their control unit, I would be very interested to see it. There's a chance it is of a balanced construction, and I could then work out some twisted pair geometries that would work well with the signal filters.

filters here   http://www.slsprr.net/technical/filter.htm

Susan,

The reason for not recommending that wires be twisted together is that many times, more than the appropriate two wires get twisted together. 

So if making a coil is a bad idea then what do you do if you want to leave some slack in case either end needs moved in the future?  Thanks for all the informative replies. I'm a techie at heart so in depth responses are great. Dave

Barry, when "twisted pair" is working it's fine.  I'm for playing with what works and using all the technical information we can get as advisory for individual applications.  There are way too many variables in our toy train wiring to fit neatly into engineering theory.

Cruikshank, with experimentation, you would probably discover that your coils have no affect.  My wiring "accepted practices" always includes enough slack at terminations for future adjustments.

Susan,

So, just to be clear, the bottom line is that twisted pair is fine for DCS and coiling wire presents no problems to DCS.

I would still avoid any tight bundling of 120 volt AC with low voltage layout wiring, on general principles.

What to do with the extra wire? Below the point where you intend to connect to the track terminate your wire in a two terminal screw type barrier terminal strip. Make the connect to your track with a wire that is about 10 inches longer than you need to reach the terminal strip; this will give you some slack should you ever need to lift up the track for maintainence. This extra length is not enought to be a problem under the layout so I just leave it hang. Now, if at some time you feel you need to move the track connection simply disconnect the wire from the track at the terminal strip, connect a new wire to the terminal strip and run that to the new track connection. If this is a long extension then install a new terminal strip at the new location.

Al

I use plastic "wire stackers" to run my wires under the benchwork.  A couple extra well-placed stackers will let you leave a neat loop of extra wire without it hanging down and getting in our way.  They keep the wire work under the tables manageable and also avoid coiling and too-tight bundling, as long as you don't put too many wires in the stackers.