Summary: Command Stations are the very heart of the Digital Command Control (DCC) system. They receive commands from a throttle network (such as Digitrax's Loconet or Lenz's ExpressNet) and send commands, using standardized DCC packet, to various items connected to the track, such as switches, track accessories, and locomotives. The command stations don't do the actual work; they tell other things to do something. All DCC layouts require a booster (which is integrated in many systems) as the command station will send the packet to the booster. The booster's job is to combine command packets from the command station with power and send this to the layout. On larger layouts, you'll have many boosters to power all the various devices, but only a single command station as there cannot have more than one command station per layout.
In the early days of DCC, the command station was its own entity. Two components were required: a command station and a booster. The throttle network connected to the command station, the output of the command station was connected to the booster. The booster's output connected to the track. In the mid 1990s a small company called RamFixx Technologies introduced their revolutionary RamTraxx DCC system featuring the command station and booster combined into one unit. Combining both units into a single package reduced the cost by about 20%. Many people refer to the integrated device as the command station.
With command stations being combined with boosters, care must be taken when powering up the command station that the booster portion doesn't automatically power up and energize a layout unexpectedly. Dedicated boosters are better options to control when the track or layout accessories are energized. Older command stations with program outputs can always be repurposed to a dedicated program track.
Summary of System Components
A layout can have only one command station, although it can have multiple boosters.
Another type incorporates not only the command station and booster, but includes the throttle within the same unit, such as the Digitrax Zephyr system. CVP's EasyDCC command station has the throttle and command station in the same package, but requires an external booster such as their ZoneMaster. Many entry level DCC systems use this approach.
Purpose of the Command Station
The command station controls the throttle network, accepting commands from throttles and other devices, processing them, sending the resulting instructions to the booster or other devices. The booster amplifies the digital data stream from the command station to the required voltage. The booster's output is a completely digital signal which is applied to the track, or layout accessories. On the track are locomotives, with their multifunction decoders. Other accessory decoders, controlling turnout motors and crossing gates, may also be connected to the track or the throttle network to receive commands.
The command station can be the limiting factor of a DCC system. It is responsible for keeping track of which trains are controlled by various throttles. They are also responsible for a host of other items, such as interpreting additional functions sent by the throttle.
For example, if your command station supports 2 digit addresses only, your decoders will only see 2-digit addresses on the track, even though the decoder is capable of understanding a four-digit address. (These are hexadecimal, not decimal numbers. The actual range of addresses available are determined by the software in the command station). Some budget oriented DCC Starter Sets may limit the total available addresses to 10. The same applies to features like speed steps, consisting, and the maximum number of locomotives which can be controlled by the command station.
As a rule, any limitations of the command station must be identified on the packaging so the buyer can make an informed decision.
Command stations vary from manufacturer to manufacturer, offering different levels of features. Differences may include the amount of memory allocated (or slots in a Digitrax system) for running trains, range of functions supported, upgrade or expansion possibilities, output current (expressed in amperes) available to the track and the total number of throttles supported. Make sure you understand what you are getting, so you will not be disappointed or overwhelmed with capabilities. Keep in mind you do not have to use all the features of a command station to get a basic layout up and running. It's easier to not use functions and features than it is to add these later on - adding features usually requires a new command station!
Of course, prices vary based on the features of the command station. Be sure to do your research on various brands and models before making a selection. Some command stations will limit the features available, whereas others are only limited by the features of the throttle connected to it.
Note that in most cases, Command Stations offered by various vendors are incompatible with other brands.
The command station is able to transmit between 150 and 200 packets per second to the decoders. You might be asking "Is that a lot?" Let's put this into human terms:
Example: Ten decoder-equipped locomotives on the track. Each decoder will receive data packet addressed to it 15 to 20 times each second. This is important because in the absence of a packet bearing its address, a loco will continue doing what it was doing...indefinitely. If a packet of information is corrupted for some reason, it simply takes roughly a tenth of a second longer for the locomotive to respond to the change. Most people will be unable to tell that an error took place in such a short amount of time.
The software in the command station can optimize transmission rates, by prioritizing packets. This minimizes lag when a large number of locomotives need to be handled in a timely manner. Addresses with no change in status are transmitted less frequently so that addresses with changing data can be handled quickly. For example, a mainline freight with the highball compared to a shunting engine, one has little changes to the throttle, the other has many.
The command station is the center of the throttle network. This network connects both throttles and boosters, as well as other accessories, to the command station. There are different types of throttle networks, depending on the manufacturer. This means that one brand of DCC equipment will not be compatible with another, due to differences in the throttle network.
- The type of throttle network has no impact on the interoperability of decoders on the track. The NMRA DCC standards apply to the DCC signals on the track only, and do not include throttle networks.
- Main article: Command Station
Command stations are the heart of the DCC system. They receive commands from a throttle network (such as Digitrax's Loconet), process them, and decide if it needs to make a 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.
Short answer: Yes. It's possible to control two (or more) separate 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 receive it's commands through the throttle network (LocoNet, XpressNet, etc). This allows you to have same, or different voltages for different layout sizes.
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, and the voltage is 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 its 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 controlled from any throttle, at any location, with a cost savings by not having to purchase two command stations.
In service mode the digital decoder can acknowledge a packet with a brief pulse of current. This can be used to find out the digital decoder’s address, for example - send a packet asking the digital decoder if it is address 1 and wait for a response. If no response, try address 2, etc. There is currently no defined way to send information back to the command station in normal running mode, although some work is being done in this area to determine what is feasible. See the extended packet format RP for a description of feedback in service mode.
However, some systems like Digitrax can have limited feedback locomotives, called transponding.