With DCC, there are four ways to build MU consists: Decoder-Assisted Consisting provides two of four ways to do it. The other two ways are Command Station Consisting (what Digitrax calls UniVersal™ Consisting), and Basic Consisting (giving multiple decoders the same address).

All systems provide at least one or more of these ways to do MU consisting. Some systems provide for all four ways. All but Basic Consisting can be done anywhere on the layout without need of special trackage and gapping. And even Basic Consisting can be done anywhere if your system and decoders support OPS-Mode Programming.

With Basic Consisting, you simply program the same address into two or more locos. When you operate that address, all the locos with that address will respond. If you want one loco to go backwards when the others are going forward, you can reverse the Base Direction of that loco with CV29. The problem with Basic Consisting is that it's not very versatile - requiring re-programming the loco address each time you want to build a consist or take it apart.

Command Station Consisting, the way Digitrax has it implemented, is ultimately versatile. You not only can MU as many locos together in any direction you want, but can even MU an analog loco to the consist as well. While other systems provide Command Station Consisting, most have limitations as to how many can be consisted, and I don't know of any others that allow consisting of an analog loco.

The only disadvantage of this method appears when you run more than about 30 or 35 locos on the layout at one time. Because the command station keeps track of which locos are MUed together, the command station must send new command packets to each loco in the consist any time there is a speed or direction change. This is no big deal until you get to around 30 or 35 locos running at one time - especially if you're operating sound, an analog loco, or have dirty track.

With the command station sending command packets to 30 or 35 locos, command packets can be slightly delayed. Under non-sound conditions, the slight delays wouldn't be noticed. But with sound, the horn might not start blowing until a split second after you press the button. If just blowing the horn is all you're going to do, it's no big deal - a slight delay is of no concern. But if trying to blow the grade crossing tune, for example, it can be frustrating. Again, for most people this disadvantage is no big deal because most people don't have a layout large enough to run 30 or 35 locos at one time.

With Decoder-Assisted Consisting, the decoder keeps track of which consist the decoder belongs to. For a decoder with this feature to work seamlessly, you must also have a system that is capable of Decoder-Assisted Consisting. 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 decoders are capable of DAC and which aren't. If you try to consist a non-DAC decoder with DAC, it just won't work.

With Digitrax's Chief and Empire Builder, the consisting process is the same for both ways. If you have the decoder address status edited for DAC, the system will automatically consist those with DAC that are capable of it.

Status Editing is the process of telling the command station the details about how to use the decoder. For most people using Digitrax, Status Editing 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. This is normally done right after programming the decoder address - when you still have the decoder's capabilities fresh in your mind.

Once Status-Edited, you don't have to do that again - unless you want to change it. The system will remember which addresses are to use Decoder-Assisted Consisting and automatically use it when you MU those locos to a consist.

Just because a decoder is capable of DAC doesn't mean you have to use DAC. As previously stated, Digitrax Command Stations are fully capable of handling most people's consisting needs, and therefore it's not important. However, DAC is something that may be important if you plan to do a lot of consisting on a system that has limited Command Station Consisting.

If your system isn't capable of DAC, but has the capability of programming CV19 in OPS-Mode, you can still use DAC for consisting if your decoders have this feature. You'll just have to do it manually by programming the consist address into CV19 when consisting time comes. If you want the consisted loco to run backwards, you simply add 128 to the consist address and program that into CV19 instead of the address.

However, because CV19 is limited to two-digit addresses, you cannot use a four-digit address for the consist. Digitrax's Command Station Consisting allows the use of four-digit addresses mixed with two-digit addresses and even an analog loco.

1) As mentioned before, if you're running more than about 30 or 35 locos at one time, consisting with DAC will lessen the command station's command packet load. Instead of having to send command packets to every loco in the consist, it sends command packets to the consist address for all locos to respond to. This allows the command station to send out more command packets per second to each active address.

2) With DAC, a consist stays consisted even when you take it to another layout. Because the decoder keeps track of the consist, the command station doesn't even need to know that it is consisted. Select the consist address and the system will start sending command packets to it. Since all locos respond to the consist address, all locos respond. This eliminates the need to re-consist the consist when taking it to other DCC layouts.

3) Some DAC decoders allow for Function Control of consisted locos. Normally, only the lead loco of a consist has function (light) control. 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. This can be very important when using reasons 4) or 5) below.

4) Decoder-Assisted Consisting can be used as a temporary address or base direction setting, even if your system doesn't support DAC. When programming a two-digit address into CV19, the decoder will ignore the normal base address in favor of the address in CV19. This can be useful when taking your loco to a club or friend's layout where your normal base address is already taken. If you add 128 (decimal) to that address, it will reverse the Base Direction. This can be useful as a form of basic MUing where you want the loco MUed tail-to-tail or head-to-head. But to also have Function Control of the lights with the CV19 address, CV22 (if available) also has to be programmed, and CV21 (if available) allows other Function Control for the CV19 address - refer to DAC Function Control for more information. To revert back to the normal base address and direction, simply program "00" into CV19. You can leave the programming in CVs 21 and 22 for the next time.

5) Another use is getting around a programming problem when installing a DSX (SoundTraxx decoder) in a loco along with another decoder to run the motor and lights. Some people install them with a low cost decoder for no other reason but to save a few dollars. What they don't realize is that they are setting themselves up for some unnecessary hassles. Fortunately, DAC can help with one of the disadvantages, but not with all of them.

To do DAC, the decoder must have that feature. Your system must also support that feature to do it automatically, or at least have the capability to program CV19 to do it manually.

All decoders carried by Loy's Toys have Decoder-Assisted Consisting.

Train Control Systems, NCE, and Throttle Up! SoundTraxx decoders have the DAC Function Control feature. Digitrax decoders do not.

Digitrax's Zephyr, Empire Builder and Chief systems support Decoder-Assisted Consisting. They can also program the DAC CV manually.

This topic may have been somewhat complex for some beginners. As you can tell, there are several ways of doing things with DCC. Which way you do it depends on your equipment and desires. That's the way the more versatile DCC systems and decoders are. With lesser systems and decoders that provide fewer options, the complexity is lessened because they don't provide some of these high-end options. But just because a decoder or system has a high-end feature doesn't mean you have to use it. If you're not comfortable with it, just don't use it and the complexity goes away. Trust me, as you get into DCC and begin to understand it a little more about how things work together, this topic won't be so intimidating. Refer to System Complexity for more information about that.

Decoder-Assisted Consisting is not intrusive. There is nothing to know about it unless you want to use it.