What will the locomotive start-up acceleration look like with a high-res step loadable speed table programmed into the decoder?
DCC is new to me. Specifically, this particular question pertains to a Digitrax Super Chief Xtra, three Broadway Limited Blueline loco each with Digitrax DN146 decoder.
To me initial acceleration/movement from a complete stop seems kind of abrupt. In other words, when I turn the throttle just one step, the train moves (IMO) too quickly to minimum speed. Will using a high-res loadable speed table allow a much slower initial startup acceleration or is this just a waste of time? (lots of programming)
Also, what issues do I need to be aware of if I want to MU these three locos together?
Not initially. The first things you should do are to check CV2 and make sure it's 0 or 1 (CV2 is the start voltage, the higher it is, the faster the loco will move on step 1) and make sure the loco is working propely mechanically - no binding in the gears or drivetrain, no side rod bind on a steam loco, etc. The DN146 decoder has back-EMF motor control - if you look on the Digitrax Tech Support Depot there are several articles on how to tune it for best performance. These are what to look for for best startup speed. Assuming all is adjusted as best as can be, a speed table will do nothing to enhance slow speed operation. If everything is set as best as can be, the loco that runs the fastes on speed step 1 is your benchmark for smeed matching - a slower loco you can make faster, but you can;t make the faster one any slower (again assuming you've eliminated mechanical issues and have adjusted the decoder's motor control)
At full throttle, the situation is reversed. Assuming you currently have no limit on top speed (CV5 being either 0 or 255), the slowest loco at full throttle is your benchmark. You can adjust a faster one to run slower at full throttle, by putting a value less than 255 in CV5, but the slowest one is already going as fast as it can.
Once you have the slow speed and top speed adjusted, you can alter the middle range (50 on the Digitrax throttle, since it goes 0-100) using CV6. Not all decoders support CV6, but the DN146 does.
This is normally more the sufficient for speed matching locos to run together, except when dealing with a decoder that doesn't support CV5 and 6. Then you will have to resort to speed tables. Using JMRI and either the PR3 or Locobuffer USB with your system will make that a whole lot easier.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Is a "hi-res" speed table something with more than 128 steps or one that just goes to 128 steps? Or is this some Digitrax only feature?
Mike Lehman
Urbana, IL
mlehman Is a "hi-res" speed table something with more than 128 steps or one that just goes to 128 steps? Or is this some Digitrax only feature?
Not Sure. The Digitrax Mobile & Sound Decoder Manual (second edition) describes it in section 8.3/pg43 ~ High Res 28 Step Loadable Speed Tables CV65-95. "When speed table is enabled and 128 speed step information is recieved from the command station, the table is interpolated to generate 4 in-between steps give full 128 step resolution."
4 times 28 doesn't equal 128. And I have 128 step resolution enabled. So I am confused.
But as Randy pointed out, this may indeed not be the issue. I'm not looking to change the "feel" of the throttle right now. In my opinion the loco(s) should start moving much slower at the first "notch" on the throttle than what they are currently doing.
I might also add that in order to "turn-on" the loco(s) sound after track power is applied I have to turn the throttle "one notch" to enable sound, and then of course the loco starts moving and I have to back it off to "zero". Then when I turn the throttle up one notch again the train speed starts too abruptly (IMO).
SOme decoders, the 28 step speed table only works in 28 spped step mode. I guess Digitrax is one of the few ehere the default out of the box is to use 128 speed steps - a lot of systems default to 28 and can be set to 128, Digitrax is 128 by default and you can set a specific loco to 28 if for some reason you like lower resolution speed control.
Digitrax decoders will still use the 28 step speed table even when driven with 128 speed steps. They calculate the in-between values so if in your 28 step speed table you have entry 16 set to 80 and entry 17 set to 90, it will use values between 80 and 90 to set the speed for the in-between speed steps, since there will be 4 steps (in 128 mode) between the two 28 step settings. It acts like there are 128 entries in the speed table, so it would go something like 80,82,84,86,88,90 by calculating the in between values.
No big deal. Other than experimenting with the feature when I was new to DCC, I have never set a speed table in any of my locos. I only use decoders that support the 3 step option with CV2-6-5, and I will set CV5 to limit top speeds to prototypical ranges, and sometimes adjust CV6 to get the overal speed range I want - more in the low end for a freight loco or switcher, more in the high end for a passenger loco. The effect of CV6 mid speed is to adjust the throttle response, say CV5 is set to 200 to limit top speed. Setting CV6 to 100 (or leaving it blank) results in a normal linear throttle response fro stop to full speed, just a slightly reduced top speed. Make CV6 150, and you have a lot more of the throttle in the lower speeds, and in the top 1/4 of the throttle, the loco will accelerate more per speed step than it does in the lower 3/4. Until you get past 3/4 throttle, the loco will be moving realtively slowly, then quickly accelerate in the last 1/4. Set CV6 to 50, and the lower 1/4 of the throttle will have a greate speed change per step, witht he loco moving slowly, but then once you get past 1/4 throttle it will start accelerating, but with a lesser change per speed step. The former is for a loco that spends most of its time running slower or pulling heavy trains, the latter is for a loco that accelerates rather quickly from a station stop and spends most of its time running at high speed.
Randy and Arto,
Thanks for the explanation. All I ever see in JMRI is 28 steps. Most of my decoders are NCEs, with some Tsunami steam. NCE indicates it automatically interpolates the intermediate steps, as does the Tsunami. But there's nothing obvious to indicates a way to manually alter them to manipulate the values in between the 28 steps typically available for custom user adjustments.
The mechnical, etc variation in my motive power is wide enough that I depend on custom speed tables to get them to play together happily.
Yes, that's all there ever is - 28 entries in the table. It's how the decoder uses them that varies, some will only respond to them if driven in 28 step mode, others like Digitrax do interpolation so the table works even when using 128 actual speed steps from the throttle side (which is really like 126 steps since a few are reserved).