Measurement Computing WaveBook rev.3.0 - Manual (Page 137)

WaveBook User’s Manual. 6-24-99 Theory of Operation 8-3
Specifying the Trigger Source
The trigger is a recorded event to correlate the acquisition with other processes. The trigger may be
used to start the acquisition or to serve as a marker within the acquisition. Every acquisition must be
triggered. If there is no valid external trigger signal, the acquisition can be triggered by the PC.
WaveBook supports several trigger sources: a software trigger from the PC, the channel 1 input level, the
TTL-trigger input, or a combination of analog input samples.
• The daqAdcSetTrig command is used to choose among the first 3 trigger sources.
• The daqAdcCalcTrig and daqAdcSetTrigComplex are used to select the combination of
analog input samples as the trigger source. This "multi-channel trigger" capability and others are
described later.
• The daqAdcSoftTrig command generates a software trigger. The software trigger takes effect
regardless of what trigger source has been selected and so can be used to force a trigger if the desired
trigger has not arrived.
Specifying the Number of Scans
During an acquisition, several groups of scans are acquired, depending on the selected acquisition mode.
Acquisition modes include:
• In the N-shot mode, a single, fixed-length group of scans is collected after the first trigger.
• In the N-shot with re-arm mode, a single, fixed-length group of scans is collected after each trigger
until the acquisition is stopped.
• In the infinite post-trigger mode, a single indefinitely long group of scans is collected after the trigger
until the acquisition is stopped.
• In the pre/post-trigger mode, three groups of scans are collected: a fixed-length pre-arm group, an
indefinitely long pre-trigger group, and a fixed-length post-trigger group.
The daqAdcSetAcq command specifies the acquisition mode and the length of the fixed-length groups.
In the N-shot mode, the length specifies the number of scans to acquire after the trigger. In the infinite
mode, no length need be specified—the acquisition continues until explicitly stopped. And in the pre/post-
trigger mode, both the pre-arm and post-trigger lengths must be specified. The acquisition modes are further
described later. Note that in the pre/post-trigger mode, the pre-arm group length (also called the pre-trigger
length) is the guaranteed number of scans that will be collected before the trigger.
Specifying the Scan Rate
During a scan, the WaveBook/512 series takes samples once every microsecond (1µs). The minimum scan
period (the fastest scan rate) is 1µs times the number of channels. The maximum scan period (the slowest
scan rate) is 100 seconds per scan. In the pre/post-trigger acquisition mode, two different scan rates may be
specified, one for before the trigger and another for after the trigger.
During a scan, WaveBook/516 series takes samples at one of three possible rates: once every microsecond
(1 µs), once every 1.05 µs, or once every 1.1 µs. The minimum scan period (the fastest scan rate) is 1 µs
times the number of channels. In the pre/post-trigger acquisition mode, two different scan rates may be
specified, one for before the trigger and another for after the trigger.
The daqAdcSetRate and dacAdcSetFreq commands are used to set the scan rate(s) either in
nanoseconds (ns) per scan, or scans per second, respectively. The scan rate may be set within its limits to
50 ns resolution.
The daqAdcGetFreq command retrieves the current actual settings of the scan rate. This may be
different from those specified with daqAdcSetRate and daqAdcSetFreq because they include the
effects of the trigger source, scan composition, and a 50 ns timebase resolution.