DCC Myths

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Summary: With all new technology, a number of related myths and half-truths will appear. Digital Command Control is not immune to mythology. They are spread by those who don't understand how this new control system works, or just need a reason not to embrace DCC. If you are unsure of DCC, read on and explore the rest of the DCCWiki as well. understand how this new control system works, or just need a reason not to embrace the technology.

Digital Command Control Myths

There are many myths surrounding Digital Command Control and how it works. These are just a few of the common myths you may encounter. Some have been around for a long time and just will not die, despite evidence to the contrary. Don't let these myths deny you the enjoyment Digital Command Control can bring to your layout operations. You have highly detailed locomotives, why not have realistic operation to match?

Many of these myths are a result of analog thinking being applied to a digital technology. The rules of analog do not apply in a digital world.

Be sure to explore the DCCWiki, you can click on the highlighted terms to learn more. The DCC Tutorials are also an excellent resource.

Origins of NMRA DCC

One of the biggest myths involves the creation of DCC. Many attribute the invention of DCC to Lenz, but the truth is different.

While it is true that the NMRA examined a digital command control system supplied by Marklin (developed under contract by Lenz) for its two rail trains, it is not the entire story.

The NMRA's DCC Working Group examined a number of systems and settled on digital technology early in the process. Marklin was invited to demonstrate their system, and did. The DCCWG saw a viable prototype, with useful features. What impressed them the most was the signalling technique, which was much more robust than other systems used. This method combined both data and power in one.

The NMRA then began developing the DCC Protocol around this signalling technique, and added a number of new features and other enhancements to that of the Marklin digital system. By doing so they eliminated the possibility of legal challenges over patents and intellectual property issues, while avoiding future licensing issues which could derail the DCC Standard before it got moving.

Primary Address

One persistent myth involves the Multifunction Decoder Primary Address. When a Decoder Reset is completed, all the CVs in the decoder will be reset to their factory default values.

Ask any modeller and they will confidently state that after a reset, the Primary Address will be set to "3" (three).

True, yet not true.

Per NMRA S-9.2.2:

Configuration Variable, 1 Primary Address

Bits 0-6 contain an address with a value between 1 and 127. Bit seven must have a value of "0". If the value of Configuration Variable #1 is "00000000" then the decoder will go out of NMRA digital mode and convert to the alternate power source as defined by Configuration Variable #12. This setting will not affect the Digital Decoder's ability to respond to service mode packets (see S 9.2.3). The default value for this Configuration Variable is 3, if the decoder is not installed in a locomotive or other unit when shipped from the manufacturer.

This clause in the standard allows a manufacturer to set the primary address in their DCC Equipped manufactures to whatever value they wish. They can, if they wish, customize a locomotive's decoder to have the cab number as the default address.

The same is true for LokSound multifunction decoders by ESU. These decoders ship with very rudimentary software installed, for test purposes only. The dealer will load the desired firmware into the decoder when it is sold. The programming software can set the decoder's factory defaults to whatever the customer wants. The end user can customize and set the defaults using a LokProgrammer as well.

So, no, the default address is not 3 in all cases.

Note: Some multifunction decoders have additional options when resetting, allowing a complete or partial reset to factory defaults

