ADLINK PXI-7921 - Manual (Page 29)

Operation Theorem • 23
T
S
is the default debounce time for a switch module, i.e. 5ms for PXI-7921. T
An
is the user specified scan delay time in the scanlist entry, indicating the time
between the relay being debounced and the exact moment that a measurement
device takes a new measurement. The actual delay time would be the greater
of the two times, to guarantee that measurement devices take measurements
after the signal path is fully settled, and the relays switch as close as possible to
their maximum operating speed.
As the scan delay time elapses, the switch module generates Scanner
Advanced
signal to inform the measurement device to take a new
measurement.
After the measurement completes, the measurement device will generate
another Trigger In signal to have the switch module update the relay pattern to
that specified in the second entry of scanlist.
The handshaking process will continue, until it reaches the end of the scanlist
(if one-time scanning mode is selected), or when a software scan-abort
command is received.
Connecting, Trigger and Synchronize with External DMM
In this example, Agilent
®
33401A 6-1/2 Digital Multimeter (DMM) is used to
demonstrate signal connection for handshaking operation.
The DMM provides two terminals on the rear panel for the handshaking
operation process, Trig In and VM Comp. The Trig In connects to the Scanner
Advanced
output on switch module, while the VM Comp to Trigger In. If the
terminal board is used, wire Trig In to TRG_IN on terminal board, and the VM
Comp
to S_ADV.
Follow the instructions below to ensure the handshake functions properly:
1. Configure ADLINK PXI switch module’s Trigger In to rising-edge
triggered, Scanner Advanced output in active-low pulsating
mode with pulse width of at least 2us.
2. Configure the DMM to wait for external Trig In before a
measurement, and generate VM Comp after a measurement.
Arm the DMM to wait for the first trigger.
3. Setup the scanlist and auto-scan mode. The first entry in the
scanlist should be set disable wait for the Trigger In, but enable
Scanner Advanced
output. Succeeding entries should enable
both Trigger In and Scanner Advanced output. Download the
scanlist to the switch module afterward.
4. Start auto-scan by sending scan start command to the switch