Multiple Unit Consisting/Decoder-Assisted Consisting

MU stands for Multi-Unit, or multiple locomotives. MU consisting is a railroading term describing putting two or more locomotives together which will 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, it is better to use command station consisting, unless there is a specific reason not to, or the DCC system simply doesn't offer the option. 
Many command station consisting solutions will typically handle most situations. Most can MU as many locomotives together in any direction, some can even MU an analog locomotive 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 with more than about 30 to 35 locomotives on the layout at one time. The cause: The command station tracks which locomotives are MUed together and sends separate commands to each individual locomotive with every speed and direction change. There is no "data rate" issue as DCC systems can accommodate the required data rate (the system's ability to quickly update all the locomotives) until there are 30 or 35 locomotives running at one time. With sound, additional lighting commands or dirty track, then slight delays will beging to occur with horns, grade crossings, light commands, etc.
Some command stations can only keep track of a limited number of trains. If a consist exceeds this limit, or other trains are in operation 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, the DCC system must support DAC. If it does not, DAC cannot be used 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 is 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, 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 task 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 Decoder-Assisted Consisting is necessary, 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.
==== How Does Decoder Assisted Consisting Work? ====
Decoder Assisted Consisting (or Advanced Consisting) works by writing a consist address into CV19CV19 is optional.. The command station will choose an addressConsist Address used will be within 1-127, usually beginning with 127 and decrementing that value as more consists are created. and make that alteration to every decoder selected by writing that value to CV19. The first locomotive selected is usually considered the top locomotive in the consist.
Once the consist is created the command station addresses it by the Consist Address in CV19. Instead of having to send packets to 2, 3, 4 or more locomotives in a consist, it reduces the traffic on the throttle network by only needing one packet for the entire consist.
The difference between Decoder Assisted and Basic ConsistingBasic Consisting was used with early decoders. is the consist address written to CV19. With Basic Consisting, each decoder in the consist is manually and individually given the same Primary (CV1) or Extended Address. With Decoder Assisted, consists can be assembled and dismantled quickly as needed. 
These consists will not work on another layout as the command station will not be aware of the consist or its address. The home layout's command station will remember the consist exists. There are also some peculiarities between DCC systems where an advanced consist may not work when moved between differing DCC systems.
If you have a locomotive which will not respond to its address, write a value of 0 to CV19. Clearing the consist address in CV19 often solves that problem.
=== Advantages of Decoder Assisted Consisting ===
# As mentioned above, when running more than 30 or 35 locomotives 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 sending command packets to every locomotive 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.
# With DAC, a consist stays consisted even when taken 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. 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.
# Some DAC decoders allow for function control of consisted locomotives. For example, typically only the lead locomotive 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 locomotives that have this address in CV 19 are automatically and instantly added to the consist. When the consist address is released, each of the locomotives 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 locomotive to run after it has been consisted, use OPS mode programming to set CV 19 to 00.
== See Also ==
* Consisting - General information about consisting
* Consisting Tsunami decoders with Lenz DCC system - Specific information for controlling functions on Soundtraxx Tsumani sound decoders from Lenz DCC systems.
* Decoder - Details about decoders.
Category:Multifunction Decoder Features
Category: Consisting