x-zactly what is D-C-C?

Discussion in 'DCC & Electronics' started by cidchase, May 23, 2004.

  1. cidchase

    cidchase Active Member

    how it works:
    (quoted from Atlas forum)

    "DCC (Digital Command Control) operates on a square wave alterating current
    signal divided into a units called packets.
    The packet consists of a header (a signal to tell the decoder that this is the
    start of the packet), the body (basically the command), and a trailing set of bits
    at the end of the packet.
    Each time the voltage goes from 0 to + back past 0 to - and back to 0 is called a bit. The decoder basically ignores the trailing bits of the packet (there is no information there, just AC current) "

    Power for the DC motor is rectified from the track AC voltage.
    Presumably the header also includes the loco address, since multiple
    locos can be operated simultaneously.
  2. Woodie

    Woodie Active Member

    I'm not sure, since it's while since I really examined DCC, but the voltage at the track, I think, is a constant 12 V DC (not AC). I understand it to be PWM (Pulse Wave Modulated). i.e. The voltage is "pulsed" onto the track. The bit stream for DDC control is placed between the "pulses" of 12V DC, and the decoder switches (very rapidly) on each pulse to filter out the DCC bit stream from the 12 V DC that it uses to drive the loco (or "accessories") with. I''ll see if I can find a detailed link re this.
    This may not be actually what happens, but conceptually.

    <---12 V DC PULSE---> 10110011110101001 <----12 V DC PULSE---> 1011101010001 etc etc etc.....

    The 1011001111010 etc being the "bits" of information instructions to control the loco and accessories.
  3. Woodie

    Woodie Active Member

    It is AC.

    I do actually correct myself. It is AC current applied to the rails, but a "modified" type of AC current. You still, however, need to be aware when wiring of the "polarity" of each rail. This is not necessarily in the ture sense of "polarity (i.e. + or -), but as in left/right, inner/outer and reversing loop mechanisms are still required to "switch polarity" in a similar sense to existing DC mechanisms. However, with DCC it can be automated (with apporpriate "reverse loop detection decoders" to an extent it is pretty transparent to operations.
  4. Fred_M

    Fred_M Guest

    Then that brings up a question, when I took physics in college the definition of AC mentioned two pesky little words that don't seem to be in the definition today, "sine wave". So when did they rewrite the dictonary? Anyone know? FRED
  5. Russ Bellinis

    Russ Bellinis Active Member

    "Sine wave" isn't mentioned with dcc because a sine wave is a sideways "S" curve if memory is accurate, dcc uses a "square wave."
  6. Fred_M

    Fred_M Guest

    That's correct Russ, but they define DCC as AC and the older definition of AC included the words sine wave, but not anymore. :D AC now doesn't include "sine wave" in the definition. I'm looking in the Glossary of a 1987 Fundamentals of Physics textbook, it says sine wave. I would define DCC current as "multiplexed bipolar asymmetrical pulsed DC", but that's just something I made up. I'm just funning, but it is a point to ponder? FRED
  7. shamus

    shamus Registered Member

    WOW -multiplexed bipolar asymmetrical pulsed DC, :D thats interesting Fred, next your going to tell me you need a Slide rule and a book full of logarithms. :thumb: :wave:

  8. Fred_M

    Fred_M Guest

    Already have them. :thumb: someplace? I think I could still use them? I think they are over by the gear driven computer and telegraph. I'll wire you if I find them. FRED
  9. RailRon

    RailRon Active Member

    I would say that AC is ANY sort of electric current which reverses polarity from time to time.

    In a technical sense it's clear that this can be done in the form of a sine curve like our normal household current. But it could also be in the form of a rectangular curve (square pulse), a triangular sawtooth curve or even a completely irregular curve - as long as the voltage drops from + to - and vice versa within reasonable time limits.

    In DCC a basically square pulse curve is modified so that it carries coded information to the loco - and that's where the "multiplexed bipolar asymmetrical pulsed DC" comes in. :D :D :D

  10. Ralph

    Ralph Remember...it's for fun!

