What is a
speed step?
That is the
way that the
command
station tells
the locomo-
tive where
you have set
the throttle
controlling
a particular
loco.
To make
things easy,
consider how
it would work
if there were
10 speed
steps. As soon as you move the throttle off zero, the locomotive
would be told to run at 10% of top speed. As you advanced the
throttle, nothing would happen until you reached about 15% of
the total throttle range. Then the loco would be told to run at 20%,
until you turned the throttle up past about 25% of the range. So,
the loco would immediately try to jump from 10% to 20% of top
speed, its acceleration limited only by the momentum setting in
the decoder, the loco's flywheel and the load of whatever cars it is
pulling. Not very realistic.
From Mr. DCC’s workbench ...
SPEED STEPS IN PERSPECTIVE
11: Diesel control stand on an EMD MRS-1.
12
DCC Impulses Column - 11
Folks have said, "Well, locos have 8 notches, so why to I need more
than 8 speed steps?" It is true that the diesel motor may only run
at 8 different speeds, but there is a throttle that controls howmuch
of the power being generated is directed to the drive motors.
When you are running a prototype diesel, the throttle determines
speed with infinite variability. The motor (and its associated sound)
does, indeed, have 8 distinct steps. By the way, steam locomotives
are infinitely variable, too. So, the more speed steps you have, the
more accurately you can reproduce the nuances of loco speed.
Way back in time, when DCC was new, there were several designs
of speed steps. Quickly, 14 speed steps became the norm. Then
came 28 and 128. Folks pretty much decided that the less than
1% difference available from 128 speed steps was enough, so the
standard stopped there. DCC command stations must be able to
speak all three versions (14, 28 and 128) to accommodate decod-
ers of all vintages.
Inside modern decoders there are two options: 14 and 28 speed
steps, adjusted by setting CV 29. What happened to 128? Well,
decoders are kind of bilingual. If they are set to 28, they can under-
stand the command station if it speaks 28 or 128.
"So, with all these choices, what should I do?” is the question on
your lips. That's your decision, but here's what I do. Firstly, forget
14 speed steps. There are all sorts of issues that are tied into the
speed control with that language. So, ALWAYS, set your decoder
to 28 (some call it 28/128, for reasons that you now understand).
Then you can go between 28 and 128 by only changing your
command station. I always use 128 speed steps when setting up
MRH-Dec 2013
