Simplex 4100-0136 Installation Instructions Manual page 7

How the Decoder Works,
Coder Timing and
Rules
Leading Edge
Detection
Continued
One complete round of a code is shown in Figure 1. The various parts of the code are labeled, and
the code is identified as 2-3-2.
Figure 2. ON/OFF Pulse Train Code Round
There is a second type of pulse train that is prevalent with coded initiating devices that does not
correspond to the simple "ON/OFF" type. This pulse train system is used with single stroke
devices, and therefore does not have the "OFF" times required to determine the difference
between pulses, digits, and rounds.
The determining factor with this pulse train system is the time between leading edge transitions
from low to high. The pulse train in this equipment goes high with the first code digit pulse, and
stays high until 200 mS before the next sounding. The positive transition results in the sounding of
the alarm bell. A "leading edge" pulse train corresponding to the code 2-3-2 is shown in Figure 2.
Figure 3. Leading Edge Pulse Train Code Round
The way to decode this pulse train is to measure the time between the leading edges (or positive
transitions) of pulses in this train. After the final sounding of the alarm bell, the equipment
holds the line high for two to three seconds, and then returns the line to its "normal" low state. The
decoder module uses this leading edge detection method.
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