BEMF in consists
Let’s finish off the week with another consisting post. In my July 2016 DCC Corner column I laid out the basic pros and cons of the various consisting methods and then in the September 2016 column I went over a specific example with my SW1 consist. As I pointed out BEMF can create some problems with speed matching. In some cases BEMF can be disabled or dialed down to a minimum. However in some decoders like the TCS WOWSound it cannot be changed.
Having a feature with this much variability among decoder brands can make speed matching a challenge. It really requires some experimentation to gain enough familiarity with the various decoders you own to actually make predictable modifications. Also turning BEMF off isn’t always the best solution either as it may greatly alter operational characteristics. Probably, your best bet is to plan your DCC decoder installations so that locomotives you plan to operate in consists have the same types of decoders. Consisting several different kinds of decoders with radically different BEMF setups can be a challenge, especially in cases where interactions are poorly documented.
One example David Reese and I ran into was with the Digitrax SDXH166D sound decoder I installed in the Atlas RS1 back in the November 2015 DCC Corner column and the WOWSound Atlas RS3s from the June 2016 column. After tweaking the settings to get the locomotives to individually run at essentially the same speeds, they seemed to work quite well when using universal consisting. However as soon as we entered an address in CV19 for advanced consisting, the Digitrax equipped locomotive would hardly move even though the WOWSound equipped locomotives behaved the same as in universal mode.
After a bunch of research I found out that with the factory default setting, as soon as you enter an advanced consist address, BEMF is disabled and that in turn causes the speed to drop to about a tenth of normal. The fix was pretty easy, just enter a value in CV57 that keeps BEMF turned on in advanced mode. In the Digitrax decoder technical manual there is a table giving values for CV57 that will provide varying degrees of BEMF in both normal operations and advanced consists. In this case a value of 85 provided a good speed match between the RS1 and RS3s.
The problem with a lot of this stuff is finding what you need to know. I had to really dig through the Digitrax literature to find out about CV57. And David had to turn to the LokSound YahooGroup for his answer about getting his locos in sync with respect to how they respond to F8. As manufacturers cram more and more features into decoders it is becoming more difficult to keep track of undocumented or poorly documented features making it even more difficult to mix different brands of decoders in consists. The good thing about DecoderPro is many of these hidden features are right there waiting for us to discover them. However you have to have the right map to know what to look for.