Chapter 2
Hardware Overview
©
National Instruments Corporation
2-11
An example of using general-purpose outputs is driving a relay that turns
on an
Inspection in Progress
light for an operator to see while the
inspection sequence is running.
I/O for Fault Conditions
The NI 8255R recognizes the following fault conditions:
•
External shutdown, when Shutdown mode is enabled
•
Watchdog timer expiration
In the event of a fault condition, the behavior of the NI 8255R is dependent
on configuration settings of the software-enabled Shutdown mode. To
resume operation, address the fault condition and cycle power on your
computer.
Table 2-4 summarizes how user configuration affects the behavior of the
NI 8255R in the event of a fault condition.
The following sections describe each fault condition.
Shutdown
Shutdown mode is a software-enabled feature that, when activated, allows
an external device to halt the NI 8255R processing operations.
Additionally, Shutdown mode allows you to specify user-defined
shutdown states for all fault conditions.
When Shutdown mode is enabled and the shutdown input signal,
ISO Input 11, turns off, the NI 8255R registers an external shutdown
condition. When a fault occurs, outputs operate according to user-defined
shutdown states. Each TTL output is configurable to drive high, drive low,
or tri-state, and each isolated output is on/off configurable.
Table 2-4.
Fault Condition Behavior
Fault Condition
Shutdown Enabled
Outputs Change to
User-Defined States
External Shutdown
On
Off
Yes
No
Watchdog
On
Off
Yes
No