The Myths

  1. DCC track signal is AC or DC: FALSE. In truth, it is neither. The DCC signal is digital, with data sent in the form of pulses on the rails.
  2. DCC is a special form of AC: FALSE. DCC is a series of fixed amplitude pulses on the rails. It could be described as a pseudo form of AC, in that the current does not follow the voltage, but appears as an AC waveform on an oscilloscope.
  3. DCC uses a carrier signal: FALSE. DCC does not use any carrier signals to transmit information.
  4. DCC uses Frequency Shift Keying (FSK): FALSE. FSK is an analog technology. There are no analog carriers or subcarriers signals present. [1] DCC signals carry their data in the time domain.[2][3]
  5. Direct Current with the DCC signal riding on top controls the locomotive: FALSE. The DCC signal on the track is composed of both Power and Data.
  6. The DCC Signal is a low frequency square wave with a high frequency control signal. FALSE. The DCC waveform is both power and data, where the binary data is encoded in the form of long (100µSecond) or short (58µS) pulses. The time between the leading edge of the pulses determines the value.
  7. The DCC signal has Polarity: FALSE. There is no concept of polarity with DCC. DCC has phases. Phase mismatch causes a short circuit. The NMRA's DCC Standard states that the rail considered positive is impossible to define[4].
  8. DCC uses a Differential Signal on the rails. FALSE. While the NMRA DCC Standard describes the signal as a Differential Signal with no ground, this is only a description of what the waveform looks like on an oscilloscope[5], not the signals themselves. [6]
  9. There are Positive and Negative voltages on the track. FALSE. As it is binary[7], only positive voltages present. One rail is energized while the other is held to 0V (Ground). The two rails are always opposite in phase/state. When the phase change happens, the relationship between the two rails is inverted. The direction of current flow changes based on the state of the rails. Unfortunately this fallacy has led to the belief of voltage doubling at a gap when the phases are mismatched.[8][9]
  10. DCC uses a negative voltage to reverse direction. FALSE. The multifunction decoder determines direction, not track voltage. Motor control is achieved by the switching sequence of the motor driver circuit.
  11. You can read DCC voltages correctly with a multi-meter: FALSE. Only a purpose built DCC meter such as the RRampmeter or an oscilloscope will give accurate readings. You can get an approximate voltage with a regular meter set to AC Volts.
  12. You can run Analog and DCC on the same layout: FALSE. When direct current and DCC meet, only bad things happen. If you must use analog power, wire the layout so you can only connect one source of power to the layout.
  13. Only two wires are needed for DCC: FALSE. DCC eliminates a lot of the wiring needed for analog operations. See the article on Wiring for more information.
  14. You must use terminators if your Track Bus run is long: FALSE. Most DCC manufacturers do not recommend nor insist on terminations. The track signals should be checked with an oscilloscope to determine if there is a problem, which may require a different solution. Many signal integrity issues are directly related to inadequate wiring.
  15. DCC is much more expensive than Analog: FALSE. In the long run, the added complexity of analog wiring schemes cost more in time and money than your DCC system. The big expense is the DCC Starter Set, a one-time cost. How much you spend is up to you.
  16. Boosters are brand specific. FALSE. Many boosters can be interfaced to a competing brand of command station. Many boosters have a low voltage or logic level input, while some can work with track voltage. It is just a matter of determining how to connect it.
  17. Costly additional boosters are needed. FALSE. Additional boosters provide additional power, as required. A typical DCC system includes a booster.
  18. Larger layouts need additional boosters: FALSE. The number required is based on the power consumption. Factors: How many locos are running at the SAME TIME. Do they have sound or lights? Do you run lighted passenger cars? Do you have accessory decoders on the track bus?
  19. Boosters with higher current ratings are better: FALSE. An oversized booster has too much current available for smaller scales. Unless you have correctly set circuit protection, locomotive damage is possible. If employing a power management device and dividing the layout into power districts, with a lower current (for example, 4A) setting, this may make sense. Remember, high current boosters can deliver a lot of current into a short circuit, as much as 60A for a brief period of time. Which results in damage before a circuit breaker can react to it.
  20. Sound Decoders require more power: FALSE. High inrush current is only at cold start up. Otherwise, sound needs about 20% more power than a silent decoder. Adding an energy storage device to any multifunction decoder increases inrush currents.
  21. BEMF never works in a consist: FALSE. BEMF is sensitive to mismatches of multifunction decoder/loco/manufacturer and can be difficult to setup correctly. However, it can be done. Most users do not need BEMF, and it is ok to disable if causing issues. True, it can be an issue. But this is not an absolute TRUE/FALSE situation. Some multifunction decoders disable BEMF when consisted.
  22. The command station will shut down when a booster is shorted: Maybe. If you only have one command station/booster and no circuit breaker at all, then yes. Remember that the protection in ANY booster or Command station is designed to protect the equipment it is part of. Other boosters will continue as if nothing is wrong.
    1. Exception: The NCE Power Cab. Unfortunately, due to its integrated design, a short will cause the Power Cab to reboot. For this reason, NCE offers a protection module called the CP6, its purpose is to limit current and prevent a reboot.[10]
  23. Programming on the main/(OPS mode) is dangerous: FALSE. Used for specific multifunction decoder address/specific CV. Programming On the Main (POM) instructs the command station to send instructions to change a specific CV to a specific decoder address.[11] It is the same as sending a horn or light instruction. Blast Mode, often initiated by using address 0, causes instructions to be sent to EVERY multifunction decoder in every locomotive on the rails. OPS and Blast Modes are not used with a dedicated programming track, which frequently leads to confusion. Remember, there is NO read back of CV in POM/OPS modes. [12]
  24. You can read CVs on the program track. FALSE. The throttle may say yes, but it is only indicating the decoder acknowledged the write operation. It does not indicate success.
  25. You must convert all your locomotives to DCC: FALSE. You only need to convert those you wish to use on a regular basis. Some may not be easily converted to DCC. Others may not be worthwhile to convert. Unless you have real attachment to them, they can be sold off and newer motive power purchased.
  26. Stall Current is important: Maybe. Older locomotives, yes. Newer locomotives have improved drivetrains and motors which are more efficient and do not draw as much current. It does not hurt to check. Most multifunction decoders shut down if they overheat, many modern decoders have the ability to handle an amp of current to the motor.
  27. DCC Ready means the locomotive has a multifunction decoder: FALSE. The term DCC Ready has many meanings. A DCC Ready locomotive lacks a multifunction decoder but can be converted.[13]
  28. The selection of a multifunction decoder is determined by the scale: FALSE. Many N scale decoders will work in an H0 scale locomotive without problems. Often the cost is higher with fewer functions.
  29. Digital Command Control decoders will reset to their default address[14] when running on a continuous loop. This myth is FALSE. This myth first appeared in the United Kingdom and was promoted at train shows. Despite the fact that many exhibition layouts are loops and running DCC, their decoders did not reset to "03". A decoder can be corrupted, but running in a loop isn't a cause.
  30. Digital Command Control is a benefit to large layouts only. FALSE: DCC benefits any layout, large or small.
  31. You need a computer to be able to use DCC. FALSE. You do not need a computer. They can be a very useful addition to your DCC system.
  32. Programming requires a computer. FALSE. Programming is an unfortunate choice of words, as it refers to configuring a decoder with a throttle.
  33. You must purchase a decoder testing device. FALSE: While a Decoder Tester is a useful item, much like a computer it is not a requirement. Although they can be very useful to test and configure a decoder before installation.
  34. The track must be absolutely perfectly clean or DCC will not work. FALSE. Track must be kept reasonably clean for reliable operation, regardless of the control system used.
  35. Turnouts must be replaced with DCC Friendly ones. FALSE. There really is no such thing as a DCC Friendly turnout. If a turnout worked fine with analog operations, it will work fine with DCC as well. The issue is strictly electrical, and some turnouts may require mechanical adjustments or modifications to eliminate troublesome problems. Click on the link for more information of issues related to turnouts and DCC.
  36. CV29 is different among decoders: FALSE: The four mandatory NMRA CV’s: CV1 Primary (short) address, CV7 Manufacturer version number, CV8 Manufacturer ID Number, CV29 Configuration Data.
  37. Automotive tail lights are necessary to protect against short circuits. FALSE. In fact, they may actually defeat the short circuit protection offered by the booster.
  38. Radio operation is expensive. The answer is complex. Yes, radio equipped sets cost more. A low-cost solution is using WiFi devices with JMRI and a Wi-Fi hub. The computer along with an interface to the throttle network can do the same thing. There are standalone Wi-Fi devices available which connect to the throttle network to offer the same capability as well.
  39. To extend radio range you can use a pie pan. FALSE. Range can be improved using a ground plane, which must be designed to work with the antenna.

