VTI Instruments EX1629, Руководство пользователя

Страница: 64 / 346

VTI Instruments Corp.
64 EX1629 Triggering
The trigger model is sectioned into five layers: IDLE, INIT, ARM, TRIG, and DEVICE. The
EX1629 reset condition places it in the IDLE state. A trigger initialize command begins the
acquisition sequence by transitioning the instrument through the INIT layer into the ARM layer.
As this occurs, the reading buffer memory is cleared
Upon entering the ARM layer, the ARM Count is reset to its specified value. The instrument
remains in the ARM layer until the specified ARM event occurs or a Software Arm is issued.
Once that occurs, the specified ARM Delay (if any) is waited, the ARM Count is decremented,
and the instrument transitions into the TRIG layer.
Upon entering the TRIG layer, the TRIG Count is reset to its specified value. The instrument
remains in the TRIG layer until the specified TRIG event occurs or a Software Trigger is issued.
Once that occurs, the specified TRIG Delay (if any) is waited, the TRIG Count is decremented,
and the instrument transitions into the DEVICE layer.
In the DEVICE layer, channels in the scan list are measured the requested sample count number of
times, and stored into local memory (FIFO).
If the TRIG Count remains nonzero, the instrument stays in the TRIG layer until the specified
TRIG event (and subsequent device action) occurs enough times to decrement it to zero. Once the
TRIG Count reaches zero, the instrument then evaluates the remaining ARM Count and repeats
the ARM layer action if it is nonzero. However, since each transition into the TRIG layer resets
the TRIG Count, each additional ARM layer action results in the full specified number of TRIG
Count actions through the TRIG layer and DEVICE layer.
Once the ARM Count reaches zero, the instrument transitions back into the INIT layer. If Init
Continuous mode is enabled, the ARM layer is automatically reentered without the issuance of a
trigger initialize command. However, unlike with a trigger initialize, the reading buffer memory is
not cleared. Conversely, if Init Continuous mode is disabled, the instrument is returned to the
IDLE layer and requires the issuance of a new trigger initialize command to begin a new
acquisition sequence.
A
CQUISITION
D
ATA AND
FIFO
When a trigger event occurs and the instrument transitions into the Device Layer of Figure 4-1, a
user-configured number of samples are acquired. Main bridge acquisition data is acquired in
parallel from all channels enabled in the scanlist, using a separate ADC per channel. Acquisition
data, from both the main bridge measurements as well as the confidence subsystem, is captured,
filtered appropriately, calibration compensated, converted to engineering units, and stored in a
FIFO in the EX1629’s RAM along with a timestamp. In parallel, the instrument’s DIO values are
sampled and placed into the FIFO as well.
The digital inputs (DIO) are sampled during acquisition as well. The maximum sample rate for
DIO data is 1 kSa/s. If the main acquisition sample rate is higher than 1 kSa/s, the expected phase
error in terms of samples between DIO & Main bridge shall be ±[Main bridge samp freq/(2*DIO
sample freq)].
To maintain current data within the digital filters, data is always acquired from the main bridge
channels. When not within the Device Layer of Figure 4-1, this data is discarded before it reaches
the RAM FIFO. This serves to keep the digital filter states updated.
Applications may retrieve data from this FIFO using either the Read FIFO or Streaming Data
interfaces. Please refer to the
Retrieving Data (Read FIFO and Streaming Data) section for further
details. Once data is retrieved from the FIFO, via either method, it is no longer kept within the
FIFO.