Twisted Pairs

[Crosland]

Would you also refer to RS422 as simply 'AC' ?  I certainly wouldn't.

 

It would depend on the audience and the context, I would not let anyone get away with calling it pulsed DC or bipolar DC or anything else like that.

[Crosland]

But , the correct definition is an high frequency alternating current waveform. ( see DCCwiki )

 

When did that change? I gave up on the DCCWiki years ago due to the insistence on referring to DCC as "bipolar DC", indicating (to me) the authors lack of understanding  

[Ron Ron Ron]

I leave you boffins to argue the terminology and semantics, but I see the term Bipolar DC is often not referred to in isolation, but is used to describe how the square wave AC is formed.

 

e.g.

Digitrax  

"....Often called a DC bi-polar square wave. A fancy way to describe how the DCC power at the rails is made.

It is actually a DC source being pulsed to two poles around a centre voltage of zero volts, thus bi-polar.

This generates an AC square waveform across the rails, and as typical this results in an average voltage of '0' volts. The average voltage is the sum of the instantaneous voltages in a half-cycle waveshape, divided by the number of instantaneous voltages.

This is a very common method of generating an AC current, such as small inverters used for AC power from a DC source."

 

Wikipedia (for what it's worth)...

"....The voltage to the track is a bipolar DC signal. This results in a form of alternating current, but the DCC signal does not follow a sine wave".

 

NMRA use the term "bipolar" everywhere....

 

"....the bipolar NMRA digital signal....". 

 

"....Bipolar signal - An electrical signal that carries the NMRA DCC information that has equal voltage components above and below 0 volts. In the context of the NMRA DCC standard, this signal will have a positive half-cycle and a negative half- cycle. (+ volts to - volts) '.

 

 

So is it both - i.e. a Bipolar DC signal, producing an AC square waveform?

 

(Don't get too heated....)

   

 

 

 

 

 

[Crosland]

This generates an AC square waveform across the rails, and as typical this results in an average voltage of '0' volts. The average voltage is the sum of the instantaneous voltages in a half-cycle waveshape, divided by the number of instantaneous voltages.

 

No, you need to measure over a full cycle.

 

NMRA use the term "bipolar" everywhere....

 

"....the bipolar NMRA digital signal....". 

 

"....Bipolar signal - An electrical signal that carries the NMRA DCC information that has equal voltage components above and below 0 volts. In the context of the NMRA DCC standard, this signal will have a positive half-cycle and a negative half- cycle. (+ volts to - volts) '.

 

That's just sloppy language. From the context it's used in the power station interface RP, what they are trying to describe is the difference between a differential DCC signal, where the current flows both directions between the two wires, and a single ended DCC signal where one wire is 0V and the other switches between 0V and some positive voltage. The latter could be called pulsed DC but it would be no use as the track power.

 

(Don't get too heated....)

Getting (something) heated is one way to measure AC signals. The RMS voltage, for example, is the equivalent DC voltage that would cause the same heating, in a given resistance, as the AC voltage being measured   

[Crosland]

 

Digitrax  

"....Often called a DC bi-polar square wave. A fancy way to describe how the DCC power at the rails is made.

It is actually a DC source being pulsed to two poles around a centre voltage of zero volts, thus bi-polar.

This generates an AC square waveform across the rails, and as typical this results in an average voltage of '0' volts. The average voltage is the sum of the instantaneous voltages in a half-cycle waveshape, divided by the number of instantaneous voltages.

This is a very common method of generating an AC current, such as small inverters used for AC power from a DC source."

 

Now that gives a clue to the source of the confusion. One way of generating DCC power to the track is to use a centre-tapped transformer and connect one rail to the centre tap. The other two wires are rectified and used to generate +ve and -ve supplies which are used to drive the other rail alternately +ve and -ve.

 

From the boosters point of view, where there is a 0V reference, I guess you could, at a stretch, say that the DCC signal is bipolar DC.

 

The decoder has no 0V reference and all it sees is AC.

 

Nowadays any decent booster will use a single DC supply and a "H-bridge" to generate the track output centered around half the supply voltage.

[Suzie]

DC is very clearly defined as a voltage that does not change, anything else is AC. Simple test is if you shove a big capacitor across it and it stops working it is because you have removed the AC.

[Mike Friedman]

To get back to the posters original question.

 

I have several busses on my layout: Main track control bus, accessory control bus, accessory power bus, XpressNet bus and feed-back bus. The only one that is twisted is the feed back bus.

