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Chapter 2 NI 2590/2591 Operation
NI 2590/2591 User Manual 2-6 ni.com
Switch Control Circuitry
The switch control circuitry (SCC) is responsible for opening and closing
relays on the NI 2590/2591. You can load commands for the SCC into scan
memory using the driver software you received with your kit. Consult the
online help or your software documentation for specific information on the
appropriate commands. There are two reasons for storing the commands in
memory before the SCC can process them:
• Both the operate and release times for the NI 2590/2591 are 15 ms.
Using memory storage, the software can send multiple commands to
the SCC without having to wait for a relay action to complete.
• The memory is used to store a scan list necessary for hardware
random scanning.
Random Scanning
The NI 2590/2591 can perform random scanning. In random scanning,
the switch module can open or close relays in any order.
The scan list is downloaded to onboard memory. Commands in the scan
list can perform the following actions:
• Open or close relays
• Wait for an external trigger
• Generate a scanner advanced trigger
• Generate a breakpoint interrupt
You can use the driver software to configure the switch module for
continuous or one-time scanning. In continuous scanning, the switch
module cycles through the scan list until you disable scanning. For
one-time scanning, the switch module runs through the scan list only once.
You can also use software commands to clear the scan list or reset it to the
beginning at any time.
The onboard control logic for the NI 2590/2591 switch module gives you
direct access to open and close the relays, and also the ability to download
up to 1,024 random scanning instructions. The software included with the
module automatically configuresthe NI 2590/2591 anddownloads the scan
list to hardware for you. The scan list itself downloads directly into the
memory of themodule to deliver the fastest scanpossible with no controller