See Also

  1. Frequency Shift Keying modulates an analog carrier to carry binary information. It uses defined analog frequencies to represent binary values. If it were used, why transmit two values when multiple values are possible? Why interoduce more complexity and cost than necessary?
  2. The time between the leading edge of the pulses determines the value.
  3. Some incorrectly argue it is Pulse Width Modulation. PWM is useful if an analog recreation is desired, such as audio. Again, why send two values when multiple values are possible, while increasing costs and complexity?
  4. The NMRA definition of the positive rail is the right hand rail as seen from the engineer's position in the cab.
  5. The oscilloscope measurement is floating, meaning there is no reference to ground (a differential voltage), resulting in the typical DCC waveform many use to support their claim it is an AC signal, or that a positive and negative component are present. As DCC is Digital, the only valid states of On and Off, ON represent by the presence of a positive voltage, and Off being the lack of voltage.
  6. Differential Signalling requires two complementary signals, each with their own conductor, which are summed to eliminate/reduce noise induced errors when using high speed communications between devices. DCC uses a higher voltage with a slower data rate, therefore much less susceptible to noise. The DCC protocol also includes error detection.
  7. Binary, meaning only two states are permissible, On and Off.
  8. Each output of the booster alternates between Source or Sink. Its state is determined by the DCC signal from the command station.
  9. This argument for AC fails when one considers the power supply voltage is inadequate and/or the type of power supply.
  10. Allowing excessive current flow with the Power Cab for an extended period can result in the case melting or destruction of its booster, as the Power Cab has no overcurrent protection built in, unlike many other boosters.
  11. Some multifunction decoders will not accept extended address changes in Operations programming mode.
  12. Unless you have Lenz’s Railcom or Digitrax’s Transponding technologies implemented on your layout.
  13. DCC Ready can have multiple meanings. In many instances the vehicle can be converted to DCC with minimal effort.
  14. Default address of 3 is true for retail multifunction decoders only.