National Instruments NI 6520, User Manual, Page: 10 / 23

NI 6520/6521 User Guide 10 ni.com
• Bit 0 is connected to a switch. The software can react to any switch
closure, which is represented by a falling edge. If the switch closure is
noisy, enable digital filtering for this line.
In this example, the DIO device reports rising edges only on bit 1, falling
edges only on bit 0, and rising and falling edges on bits 7, 6, 5, and 4. The
DIO device reports no changes for bits 3 and 2. After receiving notification
of a change, you can read the port to determine the current values of all
eight lines. You cannot read the state of any lines that are configured for
change detection until the change detection interrupt occurs.
Watchdog Timer
The watchdog timer is a software configurable feature used to set critical
outputs to safe states in the event of a software failure, a system crash, or
any other loss of communication between the application and the
NI 6520/6521 while power is provided to the PC/board.
When the watchdog timer is enabled, if the NI 6520/6521 does not receive
a
watchdog reset software command within the time specified for the
watchdog timer, the outputs go to a user-defined safe state and remain in
that state until the watchdog timer is disarmed by the application and new
values are written, the NI 6520/6521 is reset, or the computer is restarted.
The expiration signal that indicates an expired watchdog will continue to
assert until the watchdog is disarmed. After the watchdog timer expires, the
NI 6520/6521 ignores any writes until the watchdog timer is disarmed.
You can set the watchdog timer timeout period to specify the amount of
time that must elapse before the watchdog timer expires. The counter on the
watchdog timer is configurable up to (2
32
– 1)
×
100 ns (approximately
seven minutes) before it expires.
Digital I/O
I/O Connector
The 37-pin D-SUB connector on the NI 6520/6521 provides access to eight
isolated digital inputs and 8 relay outputs. For easy connection to the digital
I/O connector, use the National Instruments 37-pin D-SUB accessories. For
a list of compatible 37-pin accessories, refer to the
Cables and Accessories
section.
Caution Do not make connections to the digital I/O that exceed the maximum I/O
specifications. Doing so may permanently damage the NI 6520/6521 and the chassis. Refer
to the
Signal Descriptions section and the device specifications for information about
maximum input ratings.