Digital Command Control Frequently Asked Questions: DCC Basic Theory

What is a Packet and why do we need them?

A packet based system is flexible and easily extended in the future as new features are deemed necessary. Each packet corresponds to a command addressed to a particular digital decoder (mobile or stationary). Each packet contains the address of the digital decoder it is intended for, and data that tells the digital decoder what to do. For example a packet might be addressed to mobile decoder address 87 and the data might tell that decoder to change its current speed to speed step 10. The NMRA Standard specifies the baseline packet format that allows for up to 99 addressees and 14 speed steps. The Recommended Practices specify an extended packet format that allows for up to 9,999 addresses and 14, 28 or 128 speed steps. This is an example of the flexibility afforded by a packet based system.

What is meant by a bi-polar signal?

Each bit is divided into two halves, which are "mirrored" around zero volts. The bottom half of the signal (below zero volts) is a mirror image of the top half, but shifted over half a bit width.

Why use a bi-polar signal instead of DC with a superimposed signal?

Several reasons

• Because the signal is the power, if you have power to any given decoder equipped locomotive, you can control it.

• The track stays cleaner because there is no polarized electro-plating action like with DC systems. You will still need to clean your track, just not as often. Because the signals are "refreshed" periodically by the command station, if a loco temporarily loses contact with the rail due to dirty track it can pick up the signal again and continue running. DCC systems are much more tolerant of dirty track than DC systems.

• Locomotives can run in both directions at the same time on the same section of track because with the bi-polar signal, it does not matter to the loco decoder which way it "sees" the signal.

• The bi-polar signal allows us to use "zero stretching" to control a locomotive without a decoder. We will discuss zero stretching later.

Is the locomotive speed controlled by varying the track voltage?

No, the track voltage is constant and the loco speed is controlled by the decoder inside. The decoder varies the amount of power provided to the motor to control the loco's speed. The voltage provided by your transformer to the system may vary based on the power needs of the scale you are operating. This constant track voltage gives the added benefit of constant loco lighting, too!

If you are constantly re-transmitting packets won't that slow down the system?

The answer to this question really depends on the communications strategy and system architecture chosen by the system manufacturer. Most systems use a polled bus type of strategy and this can show signs of slowing down when more and more throttles attached to the system. Digitrax has engineered LocoNet to side step this issue. With our network strategy, bandwidth usage is greatly reduced. See LocoNet, the Digitrax Difference for more in depth information about this topic.

The Standard baseline packet allows for 127 loco addresses. Is this enough?

Realistically, most home layouts will probably not ever reach the 127 loco address limit in the standard. The extended packet RP does allow us to use 14-bit addressing to access over 9,000 addresses. Note: because certain addresses are "reserved" the full range of 9,999 addresses is not truly available.

The Standard baseline packet allows for 14 speed steps. Why would I need more?

It depends on how you run your railroad. If you really appreciate fine low speed control, like most serious modelers do, then 14 speed steps is NOT ENOUGH!!! More speed steps give you finer control for switching and serious operation. People who think that more speed steps don't give them anything are probably using handheld throttles with potentiometers or push button controls that can't do justice to the joys of 128 speed steps because of their mechanical restrictions.

What do you mean by Function Control?

All DCC Locomotive decoders can control speed and direction and most have some form of function control as well. This means that the decoders can control other "on/off" devices in the locomotive such as head lights, smoke units, cab lights, other lights, sound units, etc. Some decoders can even generate special lighting effects and sound effects without installing additional boards in the locos. Note: Function control is not part of the baseline standard, it is defined by the RP's.

Can you control turnouts?

Yes, stationary decoders are used for things like controlling switch machines, layout lighting and animation. The extended packet format allows for 2,048 stationary addresses. These stationary addresses are in addition to the mobile addressees for locomotives. Note: Stationary decoder control is not part of the baseline standard, it is defined by the RP's.

What about computer control?

You have several options if you want to use a computer. You can use a computer and an interface or you can use a computer to directly generate packets and drive the track through a booster. YOU DO NOT NEED A COMPUTER TO RUN DCC! You can use one if you choose to.

Some DCC decoders will work on conventional 12V DC power packs. How does this work? Is it part of the standard?

The "analog mode conversion" feature is not a part of the standard, it disappeared from the final draft at the last minute. We were surprised to find that is was dropped because it is a very important feature for many modelers. Decoders that support analog mode conversion, will convert to analog operation when they do not see a DCC signal.

How can an engine without a decoder run on a DCC layout?

It looks like magic but really it's not. The DCC signal is symmetric around 0 volts. This provides a 0 volt DC component. By expanding the length of the zero bits on the positive side of the signal a positive DC component can be added. Likewise, by lengthening the bits on the negative side of the signal a negative DC component can be added. Only the 0 bits can be lengthened like this.

The result is a non-zero average DC voltage that will run a loco without a decoder. However, since the complete signal gets to the motor, the stretched zero side causes the motor to run, and the following instant, the unstretched side (the opposite polarity) tries to reverse the motor. The longer (stretched) side wins, but motors do run more noisily and generate more heat.

If you are using ironless core motors, you will need to take special precautions to avoid burning them up. Usually, a ballast lamp installed in the engine will do the trick. You should run these motors on the lowest possible voltage setting on your booster.

How does this "zero stretching" affect locomotives with out decoders?

When a conventional engine sits still on a DCC layout, you will hear it "sing." This noise is caused by the signal described above hitting the motor. Once the locomotive is moving, this noise goes away. If the amplitude of the digital signal is greater than the maximum stall rating of the motor, the motor may experience permanent damage. Therefore, it is recommended that you not leave Un-converted locos sitting still on DCC layouts. This precaution will prevent stress to the motors and damage due to excess heat build up. Generally, most common open frame and can motors are not damaged even after extended exposure but it is always best to play it safe.