Measurement Computing WavePort rev.3.0 - Manual (Page 43)

WavePort User’s Manual 09-29-00 Hardware and Operation Reference 4-15
External Clock and Counter-Timer
WavePort is capable of receiving external clock input. 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.
WaveBook/516 can receive an 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 “External” for the Scanning Rate in the
Acquisition Configuration Dialog Box (see following screen shot). When the external clock is enabled,
WaveBook/516 begins a scan only after a rising edge on the TTL level occurs. Optionally, the external
clock may be divided [by a factor of 1 to 255]. This “pre-scaling” allows the user to select a reduced scan
rate.
Acquisition Configuration Dialog Box with External Clock Enabled
Note: The Clock Divider can be set from 1 to 255
The WAVEBOOK module has 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. This LoCtr only option can be used only when the
external clock frequency is greater than 305 Hz (20,000,000 MHz / 65536]. Note that WaveView does not
enforce this.