DCC FAQ
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Welcome to the DCC Faq. Please add items here which are commonly asked in forums and discussion pages.
[edit] What is DCC?
DCC is an acronym for Digital Command Control. There have been various Command Control systems since 1946, the word Digital is used to differentiate DCC from the Analog Command Control systems in use. DCC uses a digital signal on the track at all times, unlike older analog systems which superimposed their signals on a Direct Current voltage. See the DCC History page for further information on Command Control systems.
It is a method for taking input from a throttle (think: train controller) and transferring it to a command station, which then sends it out as a digital packet to the entire track layout. The locomotives on the track listen to all the digital packets, looking for their address. Once a locomotive sees a command addressed to it, it performs whatever function it's told to do - such as stop, speed up, slow down, turn lights/on and off, or turn on/off various engine sound effects.
Please see Introduction to DCC for more details.
[edit] What is a command station?
Command stations are the heart of the DCC system. They receive commands from a throttle network (such as the Digitrax Loconet), process them, and decide if it needs to make a DCC standardized digital packet to send to all the decoders on a DCC system. They don't do the actual work, they tell other things to do the work. Please see command stations for full details.
[edit] What are the NMRA DCC Standards?
In the early 1990s, the National Model Railroad Association (NMRA) began looking into a standard for command control systems. All the analog based command control systems in use had limitations, which inhibited expansion, and they were never compatible amongst themselves. The NMRA's committee decided that digital was the way to go, and the best way was to use a 100% digital signal on the track.
What caught their attention was the digital systems used in Europe, originating mainly from Germany. One system in particular was promoted by Lenz. Bernd Lenz, the developer of the Lenz command control system had published the specifications of his system, hoping others would embrace his ideas and create compatible systems. The NMRA would take control of that open standard and make enhancements to it, eventually releasing it as the Digital Command Control System.
In doing so, it created a basic standard which is compatible across most DCC manufacturers. This allows us to use a brand XYZ decoder which is controlled by brand ABC DCC system.
Compatibility of DCC components is defined at the track level. This allows decoders from different manufacturers to work together, but permits manufacturers to innovate on the user interface, throttle, and command station capabilities. Certain parameters of the decoder are defined by the NMRA, but the designers and manufacturers may add additional features (such as sound), providing they do not impair the basic decoder operations.
The complete (and rather technical) NMRA DCC standards documents can be found on their web site.
[edit] What is a decoder?
A decoder is a device which listens to digital packets for its address and performs whatever function is requested. There are two types - mobile decoders and stationary decoders. Some decoders are very simple devices providing only motor and/or a small selection of lighting options, while others are very complex with multiple special effects, tuning fine motor response, and sound effects available. Please see the decoder page for full details.
[edit] What conditions can burn out a decoder?
- Overvoltage. For example, booster is set to incorrect scale.
- Current draw exceeds decoder ratings (wiring short or other overload condition)
- Be careful when first testing a decoder installation. Put the DCC device on a test track (programming track or section with a 100-ohm series resistor for current limiting) instead of the mainline for testing. If all functions work, and it can move (assuming motor output drive from decoder) at low speed steps, then you should be relatively safe to place it on the mainline. As always, follow the manufacturer's instructions for testing in addition to the above.
- Overheating
- Imagine running a black loco in direct sunlight and hauling a heavy load at nearly the limit of the current output of the decoder. Now imagine the decoder only having current limits, and not thermal limits built in. Your imaginary decoder should running extremely hot. As you know, when things run too hot you'll let the magic smoke escape from the decoder. Avoid excessive loads for extended periods of time. If in doubt, check temperatures on newly installed decoders. If you find it running too hot, upgrade the decoder to handle larger currents and/or increase air flow around the decoder.
[edit] Can one command station control multiple layouts?
