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Chapter 2
Hardware Overview
2-10
ni.com
Table 2-3 lists the product selection ports.
General-Purpose I/O
General-purpose inputs and outputs are available as both TTL and isolated
connections. The software running on the NI 8255R can read the inputs and
drive the outputs high or low at any time.
General-Purpose Inputs
The primary difference between general-purpose inputs and trigger inputs
is that you cannot use general-purpose inputs to initiate a timed pulse
generator. In an application, use the general-purpose inputs to get the status
of the inputs at a given point and not to synchronize the NI 8255R with an
external event.
An example of how to use general-purpose inputs is reading the status of a
general-purpose input as the first step in your inspection sequence and
recording that value as part of your inspection.
General-Purpose Outputs
The primary difference between general-purpose outputs and timed pulse
outputs is that the timing of general-purpose outputs is controlled by
software rather than hardware. As a result, timing of general-purpose
outputs changes as the inspection algorithm changes, which makes
general-purpose outputs less appropriate than timed outputs for camera
control, strobe light control, and other applications that require precise
timing.
Table 2-3.
NI 8255R Product Selection Ports
Function
External Connection
Data(5), rising edge latch
ISO Input 5
Data(4)
ISO Input 4
Data(3)
ISO Input 3
Data(2)
ISO Input 2
Data(1)
ISO Input 1
Data(0)
ISO Input 0