Multifunction Decoder Assisted Consisting

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Summary: Places the burden of MU consisting on the decoder instead of the command station.

MU stands for Multi-Unit, or multiple locos. MU consisting is a railroading term used to describe putting two or more locomotives together to operate as one unit. Please see the consisting article for more details as this article specifically deals with Decoder Assisted Consisting.

Command Station as compared to Decoder Assisted Consisting

When to Use Command Station Consisting

Generally speaking, you are better off to use command station consisting, unless you have a specific reason not to, or your system simply doesn't offer it.

Many command station consisting solutions will typically handle most needs. Most can MU as many locos together in any direction you want, some can even MU an analog loco to the consist as well. You will have to check your command station's consisting capabilities for details.

The only disadvantage of the command station consisting method appears when you run more than about 30 to 35 locos on the layout at one time. This is because the command station keeps track of which locos are MUed together and must send a separate command to each individual locomotive, and it must do so with each speed and direction change. There's no "data rate" issue as DCC can accommodate the data rate (the system's ability to quickly update all the locomotives) until you get to around 30 or 35 locos running at one time. If you add sound, have additional light commands, or dirty track, then you will start noticing slight delays with horns, grade crossings, light commands, etc.

Some command stations can only keep track of a certain number of trains. If your consist exceeds this limit, or there are other trains on a large layout, you will need to use DAC (decoder assisted consisting).

Decoder Assisted Consisting

Decoder Assisted Consisting (DAC) places the consist burden on the decoder and can free the command station to handle other duties - particularly useful on large layouts where there may be MANY decoders and activities going on.

To use DAC, your DCC system must support DAC. If it does not, you cannot use DAC on your layout. With some systems, the process of MU consisting via command station consisting is different than that for consisting with DAC; the problem with this is that you have to keep track of which decoder is capable of DAC and which ones aren't. If you try to consist a non-DAC decoder with DAC, it simply won't work - you won't break it trying.

Fortunately, with some systems, (such as Digitrax's Chief and Empire Builder), the consisting process is the same for both methods. If you have the decoder address status edited for DAC, the system will automatically consist those with DAC that are capable of it. This automatically offloads the load of consisting onto the decoder without you having to perform additional steps to create the consist each time.

If your command station supports it, Status Editing is the process of telling the command station about your decoder(s) so that it can optimally use the features of a decoder. In this case, the command station will automatically configure the consisting to use DAC if the decoders support it, otherwise, it will setup a command station based consist. The operator (you) simply tells the command station to create the consist, and it decides the best method automagically.

For most people, using status editing (or even DAC) isn't necessary because command station consisting works just fine. However, if you need Decoder-Assisted Consisting, status editing is how you tell the system which addresses to use DAC with. You would typically status-edit the command station right after programming the decoder address, when you still have the decoder's capabilities fresh in your mind.

Advantages of Decoder Assisted Consisting

  1. As mentioned above, if you are running more than about 30 or 35 locos at once, consisting with DAC will lessen the command station's command packet load. This allows the command station to send one packet to the consist address for each command, speed, direction, or speed change, instead of having to send command packets to every loco in the consist for each command, speed, direction, or speed change. This frees up the command station to handle other operations or repeat packets more frequently.
  2. With DAC, a consist stays consisted even when you take it to another layout since the decoder keeps track of the consist instead of the command station. On another layout, the command station doesn't care that it's consisted. You simply select the consist address and the system will start sending command packets to it. The consisted decoders will listen for the consist address and respond to commands sent to it.
  3. Some DAC decoders allow for function control of consisted locos. For example, typically only the lead loco of a consist will have its headlight (function control) on. However, if the decoder also has the DAC function control feature, you can specify which functions will be controlled by the consist address and which ones won't (sounds, animation, lights, etc.)

Hints and Tips from Various Users

Decoder-assisted Consisting vs. Digitrax DB-150 Command Station

From Bryan on 2 August 2006 (EDT):

I'm running a Digitrax DB150 command station with JMRI, and I prefer decoder-assisted consisting. I find that, with command-station consisting, the command station seems to lose track of consists that haven't been run in a while, meaning that they have to be entered all over again (a tedious process).

With decoder-assisted consisting, the consist address is recorded in CV 19. Suppose the consist address is 50. When this address (50) is selected by a throttle, all the locos that have this address in CV 19 are automatically and instantly added to the consist. When the consist address is released, each of the locos is again separately controllable by its primary DCC address.

So, the good thing about DAC is that the consist doesn't get lost. That's also the bad thing. I think where people get into trouble with DAC consisting is that they forget that there's something in CV 19. If you can't get a loco to run after it has been consisted, use OPS mode programming to set CV 19 to 00.

See Also