LAMRS Equipment, Speed Matching Issues for 28 Step Decoders

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Speed Matching Issues for 28 Step Decoders

The club has a lot of older 28 step decoders in locos and they have been causing problems. Most older (year 2000 and older) decoders use 28 speed steps, although they will respond to 128 speed step commands. These decoders are Digitrax decoders in the "0" and "1" series, NCE decoders that are not silent running, MRC decoders and Bachmann EZ-Command decoders all have the same general issues.

The issue is that when commanded in 128 step mode, they will run, but much of their functionality is disabled. In 128 step mode, they run as "direct drive" decoders. They run 0 to max over the throttle range of 0 to 99. Any momentum, Vstart, Vmid, and Vmax settings along with any 28 step speed tables are simply ignored. To speed match them at all requires that the Digitrax throttle be "status edited" into 28 step mode.

To Status Edit an address on a Digitrax throttle, dial up the loco number as usual, but BEFORE pressing the Loco button the 2nd time, press Edit instead (lower panel, on the left). Then twist the right throttle knob through the seven possible settings and pick 14, 28 or 128 as appropriate. The others are for Marklin decoders and such. THEN press the loco button. That address will be recorded into the slot table with the notation that it is a 28 step decoder (or whatever was selected).

Status Editing impacts the address of that loco. Once the status is edited, it stays in the command station slot table until that entry is purged somehow. Status Editing impacts the way that the command station formats commands to the decoder. The decoder recognizes these formats and operates in the way that it was designed to operate. This can vary from decoder to decoder, but some general characteristics seem to exist across all the impacted decoder types.

The problem is that the Digitrax throttle has an encoder that has 100 settings, 0 through 99. Somehow, these have to be translated into 28 individual steps. NCE does it by making the speed display go from 0 to 28. Digitrax does it by making the throttle go from 0 to 99, generally with a 3% step per tick. Throttle settings of 3, 6 and 9 are typically ignored. The decoder won't make any power at all until the throttle reaches 12 or sometimes 15. Further, at a few speeds, the throttle will advance by 4% per tick so that the 28th speed step occurs at 99 on the throttle.

There is a problem when trying to speed match a loco with a 28 step decoder to one with a 128 step decoder. The club sets up the locos to just start to crawl at speed 1 on the throttle. This is done through the use of BEMF in the decoder or by setting Vstart and the Kick Start CV's such that the loco just starts to move on the first click. A 28 speed step decoder will usually not move until the throttle reads 12 no matter how Vstart is set. Vstart simply sets the motor voltage AT speed step 12. This means that a consist of locos with mixed decoders will simply drag the one(s) with 28 step decoders until a speed of 12 is dialed up.

SOME decoders, like the Bachmann ones, will act this way until some momentum is programmed. Even though they are 28 step decoders inside, they will interpret the lower speeds and start to move. However, Bachmann decoders don't have a Vmax or speed table so that it is not really practical to speed match them anyway.

A way around it is to run EVERYTHING in 28 step mode. This can be done two ways. The first way is that each user must make sure that the address is status edited to 28 step mode before the loco is selected. While technically this would work, in practice it would not. The extra steps needed to force 28 step mode would just be skipped and then the speed matching would not work again.

The second way is that the Digitrax command station can be set to force 28 step mode with option switch 22. If we did that, we'd probably have to rematch every loco on the layout.

Overall, I think that the best option is to let the old 28 step decoders go, which we have mostly done anyway, and use only more modern 128 step decoders.

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© 2010 George Schreyer
Created 2 Apr 2009
Last Updated December 5, 2010