VTI Instruments EX1000A, User Manual

Page: 73 / 320

www.vtiinstruments.com
EX10xxA Triggering 73
If the desired trigger model configuration requires the use of the digital I/O port and/or the trigger
bus, it is thus best to configure the ARM source for the required digital event, leaving the TRIG
source as Timer. The TRIG source should be changed from Timer only in the situation where
triggering is required on a two-stage digital event. An example of this is arming on a digital I/O
channel level and then triggering on a trigger bus channel edge.
M
AXIMUM
T
RIGGER
R
ATE
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 EX10xxA can completely and correctly execute the DEVICE layer. To prevent the
instrument from being overdriven, the trigger model contains an internal pacer timer. Once the
trigger event has been satisfied, this timer disables the recognition of all trigger events for a time
period of 1 ms. Trigger events that occur during this 1 ms 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 EX10xxA
with a burst of external pulses that exceeds 1 kHz 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:
Init Continuous Disabled
Timer Interval 0.1
Arm Source Immediate
Arm Count 1
Arm Delay 0
Trig Source DIO Channel 0 Positive Edge
Trig Count 1
Trig Delay 0
Once initialized, the EX10xxA 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 990 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 750 Hz. Note that the actual reading output rate is far less than the 1 kSa/s maximum rate. To
achieve the maximum output rate, the TRIG event source should be set to Immediate or Timer (with
a Timer Interval of 0.001).