VTI Instruments EX1401, Руководство пользователя

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VTI Instruments Corp.
76 EX1401 Instrument Drivers
events that occurred during the Holdoff time are ignored. When the specified Holdoff time is shorter
than one data record period, the two successive data records may be overlapped.
Waveform Record
Waveform Record
Waveform Record
Trigger Holdoff
Trigger
Trigger Trigger
Overlap
Overlap
F
IGURE
5-4: O
VERLAPPED
D
ATA
A
CQUISITION
Trigger Holdoff
The ability to configure the TRIG event source to be an external event such as a Digital I/O channel
level or a Trigger Bus channel edge is a powerful test feature, allowing measurements to be precisely
correlated with external events. External events, however, could nominally be generated at a rate far
faster than the EX1401 can completely and correctly execute the DEVICE layer. To prevent the
instrument from being overdriven, the trigger model contains an internal holdoff timer. Once the
trigger event has been satisfied, this timer disables the recognition of all trigger events for a time
period of 1 sample period. Trigger events that occur during this period are ignored, not buffered.
This is similar in concept to the fact that, for example, additional ARM events are ignored if the
trigger model is not specifically waiting for an ARM event. Due to this action, triggering the
EX1401 with a burst of external pulses that exceeds the configured sample rate will result in not
only fewer readings than trigger events, but also a reading rate that is potentially far less than the
maximum rate.
Consider an example in which the trigger model is set to the following configuration:
Start Source Immediate
Arm Source Immediate
Arm Count 1
Arm Delay 0
Trig Source DIO Channel 0 Positive Edge
Trig Count 100
Trig Delay 0
SampleRate 1000
Once initialized, the EX1401 is driven by a burst of 100 positive edge transitions at a rate of 1.5
kHz. At 1.5 kHz, the time between trigger events is 667 µs. Since this time is less than 1 ms, every
other trigger event will be ignored. Consequently, this burst will generate 50 readings at a rate of