How Does Digital Command Control Work

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How Does Digital Command Control Work?

Main article: DCC Power

Though it can sometimes be useful to understand how an underlying technology works, it is usually not necessary to understand a technology for one to use it. Most of us use the Internet without understanding the underlying TCP/IP protocol suite. Likewise, there is no need to understand the technology behind DCC in order to use it. The information below is provided for those who wish to understand DCC's underlying technology.

Is It AC or DC?

The most frequently asked questions about Digital Command Control are "Is the Digital Command Control waveform applied to my track AC or DC?" and "Is it a special form of AC?"

The answer is: No.

Many of those ideas come from trying to apply analog ideas and what is in a wall outlet to a digital signal.

  • Since the rails switch state from ON to OFF thousands of times a second, the DCC signal is clearly not Direct Current which is defined as voltage of the constant polarity. So typical DC measurements cannot be used.
  • DCC current does change direction, technically it is AC (alternating current), but it is not the classic sinusoidal AC waveform in your home's outlets. Due to this waveform's properties, a voltmeter designed to read your typical home AC voltages will be inaccurate.
  • The DCC signal is a "square wave", where the time the rail is ON or OFF contains the digital data of "ones" and "zeros". Indeed the NMRA standard defines the timing for these digital "data bits".
  • To properly measure "DCC track voltage" you need either an oscilloscope, purpose built DCC voltmeters, or a good True RMS DMM on the ACVolts range.
  • While it is possible to make measurements with any general purpose DMM, these measurements are only to be used for comparisons, not as a measure of a definite value.

Clear Your Mind

Approach Digital Command Control with an open mind. As a Digital technology, avoid thinking in analog terms. Many myths have arisen over the years, often by trying to fit Digital Command Control into an analog concept, or by those who fear Digital Command Control. There are many out there who will try to apply analog concepts to digital. It just does not work that way. Most problems related to Digital Command Control exist not with the technology but within the user's frame of reference.

Conceptually, How Does DCC Work?

DCC works by conveying operating commands, such as speed and direction, to the locomotives via modulation of the track "signal". Each locomotive has at least one pre-assigned address. Since Commands are part of the track signal, all locomotives receive them, but each locomotive only acts on commands addressed to it. All other locomotives ignore the command. (There are some exceptions, like "all stop") To ensure reception of commands, typically commands are repeated periodically. DCC commands are broadcast at more than a hundred commands per second, so each locomotive receives its command updates in a timely manner.

Electrically, How Does DCC Work?

Digital Command Control signals are encoded into the voltage applied to the track (10 to 24 volts, depending on scale (per the NMRA standard). One of the very smart ideas about this is that with a full wave bridge (or diode matrix in this application), very little filtering is needed to turn this source of power into DC to operate the decoder and the motor. The sinusoidal AC in your home needs a lot of filtering to make "good" DC. Thus the DC needed to power the microprocessor in your decoders can be done simply and compactly.

What are DCC Packets?

As stated earlier, the decoder interprets the modulated signal as "ones" and "zeros", or bits. Eight bits form a byte. Commands are made of these bytes. The command station sssembles these bytes of data into a packet. A packet is a complete DCC command, with the preamble, address, commands and error correction bytes. Packet sizes can vary slightly, so 100 to 200 packets are broadcast per second.


Operationally, How Does DCC Work?

The following links have examples of how to accomplish a move using a DCC system.

Operational Example: Moving a Locomotive

Main article: Introduction to DCC/Moving a Locomotive

Operational Example: Moving a Locomotive Onto a Siding

Main article: Introduction to DCC/Moving a Locomotive Onto a Siding