[Fergy]

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?

 

[Fergy]

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....

 

[Fergy]

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).

 

[Fergy]

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.

 

[Fergy]

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...

 

[Fergy]

Okay, sorry, didn't have the stamina initially to look.... yeah, the AC line has been used before - in my last house, it was used for intercom units - we could talk or call from several points in the house - cool but not particularly secure. This was done by overlaying a signal on the AC that could only be detected by decoding units - it was out of phase from the standard AC and very low-level so it didn't affect standard AC devices. Still, for a system as complex as digital slot cars (worse actually than an internet connection because of the real-time urgency), and problems from external and internal noise sources, it just isn't as ideal as a complete DC system that is totally isolated from the mains voltage.

...and I don't want to know about UK electric sheep...