Measurement Computing ZonicBook 618E rev.3.4 - Manual (Page 69)

Tach Channel Noise Rejection (Debounce)
A noise rejection (debounce) circuit is used to eliminate noise associated with tachometers. ZonicBook/618E’s
debounce circuit makes use of a delay time in the range of 500 ns to 25.5 ms, or no delay time at all. Related
configuration is made via eZ-Analyst or eZ-TOMAS software.
A few terms related to noise rejection, and which are used in eZ-Analyst and eZ-TOMAS, are:
Edge Detect – Edge Detect is short for “Tach Pulse Edge Detection.” The term pertains to detecting the rising
or falling edge of a tachometer pulse using either an “Immediate” or “Delayed” mode.
Immediate Edge Detection (“Before Stable”) – “Immediate” is one of two Edge Detect modes. The term is
synonymous with “Before Stable.” If “Immediate” edge detection is selected a tach pulse will be recognized on
any instantaneous measurement that meets the trigger level criteria. In this setup the “Delay Value” is the amount of
time before the next tach pulse can be recognized. If a keyway is being used as a tach trigger you must calculate the
minimum pulse width, based on the maximum RPM rate expected.
Delayed Edge Detection (“After Stable”) – “Delayed” is one of two Edge Detect modes. The term is
synonymous with “After Stable.” If “Delayed” edge detection is selected a tach pulse will be recognized when the
measurement meets the trigger criteria for the specified duration. In this setup the “Delay Value” defines the length
of time that the trigger criteria must be stable before the tach pulse will be recognized.
From the following illustration we can see that there are two debounce modes, as well as a debounce bypass. In
addition, the signal from the comparator can be inverted before it enters the debounce circuitry. The inverter is used
to make the input rising-edge or falling-edge sensitive.
Edge selection is available with or without debounce. In this case the debounce time setting is ignored and the input
signal goes straight from the inverter [or inverter bypass] to the counter module.
There are 16 different debounce times. In either debounce mode, the debounce time selected determines how fast
the signal can change and still be recognized.
Debounce Model
ZonicBook/618E 917695 Tach Channels 7-5