Measurement Computing WaveBook rev.3.0 - Manual (Page 143)

WaveBook User’s Manual. 6-24-99 Theory of Operation 8-9
Pre/Post-Trigger Acquisition Mode
Unlike the preceding modes, the pre/post-trigger mode acquires samples both before and after the trigger
(the other modes only take samples after the trigger).
Start
No acquisitions
before start
Pre-Trigger
Scan Period
Pre-Trigger Scan Count
Pre/Post-Trigger Acquisition
Time
Trigger
Armed
Trigger
Pre-Trigger
Scan Period
Post-Trigger
Scan Period
Post-Trigger Scan Count
When the acquisition starts, WaveBook begins taking scans with the trigger disabled. WaveBook
guarantees that at least a certain number of scans will be collected before the trigger is recognized. This
number is the "pre-trigger scan-count". After the pre-trigger scan-count has been collected, WaveBook then
arms the trigger. WaveBook continues scanning at the pre-trigger scan rate until the trigger occurs. When
the trigger does occur, WaveBook completes the current pre-trigger scan and then begins scanning at the
post-trigger scan rate until the post-trigger number of scans has been collected.
The collected scans can be divided into 3 groups: Pre-arm scans, post-arm scans, and post-trigger scans.
The pre-arm and post-trigger groups are each individually set to a fixed length from 1 to 100,000,000 scans.
The post-arm group (scans after WaveBook is armed but before the trigger is detected) may contain any
number of scans, depending on how long it takes before the trigger. Thus, the minimum number of scans
collected in this mode is the sum of the pre-trigger and post-trigger scan counts; but the maximum number
is unlimited. Circular data buffers are often used to capture this type of data.
Scan Composition
Every acquisition collects one or more identical scans of the input signals. Each scan consists of from 1 to
128 samples collected at a rate of 1 microsecond per sample. Each sample of a scan specifies the channel to
be acquired and the gain and offset to be applied to the signal. The daqAdcSetMux and
daqAdcSetScan
commands are used to specify the scan composition.
In general, the channel number, gain, and offset may be specified arbitrarily for each sample in the scan, but
there are some restrictions:
• The digital input port (channel 0), if read, must be the first channel of the scan.
• The first channel of the scan may not be a channel equipped with a WBK11 (simultaneous
sample/hold) option. If an SSH channel is the first channel of the scan, the software will automatically
insert a dummy channel (whose result is discarded) as the actual first channel of the scan, thus limiting
the maximum scan length to 127 samples.
• Every reading of a given SSH channel must be at the same gain because the WBK11 cannot change
gain between multiple readings of the same channel. Different SSH channels may be sampled with
different gains, and a single SSH channel may be read with both unipolar and bipolar offsets, but all
samples of a single SSH channel must be made at the same gain.
Scan Period
The scan period is the time from the first channel of one scan until the first channel of the next scan.
WaveBook supports two different scan periods: the pre-trigger scan period and the post-trigger scan period.
Because WaveBook samples at a rate of 1 MHz, the absolute minimum scan period is just 1 µs times the
number of samples in the scan. However, the actual minimum period may be more:
• If the first channel in the scan is a SSH channel (with a WBK11 installed), then a "dummy" channel is
added to the scan adding 1 µs to the minimum scan period.
• Multi-channel triggering increases the minimum pre-trigger scan period by 1 µs plus 1 µs for each
trigger sample. However, multi-channel triggering does not affect the minimum post-trigger scan
period.
The actual scan period may be any value from the minimum scan period to a maximum of 100 seconds, and
can be set with 50 ns (0.05 µs) resolution.
Note: The pre-trigger and post-trigger scan periods may be set to any valid value from their respective
minimum (which differs if multi-channel triggering is used) to their maximum of 100 seconds.