National Instruments NI VISION PCI-8254R - Manual (Page 23)

Chapter 2 Hardware Overview
NI PCI-8254R User Manual 2-12 ni.com
Caution Use the Indicator Only option only to test the watchdog timer. If software
becomes unresponsive, it cannot be relied upon to send notification to the host.
• Indicator Only—This option sends the expiration signal back to the
development machine through software. True indicates an expired
watchdog timer. False indicates an unexpired watchdog timer. The
expiration signal that indicates an expired watchdog timer continues to
assert until the watchdog timer is disarmed. Disarming the watchdog
timer resets the software indicator.
• TTL Output 0—This option outputs a signal on TTL Output 0. High
indicates that the watchdog timer has expired. Low indicates that the
watchdog timer has not expired. If the watchdog timer has expired, the
expiration signal continues to assert until the watchdog timer is
disarmed.
• Shutdown—If Shutdown mode is enabled, the outputs go to the
user-defined shutdown states.
Considerations When Connecting the Digital I/O
The isolated trigger inputs on the NI 8254R are current sinking and
optically isolated. The following are considerations you need to make when
connecting the digital I/O.
Wiring an Isolated Input to a Sourcing Output Device
You can wire an isolated input to a sourcing output device, as shown in the
following figure. Refer to Getting Started with the NI PCI-8254R for
switching thresholds and current requirements.
Caution Do not apply a voltage greater than 30 VDC to the isolated inputs. Doing so could
damage the NI 8254R.
Figure 2-5 shows an example of connecting an isolated input to a sourcing
output device.