similar to the TSU-1000. The Micro is smaller and only has two
functions, while providing less output power. You may wish
to refer to figure 4 as you read this column, to keep in mind
where things are connected outside the decoder.
The Power Supply
The most basic of needs in the decoder is to change the DCC signal
from the track into DC power to run the decoder and to drive the
motor and lights connected to it. The blue box in (3) depicts this.
Figure 3 shows the internal connection of the positive voltage to
other circuitry within the decoder. The negative voltage connec-
tions are not shown, but still exist.
The red and black track leads bring the DCC track voltage in.
The positive voltage from the power supply comes out of the
decoder on the blue lead. Some decoders bring the power supply
negative out on a wire for keep-alive circuitry or other uses.
SoundTraxx brings the negative out of their Tsunami-1000 series
decoders on a black wire that is wired to the external capacitor –
different than the black wire connected to the track.
In the Micro Tsunami (TSU-750) series decoders, the negative wire
is green-with-yellow stripe – a much less confusing color than hav-
ing two black wires that have different uses on the same decoder.
TCS brings out the negative wire on most of their decoders with a
black-with-white- stripe wire, per NMRA Recommended Practices,
RP-9.1.1.
I’ll discuss the diode shown in the positive lead in figure 3 later in
this column. To my knowledge, it only exists on the Tsunami TSU-
1000 series decoders.
DCC Impulses Column - 3
Some open board decoders provide a power supply negative
contact point on the board.
DCC Detector
The green boxes in figure 3 represent the circuitry that handles
the DCC data inside the decoder.
The DCC Detector looks at the DCC pulses on the track and
translates their timing into a stream of data pulses for the
“brains” of the decoder to use. The next few paragraphs and
figures will explain how different detector schemes work. If you
don’t care, jump ahead a couple of pages to the next section,
about the microprocessor.
The NMRA standards are that pulses that are nominally 55 micro-
seconds (µS) apart represent a data one. Zeros are indicated by
pulses more than 110 µS apart. So, to identify the pulses, the
detector needs to decide when a transition has occurred.
The most common method is to set a threshold (say 6 volts) and
presume that when the voltage goes above or below that level, a
transition has occurred. The disadvantage of this method is that
dirt on the track and other discontinuities may drop the DCC
signal level enough that transitions are missed. If that happens,
instead of a group of ones and zeros, the detector decides that
there was a long zero. Remember, zeroes are deemed to be any-
thing longer than 110 µS without a transition (5).
About 10 years ago, Lenz released their Gold series decod-
ers that used a differentiator to determine when a transition
occurred. In English, this means that, instead of looking at the
level of the DCC signal, the detector looks at the fact that there
is a change (difference) of voltage. Running the DCC signal (5)
through a capacitor creates the signal in figure 6. So why bother?
MRH-Feb 2013