    Uhhhhhh...what now?.... :)
    Electrical caveman and Atlas block components user
  11. Fred_M

    Fred_M Guest

    From http://www.thefreedictionary.com/AC, AC - an electric current that reverses direction sinusoidally; "In the US most household current is AC at 60 cycles per second" The word sinusoidally revert to:
    Adj. 1. sinusoidal - having a succession of waves or curves
    curved, curving - not straight; having or marked by a curve or smoothly rounded bend; "the curved tusks of a walrus"; "his curved lips suggested a smile but his eyes were hard"

    So AC is current that reverses direction in a smooth wave.

    :confused: :confused: :confused: :confused: :confused: :confused: :confused:
  12. cidchase

    cidchase Active Member

    Hi y'all,
    Alternating current alternates direction periodically, while
    direct current does not. The waveform is irrelevant. :thumb: :thumb: :thumb:
    There's no reason for folks to be confused about these terms! :wave: :wave:
  13. Lightbender

    Lightbender Member

    But I'm still confused! Are you saying the dc is being modulated to pass the data or is it simply being used to carry the modulated ac data signal.
  14. Woodie

    Woodie Active Member

    To AC or DC..... that is the question....


    A link that may help define what you are asking about AC/DC stuff. :)

    Click here :thumb:
  15. railwaybob

    railwaybob Member

    For a simple, easy to understand, non-technical description of what is DCC, click on this link.

    Unless you're a person who loves the technical stuff on the digital transmission of "commands" over Local Area Networks (LANs), you can get bogged down with the 1's and 0's.

    The alternating "Direct Current" that goes down the track power buss to the rails is actually "bursts" of 1's and 0's which convey a message for a specific decoder (stop, go, reverse, forward, turn on the lights, etc). If you are interested in monitoring these signals, you would need an interface between your track and your computer (eg LocoBuffer II) and some software that would save you the trouble of trying to interpret what those 1's and 0's mean.

    For a semi-technical description of how it works, read Chapter 13 of "The Digitrax Big Book of DCC".
  16. Cogent

    Cogent New Member

    Being that I am an IT professional I understand how this works.

    Comment: After further thought, DCC and DSL are not similar.

    This technology is very similar to DSL used for broadband internet connections. As an example, how is it that your computer can use your phone line at the same time as you do while talking to your friend across town??? A phone line is electrified just like a model train track. It always has the same voltage so how is a signal sent? Good questions.

    I just found the NMRA Standards page on DCC and it is very informative (http://www.nmra.org/standards/consist.html#standards-DCC). The answer to my retorical questions above is frequency. Others have basically said the same thing above but in simpler terms. With DSL your computer is actually using a much higher frequency than the human ear can hear (or in some cases, lower). But both voice and data are sent over the electrified line using various frequencies applied to the voltage, at the same time! Did you know that the tones that the buttons on your phone make are actually two tones used to cross reference the the number you pushed. A tone is of course a frequency so each button sends two frequencies at the same time.

    So, with a DCC system the instructions are turned into changes in frequency applied to the voltage to your layout. The changes in frequency are what send the data (instructions) but every locomotive actually receives the instructions. This is why each locomotive has a decoder and a specific number, or address. Only the locomotive specifically addressed will process the instructions and perform the action. All the other locomotives ignore the data since it isn't addressed to them.

    I guess a simple way to think about it would be to think about your radio. Every radio can be tuned into it's own radio station. Even though radio waves of every station reach the radio, it is only tuned to one station. It will only play signals from that station.

    Does that make sense?

  17. railwaybob

    railwaybob Member

    Actually, it's more like a PA system that broadcasts a series of announcements intended for specific persons that everyone can hear. If the announcement isn't for you, you ignore it. However, if the PA system says this message is for you, then you listen and follow the instructions.

    Similarly with DCC. The command station/booster is continuously broadcasting commands for specific locomotives. The decoder in your loco reads all of those commands. If the command isn't for your loco, the command is ignored. However, if the command is for your loco, then the decoder listens to the command, interprets what it has to do, and carries out the command.
  18. Cogent

    Cogent New Member

    Well said RailwayBob.

    I realized after posting yesterday that DSL is nothing like DCC. LOL!

    Oh well... I will add a comment to my previous post.

Share This Page