| With
DCC you can program your own speed curve if desired. Forward and Reverse
Trims are used to adjust the top speed of the User-Loadable
Speed Table without having to redo the whole thing. Forward and
Reverse Trim can also be used with Throttle Up! SoundTraxx decoders'
optional built-in speed tables.
Without these trims, one would have
to completely redo the entire speed table if the Loco's top speed
didn't come out right. Besides providing capability to adjust for
mistakes and loco running changes over time, these trims have other
uses:
Some locos do not run at the same
speed in reverse as in forward. This is usually caused by the fact
that the drive train surfaces that are used when going forward are
more worn than the surfaces used when going in reverse.
While most prototypical diesels run
the same speed in reverse as in forward, making them ambidextrous,
some don't. If your model is one that is not supposed to go the same
speed in reverse as in forward, this can be used to make that happen.
CV 66 is used for forward trim and
CV95 for reverse trim. This allows you to set the exact top speed
for the loco in forward and reverse independently. As previously mentioned,
some locos don't run the same speed in reverse as in forward. This
allows you to correct for that.
These CVs work a little differently
than most, in two ways: Programming most CVs with a zero disables
the feature. These two CVs have two "disabled" values: zero
and 128 (80 hex). Zero is basically the default "disabled"
value. 128 provides a 1 to 1 ratio of top speed, which essentially
is no change.
Next, most CVs have a range where
you start at 1 for the least effect and increase for more effect.
Some start at 1 for the most effect and increase for less effect.
These two CVs start at 128 for no effect (the same as zero for most
CVs). Adjustments are made by increasing or decreasing from 128.
For each increment below 128, the
top speed is reduced roughly 1 percent. For each increment above 128,
the top speed is increased by roughly 1 percent. The actual amount
is calculated as n/128 - that is, the value divided by 128 yields
the percentage of change. For example, entering 129 yields a 0.7%
increase, whereas 127 yields 99.2% of the power expressed in the speed
table - about a 0.7% decrease.
The easiest way to use these trims
is trial and error, rather than trying to calculate what you need.
If the loco's top speed is too fast, reduce the value. If the top
speed is too slow, increase the value. However, be aware that you
can't increase the top speed beyond what the loco is capable of doing.
Once the loco is going as fast as its design allows it to go at full
voltage, adjusting the trim faster will not make the loco go faster
than it can go, but it will attain that top speed sooner.
For example, let's take a loco that
can attain a top scale speed of 100 MPH. A User-Loadable
Speed Table has been entered that will make the loco go 90 MPH
at top throttle. Programming the trim with a value of 142 will now
make that loco reach 100 MPH at the top throttle setting (speed step
128). If the loco could go faster, programming a value of 157 would
make it go about 110 MPH. But since the loco can only go 100 MPH,
that's as fast as it will go but, it will reach that 100 MPH at a
lower speed step.
With the trim set at 142 so the loco
will achieve 100 MPH at speed step 128, it probably would be doing
about 89 MPH at speed step 115. But with the trim set for 157 to
try for a top speed of 110 MPH, it would achieve 100 MPH at speed step 115, and continue at that same speed all the way up through
speed step 128. The decoder is doing what it's supposed to do to
achieve 110 MPH, but the loco just can't go that fast with the available
voltage. |
