Measurement Computing Data Acquisition Systems rev.10.4 - Manual (Page 89)

Setting the Scan Rate
The daqAdcSetRate function sets the scan rate interval for a channel scan group. The scan rate is set by
the reqValue parameter. Whether the value is given in terms of frequency or period, the requested scan rate
is programmed into the built-in pacer clock as a scan interval timer. Once set, this scan interval timer will fire
at the requested rate to initiate the scanning of the channel scan group.
Scan Period Scan Period
Scan 1 Scan 2 Scan 3
When the scan interval timer fires, the first channel in the channel scan group is sampled. The subsequent
channels in the scan group are then sampled at the specified sampling interval for the device. The sampling of
the individual channels continues until the last channel in the channel scan group has been sampled. The
period between the sampling of the last channel in the channel scan group and the next firing of the scan
interval timer is “dead time”, where no channel sampling is performed.
Scan 1 Scan Period Scan 2
Sampling Interval Sampling Interval Dead Time
Ch0 Ch1 Ch2
This function does not set the sampling interval between individual channels within the
scan group. Channel sampling interval (if programmable) can be set globally by the
daqSetOption
function, or individually by the flags parameter of the scan-setting
API commands (see daqAdcSetScan).
Scan Rate Mode
The mode parameter allows the setting of the scan rate in either period or frequency. The possible values for
this parameter are as follows:
DarmPeriod – This value defines the requested scan rate to be in nanoseconds. In this case, the reqValue
parameter will be interpreted as the interval between channel scans in nanoseconds.
DarmFrequency – This value defines the requested scan rate to be in frequency. In this case, the
reqValue parameter will be interpreted as the frequency of the scan rate in hertz (or channel scans per
second).
DarmExtClockPacer -- Sets external pacer clock divisor. Here reqValue defines the pacer clock divisor
value (1 to 255) when the clock source is defined as external (by setting DacsExternalTTL in the
daqAdcSetClockSource
function). When used as external clock divisor, one scan will be initiated every
reqValue pulses on the external clock input.
The DarmExtClockPacer parameter value can only be used with the WaveBook/516.
Programmer’s Manual 908594 Daq API Command Reference 4.2-33