Measurement Computing Data Translation DT9857, User Manual

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Principles of Operation
81
Note: The Input Trigger LED on the module, shown in page 176 , is amber to indicate an
armed condition when the module is waiting for an external digital trigger, threshold trigger,
or Sync Bus trigger (the module must have been configured for one of these trigger types),
green when the module has been triggered, or off when the module is idle.
Start Trigger Sources
DT9857 and DT9857E modules support the following sources for the start trigger:
• Software trigger – A software trigger event occurs when you start the analog input
operation (the computer issues a write to the module to begin conversions). Using
software, specify the start trigger source as a software trigger.
• External digital (TTL) trigger – For the DT9857 module, an external digital (TTL) trigger
event occurs when the module detects a rising- or falling-edge transition on the signal
connected to the External Trigger pin (pin 11) of the Digital connector on the module. For
the DT9857E module, an external digital (TTL) trigger event occurs when the module
detects a rising- or falling-edge transition on the External Trigger BNC input of the
module.
Using software, specify the trigger source as an external, positive digital (TTL) trigger for
a rising-edge external trigger or an external, negative digital (TTL) trigger for a
falling-edge external trigger.
• Threshold trigger – The start trigger event occurs when the signal attached to a specified
analog input channel in the channel list rises above or falls below a user-specified
threshold value. Using software, specify the following parameters:
−
Start trigger source – Specify a positive (low-to-high transition) threshold trigger if you
want to trigger when the signal rises above a threshold level, or a negative
(high-to-low transition) threshold trigger if you want to trigger when the signal falls
below a threshold level.
−
Threshold channel – Specify any one of the analog input channels as the threshold
input channel.
−
Threshold level – Specify a value between ±10 V for a gain of 1 or ±1 V for a gain of 10
as the threshold level.
Note: If you choose a threshold trigger as both the start trigger and the reference trigger,
the threshold channel and threshold level must be the same. The polarity of the trigger
(positive or negative going) can be different for each trigger source.
• Sync Bus trigger – When multiple DT9857 or DT9857E modules are connected together
using the Sync Bus (RJ45) connectors, you can use the Sync Bus trigger as the start trigger
of the analog input subsystem on the slave module by setting the synchronization mode
of the analog input subsystem of the module to Slave. In this configuration, any other start
trigger for the slave module is ignored. Refer to
page 105 for more information on this
configuration.