DCC TWO ENGINES RUNNING OPPOSITE DIRECTIONS ON THE SAME TRACK

[mcddhawk4]

Is it possible to run two engines in opposite directions on the same stretch of track using dcc?

[TomDiehl]

Yes.

You can actually have cornfield meets like Gomez Adams used to do.

[Driline]

I'm getting my DCC NCE unit this week so I haven't had practical experience, BUT, I "think" the decoder determines the direction and or polarity of the motor, so I would say yes. Of course however if you switch the polarity of the DCC bus wires on a reverse track, then both engines should run opposite the direction they were heading in the first place.

Sounds confusing but I hope I've got this right. I'm sure those in the know will correct me

[Driline]

Yes.

You can actually have cornfield meets like Gomez Adams used to do.

Cool ! I'm going to blow the bridge up just like Gomez did too!

[simon1966]

It is all part of the fun of DCC.  Each loco operates completely independent of the others.  Different directions, different speeds, different lighting effects, different sounds etc.  No worries about blocks, but you do have to worry about trains hitting each other.

[Vail and Southwestern RR]

 Of course however if you switch the polarity of the DCC bus wires on a reverse track, then both engines should run opposite the direction they were heading in the first place.

Sounds confusing but I hope I've got this right. I'm sure those in the know will correct me

No, no, no.  the polarity on the bus wires has absolutely nothing to do with the loco direction.  It is one of the things that makes auto-reversers work.  The direction of the loco is determined by the voltage the decoder puts to the motor, which is totally independent on which rail is which.  So that means that you can reverse the polarity of a track at any time and NOT change the direction of the train.  So where in DC control you needed to either change the polarity of a track section that the train was not in, or change the polarity and direction control for a train at the same time, in DCC you can change the polarity of the track any time.  So you just need to change the polarity in a reverse loop at any time that the wheels of the train will not short the reverse section with the "non-reversed" section.  It's all clear in my head, putting it to words is more challenging!

 

 

[jamnest]

The normal direction of the locomotive is controled by CV29 in the decoder.  This allows you to run locomotives long hood forward, or have a cornfield meet!

Characteristics Controlled by CV29

Configuration Variable 29 (CV29 for short) is a very special CV. The value entered for this CV controls several things:

1. 2 digit addressing or 4 digit addressing (as described above

2. Normal Direction of Travel (NDOT)

3. Speed step control: Advanced Mode (28/128 speed steps) or Standard Mode (14 speed steps)

4. Analog mode conversion On or Off

5. Speed table On or Off

The Normal Direction of Travel, or NDOT for short, lets you set up your

locos to run either long hood forward or short hood forward. Because with

DCC the decoder determines which way the loco will move independent of

track polarity, you can set up either direction as forward depending on the prototype.

SOURCE: Digitrax Decoder Manual (Can be donloaded free from the Digitrax Web Site.)

 

[MisterBeasley]

I didn't invent this phrase, but I use it a lot:

With DC, your run the track.  With DCC, you run the trains.

Truer words were never spoken, not even by Shakespeare:

"Romeo, Romeo, wherefore art thou Romeo?"

"Fair Juliet, cometh though hither!  The Duke of Mantua hath bequeathed unto me a Big Boy, and therewith cometh a great wave of sound, a veritable Tsunami, and the rails rumbleth beneath it..."

 

[claycts]

Is it possible to run two engines in opposite directions on the same stretch of track using dcc?

I hate to say it BUT YES!

We tried to MATE an RS3 and a Brass Mike, Score

Brass=1

Plastic=0

Head-on scratch one RS3. Oredering a new body.

[Anonymous]

as mentioned, polarity has NOTHING to do with loco direction. If it did, your engines wouldnt move at all because DCC uses AC current to the track and the polarity is reversing 60 times a second I belive. The decoder is a lot like a miniature power pack that rides inside your locos. It takes the AC from the rails, makes it DC and controls the voltage and polarity to adjust speed and direction. Just like a power pack. And decoders also do neat stuff like lights and sound.

The only time polarity is even a consideration in DCC is on a reversing loop where the polarities have to match on both sides of the gap.

[selector]

Run into each other, away from each other, and chase each other around at different speeds on the same powered track, even though they get the same voltage and are identical engines with identical decoders.  You can make one whistle while the other injects water, or make one reverse its lighting pattern while the other remains as it was.  You can set CV's 3 and 4 so that these same two engines will accelerate and decelerate at entirely different rates, same voltage, same district of power.

So, it isn't what they're getting in the way of power so much, but what they can do with what they're getting that makes these wonderful decoders worth the money, to my way of thinking.  They make toy trains come alive in a way that is extremely difficult and ponderous in DC.  You could even say that DCC is cheaper when considering materials and time to install.

[Vail and Southwestern RR]

as mentioned, polarity has NOTHING to do with loco direction. If it did, your engines wouldnt move at all because DCC uses AC current to the track and the polarity is reversing 60 times a second I belive. The decoder is a lot like a miniature power pack that rides inside your locos. It takes the AC from the rails, makes it DC and controls the voltage and polarity to adjust speed and direction. Just like a power pack. And decoders also do neat stuff like lights and sound.

The only time polarity is even a consideration in DCC is on a reversing loop where the polarities have to match on both sides of the gap.

