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Beppe Giannini
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Discussion Starter · #1 ·
Since the issue keeps coming up from time to time, this

www.geocities.com/OzDCC/

is the best explanation I've found so far

In short, I understand that :
- you start with DC (say + 18V)
- you chop it to form the digital "message"
- what goes out is a rectangular wave +/- 18V
- the car decoder reads the message(s) - speed and lane change
- perhaps the wave is rectified back into continuous +18V DC before going to the decoder chopper, which determines the output voltage to the motor

If this is all correct, kids had definitely better NOT lick both rails at the same time !

Beppe
 

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I know a fair amount about digital in general, but not the specific implimentation with slot digital.

But I understand scalextric uses PCM - pulse code modulation. (there are a few methods of encoding digital signals) Usually, this method involves turning the current on and off, not reversing it, ie switching between (say) +12V and 0 V, or it could be implimented to switch between +12V and +10 volts. Most of the digital signals I have come across operate in the region of 48KHz or more, 48000 on and offs per second instead of 50 on AC. rates can go up to megaHz though, and each hi voltage represents a binary 1 and each low voltage represents a zero. each 'package' or set of numbers will probably include an address or car number, so the car the rest of the instructions belong to will know to pay attention to the rest of the contents.

Since a scalextric rail with lots of joints isnt the ideal conductor for high frequency square waves, the frequency might be a lot lower, dont know. But the higher the frequency, the easier it should be to filter out for the motor.

Hope this is clear, and not further obfusciating the issue
 

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Beppe thanks very much for starting this - much appreciated!


Let's see if I have any kind of handle on it so far.
I see both AC and chopped DC as 'fluctuating currents' for want of a better term.
But better check - is this a reasonable term?

Would it then be correct to say that if the current never drops below zero, it is chopped DC, but if it does, then it is AC?
eg
+18v to 0v = chopped DC
+18v to -18v = AC

I must get the hang of this!

EDITED
For the benefit of anyone trying to follow this thread - THAT IS WRONG!
I think I can handle being wrong as long as I learn something in the process!
 

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Beppe Giannini
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Discussion Starter · #5 ·
Er... , electronics is not my area of competence at all, so I do recommend you look up that Australian site (I don't know why the link did not come out active....)

As a former electrical engineer, I can say that AC (single phase or 3 phase) is classically considered to be a current that varies following a sine wave, at a fixed frequency - so I wouldn't consider the DCC shape as AC

The one thing I've grasped is that DCC was adopted by model trains because the environment is electrically very "noisy" - so that if you classically superimpose the control signal to power it will be drowned out - with DCC, signal and power coincide

Hope this helps a bit.... at the moment, I'm just too excited from astro's revelation of Scalex having used "my" method for LC activation !

Beppe
 

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chopped is slang, the term is 'pulse code modulated'. Modulation is usually the adding of one signal to another, and what is added to the dc is 'pulses'. In theory, the digital modulated signal will be a square wave with distinct changes of state from hi to low voltage, at irregular coded times.

What is usually ment by ac is a continuous regular sign wave, or oscillation, always changing voltage between +12V and -12V (for example). Main reason we have AC mains is that for technical reasons, less electricity is lost down the extremely long distances between power stations and ppls houses.
 

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Brian Ferguson
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According to the NMRA, Digital Command Control uses pulse-width-modulated bipolar DC for both the command signals and power. This means that the voltage ranges from a positive (of some level) to a negative (of the same level). Think of it as square wave AC if you like. However, the duration of the square waves (pulses) is not constant - it is the width of these pulses that are interpreted as binary signals by the decoders. To actually power motors or other devices, the input voltage is rectified and the decoder determines the amount of that rectified DC that should be fed to the device.

Probably clear as mud, right?
 

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Brian Ferguson
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Just to clarify a bit further (at the risk of boring most)...

If the range of the bipolar DC is +12V to -12V (24V peak-to-peak), the rectified voltage will be 12V. Note that it doesn't matter what the pulse widths are, the rectified voltage will always be constant since the voltage that the rectifier sees will always be either +12V or -12V. A rectifier doesn't care about pulse widths or pulse duration - it just changes the + or - V into a positive voltage level. There will always be either a +12 or -12 voltage present, so the rectifier will generate a constant 12V level, subject to further notes below.

The power fed to motors, etc., will use 0V as the common or "ground" voltage and will use 12V as the maximum positive voltage.

Due to ineffencies in rectification, though, the motor will see less than the max voltage. To compensate, the DCC system will usually operate at a voltage slightly higher than the target motor voltage. For example, if 12V is the target voltage for motors, the system may actually run at something like 25.2V peak-to-peak (12.6V positive and 12.6V negative). By DCC standards, the voltage can actually be as high as 27V peak-to-peak.

This means, though, that if any slotcar system exceeds 27V peak-to-peak, or approximately 12.9V DC for motors, then it is non-standard by DCC conventions.

