Chapter 2
Using the SCXI-1129
©
National Instruments Corporation
2-21
an SCXI-1129 with external instruments to guarantee that both the
measurement instrument and the matrix have settled before taking the
next measurement.
The second type of scanning, software scanning, is a method by which
matrix connections are initiated through the software. This method requires
no trigger connections. Software scanning does not offer the time accuracy
of hardware-timed scanning.
Hardware-Timed Scanning
In hardware-timed scanning, the software downloads a list of commands
to the SCXI-1129. The software then starts executing the scan list. The
scan list executes until reaching a wait for a trigger command. When the
SCXI-1129 receives a trigger, the scan list continues to advance until
receiving another wait for trigger command. Between waiting for triggers,
the switch state of the matrix can be changed. When the matrix has
changed state, the scan list can be instructed to send a Scanner Advanced
(SCANADVD) signal to the measurement instrument indicating that
the system is ready for another measurement to be made.
Hardware-Timed Scanning Using a DMM
When using a DMM to control a chassis, synchronous mode is the only
mode in which the DMM and the SCXI-1129 can operate. The DMM
issues a trigger to change the matrix configuration at regular intervals. You
can program the interval at which the DMM sends triggers; set the interval
as follows.
where
n
is the number of activated switches.
The module that connects directly to the DMM is referred to as the cabled
module. If more SCXI-1129 modules are added to your system, the trigger
is bused over the SCXI backplane allowing other modules to be triggered,
as shown in Figure 2-14. These additional modules are referred to as
non-cabled modules since they do not require you to cable the DMM
directly to them.
n
352
µ
s
×
n
1
–
3
------------
4 ms
(
)
debounce time
+
+
Содержание NI SCXI-1336
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