I'll try to clarify this a little, and hopefully do so in such a way as to not start the AC/DC arguement.  The DCC waveform is a rectangular waveform, in which the rails spend equal time being the plus voltage and the minus voltage.  Ones and zeros are transmitted my changing the duration of the pulses.  The inportant thing here is that a one or a zero is made up of both halves of the waveform, in other words for each "bit" each rail is high half the time and low half the time.  (There is an exception in the zero stretching case, which allows DC locos to operate on a DCC setup, but I'll ignore that for now).Whether a bit is a one or zero is determined by how long the duration of the pulse is.  If you want to look here : http://www.nmra.org/standards/DCC/standards_rps/S-91-2004-07.pdf  you can see the details of what the timing is.  The result of this is that the frquency is not constant (in fact changing the frequency is how you tell a one from a zero).  It is hard to pin down what a nominal frequency is since the zero bits are defined by a large range of time, but with equal ones and zeros and the shortest allowed zero pulses the frequency would be around 6300 Hz.  That's how you can transmit a fair amount of data to numerouse decoders an have no noticeable time lag.  If it was only 60 Hz the poor thing wouldn't stand a chance.

The techie in me just insisted on doing this.  Sorry!

 

 

[selector]

But yer a good techie, Jeff, and your effort is appreciated, at least by me.

 

-Crandell

[tomikawaTT]

As has been (over)stated, it is altogether too possible to have two DCC-equipped trains running in opposite directions on the same single track.

Unless you only operate a single locomotive at a time, the book, Rights of Trains, by Peter Josserand, should be mandatory reading for all operators.  While it goes over every possible operating rule in exhaustive detail, the basic messages are loud and clear.

YOU DON'T TURN A WHEEL WITHOUT PROPER AUTHORITY.

YOU DON'T ENTER ANY TRACK IF YOU EVEN SUSPECT THERE MIGHT BE AN OPPOSING TRAIN ON IT.

Since, unlike straight DC, it is possible to run any train in any direction at any speed anywhere with DCC, it now becomes necessary to run like a full-scale lokfuhrer rather than a toy operator.

Chuck (who runs straight DC with lots of fail-safes and auto-stop subcircuits)

[selector]

...YOU DON'T TURN A WHEEL WITHOUT PROPER AUTHORITY.

YOU DON'T ENTER ANY TRACK IF YOU EVEN SUSPECT THERE MIGHT BE AN OPPOSING TRAIN ON IT.

Since, unlike straight DC, it is possible to run any train in any direction at any speed anywhere with DCC, it now becomes necessary to run like a full-scale lokfuhrer rather than a toy operator.

Chuck (who runs straight DC with lots of fail-safes and auto-stop subcircuits)

 

aaawwwww....you had to go and take all the fun out of it.   That's it, I'm selling my stuff....

[Driline]

So I don't need to do anything special with my turntable as far as wiring goes? Good, I like DCC.

[rrinker]

So I don't need to do anything special with my turntable as far as wiring goes? Good, I like DCC.

 What kind of turntable is it? Some use a split rail which flips the polarity as you spin the table - these you need to do nothing else. Other don't do this, in which case when you turnt he loco allt he way around so it's heading back out again, you now have the rail that is on the + half of the DCC wave next to the rail that is on the - half of the DCC wave - and this will be a dead short.  What you need is an auto-reverser like a Digitrax AR-1, or Tony's PS-Rev, which will automatically switch the 'polarity' (since DC is a square wave signal what we reallty are flipping is the phase of the signal, not the polarity, but I'm trying to kepe it simple) to match as the train drives off. This has nothing to do with the direction the loco will move - if you are running forward and pull onto the turntable, then spin the table around 180 degrees so the loco is headed back out - you still leave the direction control set to forward.
 The same goes for reverse loops - if at any point the rails loop back and join up with themselves, so that the actual direction of movement of the loco changes, you have a reverse loop and need to wire it the same as you would with DC - except with DCC you flip the polarity of the loop track and not the main line, and when you do so it does not affect the direction the loco is going. Plus with DCC you can buy automatic devices like I mentioned so there are no toggles to flip, just run the train and everything is taken care of.

                                                --Randy

[Driline]

So I don't need to do anything special with my turntable as far as wiring goes? Good, I like DCC.

 What kind of turntable is it? Some use a split rail which flips the polarity as you spin the table - these you need to do nothing else. Other don't do this, in which case when you turnt he loco allt he way around so it's heading back out again, you now have the rail that is on the + half of the DCC wave next to the rail that is on the - half of the DCC wave - and this will be a dead short.  What you need is an auto-reverser like a Digitrax AR-1, or Tony's PS-Rev, which will automatically switch the 'polarity' (since DC is a square wave signal what we reallty are flipping is the phase of the signal, not the polarity, but I'm trying to kepe it simple) to match as the train drives off. This has nothing to do with the direction the loco will move - if you are running forward and pull onto the turntable, then spin the table around 180 degrees so the loco is headed back out - you still leave the direction control set to forward.
 The same goes for reverse loops - if at any point the rails loop back and join up with themselves, so that the actual direction of movement of the loco changes, you have a reverse loop and need to wire it the same as you would with DC - except with DCC you flip the polarity of the loop track and not the main line, and when you do so it does not affect the direction the loco is going. Plus with DCC you can buy automatic devices like I mentioned so there are no toggles to flip, just run the train and everything is taken care of.

                                                --Randy

I'm not sure about the turntable. Its a "Heljan" brand HO scale, about 14" long.Its nearly 20 years old. Not sure if the polarity changes or not. I don't think it does.....

[tstage]

Draline,

If it's 20 years old, you'll need a auto-reverser for your turntable.

Tom

[mcddhawk4]

Thanks for all the great advice.

[Tilden]

OK you guys, enough with the bad Karma questions.  Was running trains last night and sure enough almost had a head-on.  Now this has never happened before, so I'm sure it was the result of your discussions working on my sub-concious........