Not sure how this translates to Scaley's version. If they are following true DCC conventions though, they can't exceed this voltage level. To feed motors at 15V would require a peak-to-peak DCC voltage of approximately 31.2V, outside of the current DCC standards limit.

As an aside, the DCC convention is one of the reasons why digital control is not currently considered appropriate for large numbers of devices (cars) where rapid changes in instructions are relevant. Each car receives its instructions via "packets" of data sent from the DCC source. Each decoder recognizes it's own set of instructions. Thus, there are a finite number of "data packets" that can be sent each second and this information cannot overlap. Each car sees instructions at intervals, not constantly. It's entirely possible that if a large number of cars are involved, that a given car may not see every input that its driver initiates. Consider how fast a driver can change trigger position - it would be desirable to send a packet to the decoder for ANY change in trigger position. If too many cars are on the system though, some of those inputs may not get sent. Believe it or not, the bandwidth of DCC has practical limitations. It works well for trains, but slot cars depend on much faster, and more frequent, changes in inputs.

Ooops... getting carried away, I fear....
 

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If anyone is bored, they don't have to read it - this is good stuff!
No Fergy - you are not getting carried away at all - this is a very valuable resource and I do follow most of it. I was just getting thrown by the AC/DC business, mainly because everything I have seen on DCC until recently clearly said it was AC current. Maybe it was thought easier for our less electronically educated minds to grasp. Actually it WAS until you guys started to straighten me out. Well, I THINK you are straightening me out!
 

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Errr . . . I THINK you were right, Astro!

In any case, if you hadn't said anything, we wouldn't have this topic now and it's a good one. So ->
 

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Brian Ferguson
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Astro, is ISN'T AC!! You were right - it's DC, but it is DC that rapidly switches from positive to negative. Many don't realize that DC can have a negative value. Unlike AC's sine wave form though, bipolar DC switches almost instantly from max positive value to max negative value, and vice versa - it's a "square" wave that AC does not have. Also, AC has a constant repetitive wave form, usually 50Hz (UK and Europe, for example) or 60Hz (North America, for example), but the wave form of DCC signals can vary drastically - the width of the DC pulse can vary radically (theoretically from millionths of a second to more than a second) while AC forms have a constant width (50 Hz or 60 Hz).
 

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I'm glad you stepped in there Fergy - I thought I had understood properly but wasn't confident enough to say any more than I did! I've got it, by George I've got it!
But am not surprised that it was confusing.

Is it possible that other systems HAVE used genuine sinusoidal AC?
(That sounds as though it wants to kill itself!)
My thinking here is in connection with full blown AC power supplies, 220-240v AC in UK, that have simultaneously used the same power cables to network computers.
 

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digital signals can be modulated onto ac, prob not as DCC or PCM (there r a lot of modulation schema), and i believe on the mains systems they use very high frequencies approaching RF. For this to be legal, u have to filter ur mains very thoroughly, cos its illegal tho very easy to contaminate the grid with ur modulated signals. My understanding is that the modulated frequency has to be a good order of magnitude greater than the carrier frequency (50hz in the case of mains), and my suspicion is that due to scaly rails, joints and braid contact, the frequency for this must be relatively low
 

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Brian Ferguson
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Tropi, I'm not aware of any serious modern system that has used AC to communicate data. The problem is that AC cycles from max to min voltage with a sinusoidal wave form. Theoretically, it is possible to alter the frequency of the AC wave form, but it is more efficient to do that with the square-wave of DC. Particularly with today's digital electronic systems - by definition, digital is on or off, not slowly changing from one to the other and therefore yielding states of 'maybe' .
Digital components react to changes in voltage levels or peak voltage levels, and changes must happen very quickly to avoid erroneous interpretations of the signals - sine waves are very imprecise and lead to digital interpretations like "yes, no, yes, no, maybe, no, yes, no, yes" during the transitions. DC signals, however, change state rapidly, from one extreme to the other, and thus are compatible and friendly with digital circuitry. It's "yes" or "no" and there is no other possibility.

Writing this as Astro replied, so: yes, other signals can be added to either an AC or DC power form, but not on the AC line voltage, because, as you said, it will be fed to other mains users. The main voltage must be stepped down for our use anyway, and the motors are DC, so any attempt to use AC is pretty pointless anyway. The actual average DC frequency (DCC's PWM is imprecise as to actual frequency) can be quite a bit higher than the line frequency, but is still limited by many factors - as you say, track connections, braid contact, etc. And don't forget that a "packet" of info includes an "address" for each car, along with a "function request" (such as speed control), and a "value" (what speed?), etc. It's complex stuff! But DC is the only efficent way to address it.
 

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Brian Ferguson
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Tropi, any particular aspect of that site that we should look to?


Seems like lots of people patting themselves on the back, but I can't really find out what they're talking about.... maybe I'm lazy...
 

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Yep you're lazy!

Internet via power line
The connection is that the system transmits digi info via an AC supply.
It's 1 am here and I am crawling off to bed - will probably have digital dreams.
Do androids dream of electric sheep?
 
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