 

From the Lenz LZV100 manual:

 

"The feedback bus is used for advanced layout control functions.  Via these wires, which also must be a twisted pair, the Command Station asks for the state of, for example, turnouts or track occupancy detectors."

 

In the picture below the feed-back bus is the twisted pink and blue wires

 

[Mick Bonwick]

To get back to the posters original question.

 

I have several busses on my layout: Main track control bus, accessory control bus, accessory power bus, XpressNet bus and feed-back bus. The only one that is twisted is the feed back bus.

 

 

 

 

Typical. You wait ages for one and then several come along together!

 

I have not twisted mine and have never had any problems, but the layouts I have used DCC with have only been going for 8 years, so there's time yet, I suppose.

[Junctionmad]

DC is very clearly defined as a voltage that does not change, anything else is AC. Simple test is if you shove a big capacitor across it and it stops working it is because you have removed the AC.

I know , we have descended into pedantry here. But

 

DC stands for direct current , that is the unidirectional flow of current , there is no mention of voltage. Similarly AC is defined as alternating current , ie current flows in both directions.

 

PDC ( pulsed DC ) means a waveform where current does not reverse during the cycle, and is common in logic circuits. These signals are NOT AC waveforms. ( even though they pass your capacitor test ! )

 

Bipolar DC is a term used to describe positive and negative DC supplies typically centred around ground , in a bipolar pulsed DC system , each resulting waveform referenced to ground , is a ordinary pulsed DC wave train, even though each leg has current flowing opposite to the other leg. However if you then reference the output of the two legs without ground. The resulting bipolar waveform is a AC waveform. ( because current is seen to be alternating ) l

 

Since DCC is clearly seen to have a reverse current each half cycle, it's clearly an alternating current waveform, ie AC.

 

I accept that in many cases DC is seen ( by many ) as a fixed constant voltage however the reality is the definition of DC has a far wider use then that.

 

 

The terms get misused of course even by the experts.

Dave

[Crosland]

"The feedback bus is used for advanced layout control functions.  Via these wires, which also must be a twisted pair, the Command Station asks for the state of, for example, turnouts or track occupancy detectors."

 

That's a low signal level differential bus and should definitely be twisted to limit noise coupling from high current traction supplies.

[Crosland]

PDC ( pulsed DC ) means a waveform where current does not reverse during the cycle, and is common in logic circuits. These signals are NOT AC waveforms. ( even though they pass your capacitor test ! )

Like all waveforms, they are a combination of AC and DC components

[jaym481]

I'm compelled to ask when you lot are going to admit it's all actually magic? At what point during your stay at Hogwarts do they start to teach you how to convince us muggles it's something called "science?"

[Griffin Road]

That's a low signal level differential bus and should definitely be twisted to limit noise coupling from high current traction supplies.

 

 

 

Sorry - still haven't used twisted wire and has worked fine on three layouts - oldest one now probably 7-8 years old .

 

Why is it working okay if all you have written is true ???

 

Mike b

[Griffin Road]

I'm compelled to ask when you lot are going to admit it's all actually magic? At what point during your stay at Hogwarts do they start to teach you how to convince us muggles it's something called "science?"

 

I missed the class and am working blind to the fact that all this ' science ' knowledge is required . I really must catch up .......

 

I inadvertently wired up my trainset like a dc one - missing out all the switches etc - and plugged a Lenz set into it . As Lenz developed I tried to keep up .

 

 

M b

[Oakydoke]

Sorry - still haven't used twisted wire and has worked fine on three layouts - oldest one now probably 7-8 years old .

Why is it working okay if all you have written is true ???

Mike b

I smoke 40 tabs and drink a bottle of whisky every day and it hasn't done me any harm.

I've been driving on bald tyres for years and I haven't had a crash either, so all that safety stuff must be tosh.

[Dagworth]

I smoke 40 tabs and drink a bottle of whisky every day and it hasn't done me any harm.

I've been driving on bald tyres for years and I haven't had a crash either, so all that safety stuff must be tosh.

If you gave up the fags and booze you could afford new tyres

 

Andi

[Crosland]

Sorry - still haven't used twisted wire and has worked fine on three layouts - oldest one now probably 7-8 years old .

 

Why is it working okay if all you have written is true ???

 

Mike b

 

Just as many people don't bother wiring point frogs correctly, What works for you, works for you. That's fine, but I will always recommend best practice, especially when it's also the manufacturer's recommendations, as in this case?

 

A lot of people would have a lot less problems if they learned to differentiate between well reasoned advice and "do it my way, it works for me".