Short answer: Yes. It's possible to control two (or more) seperate layouts using a single command station. For instance, if you wanted to operate an N scale layout inside and a G scale outside. Most systems combine the command station and booster into a single unit. Either case, you connect one layout as normal, that is, connect the command station/booster to one layout. Then, you simply purchase a second power supply and booster for the other layout. The second layout will recieve it's commands through the throttle network (LocoNet, XpressNet, etc). This allows you to have same, or different voltages for different layout sizes.
For example. Assume we have an N scale layout in the garage. Since power requirements are low, we purchase a DCC system that outputs about 2 to 3 amps of power, and the voltage set for N scale. We setup this layout as described in other parts of this website. A year later, we want to setup a G scale, or garden railroad in the backyard. To do this, all we do is purchase another power supply and booster for the second layout. Because the power is independent of the layout, we don't need to worry about the higher voltage from the garden railway making it's way to the N Scale layout. To get the commands from the command station to the garden railroad booster, we simply connect the throttle network (such as ExpressNet, or LocoNet) to the booster. We now have two railroads being controled from any throttle, at any location, with a cost savings from not having to buy two command stations.
[edit] Can I quickly change from DC to DCC and back again?
Yes, simply wire for multi-cab control as you normally would for DC. However, do not use common rail wiring. Also, use #14 or better bus wire (see Track wiring for details). When you want to change from DC operations to DCC, simply hook up the booster in place of one of the cabs and switch all blocks to that cab. When you're ready to go back to DC, simply flip to the block toggles back to the DC cabs.
[edit] 128 vs 126 speed steps
The number 128 comes directly from the use of 7 binary bits to set the locomotive speed plus one more bit used for indicating locomotive direction within the command. Together they make up a total of 8 bits or one byte which is a standard unit of binary information for computers.
Of the 128 speed steps, two are reserved for special commands such as Emergency Stop. This leaves 126 usable speed step commands to control the locomotive normally. Thus any throttle or cab with a display will only show 126 speed steps at full speed and not 128.
For more information on 126 versus 128 speed steps, see the speed steps article.
[edit] How do I measure DCC voltage?
Because DCC is a square AC signal, you cannot use a simple AC / DC meter. The cheapest solution is to build a simple circuit. Here's one circuit diagram, and here's a second one. Although it won't be 100% accurate, it will get you pretty close. A second solution is buy a DCC specific Volt/Amp meter from various DCC resellers.
[edit] How do I control track switches?
You'll need some turnout motors and a mobile decoder. See Turnout Motors for details.
Add your question here, someone will answer...sooner or later. ;-) TazzyTazzy 14:18, 12 January 2006 (EST)
[edit] Limit speed of a loco?
For those of us with children, or grand children around, that like to run trains at top speed, there is a quick an easy way to slow the train down.
Simply, program the decoder CVs to use a lower "max" voltage, thus limiting the speed. You'll need to change two CVs - CV05 (max voltage) and CV06 (midpoint voltage).
CV02 should remain unchanged from "normal speed" to "limited speed". Some people have an extra locomotive just for these situations and always leave them set for slower speeds.
[edit] How much track can my booster/command station power?
Many people just getting into DCC often wonder how much track their command station and/or booster can power. However, as long as you adequate buss and feeders, the length of track isn't an issue. Whats needs to be considered are just two factors:
- The number of engines (motors) you will be running simultaneously; and
- The number of accessory loads on your booster.
It then comes down to how many trains your system can power, not how much track. Simply add up all the power needs of the trains you want to run and make sure you have enough boosters distributed throughout your layout.
[edit] Rails dangerous to kids?
The maximum voltage regarded as safe for human contact is nominally anything less then 32 volts. The maximum voltage used to be 48v, but has been reduced over the last few years. For this question, AC and DC are considered to be the same risk.
However, there is more danger from heat generated by shorting the tracks with metal objects, such as bracelets or necklaces. However, your booster should detect the short and cut the current. But, before that happens, the metal may get warm or even hot. The child may be scared and even slightly burned, but not seriously harmed.
This is yet another reason to test your wiring to make sure your booster's short circuit protection works on all sections of track.
[edit] See Also
- DCC History, an introduction to command control systems.
