Measurement Computing StrainBook/616 - Manual (Page 95)

External Clock and Counter-Timer
The Internal and External buttons located in the rate section of the screen (previous and following figures)
are used to select Internal Clock or External Clock.
The external clock is useful when data collection depends on rotational speed or axial position. By
synchronizing the system with an external event for correlation of data, you can collect event-dependent
data instead of time-dependent data.
Make sure to adhere to the minimum slew rate requirement of 20 v/µs.
StrainBook can receive external clock input through pin 20 of the DB25 connector labeled DIGITAL I/O,
EXTERNAL CLOCK, TTL TRIGGER. This enables data scanning to be correlated with an external pulse
train.
To enable the external clock select for the Scanning Rate in the Acquisition Configuration
Dialog Box
(see following screen shot).
With the external clock enabled StrainBook will begin a scan after a rising edge on the TTL level occurs.
Optionally, the external clock may be divided by a factor in the range of 1 to 255. This “pre-scaling”
allows the user to select a reduced scan rate.
Note: The Clock Divider
can be set to a value
from 1 to 255.
Acquisition Configuration Dialog Box with External Clock Enabled
StrainBooks have a 32-bit internal counter that calculates and reports the external clock’s period. The
counter can be read with each scan of the analog data. This is often beneficial in later analysis when there
is a need to correlate physical phenomena with speed.
The counter channel actually consists of two independent channels (CtrLo and CtrHi). These can be
turned “On” in the Channel Configuration Spreadsheet. When enabled the low (CtrLo), then high (CtrHi)
words of the counter will be configured in each scan. Note that the spreadsheet’s Units column can be
used to view a predefined period in units of seconds, ms, or µsec.
WaveView can be configured to read only the low word of the counter data
(CtrLo:“On,” CtrHi: “Off”). This decreases the minimum scan period by 1 usec.
The LoCtr only option can be used only when the external clock frequency is greater than
305 Hz (20,000,000 MHz / 65536]. However, WaveView does not enforce this.
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