National Instruments NI PCI-8254R - Manual (Page 20)

Chapter 2 Digital I/O
© National Instruments Corporation 2-9 NI PCI-8254R User Manual
example, an upstream NI 8254R device programmed for part classification
might drive the product selection port of a downstream NI 8254R device.
Alternatively, a PLC with information about which part is being inspected
can drive the product selection port of the NI 8254R.
Using ISO Input 5 as a Latch
You can configure the product selection port to use ISO Input 5 as a latch.
A rising edge on ISO Input 5 can latch the data into a data register on the
NI 8254R. Before each inspection, the software checks the status of the
product select inputs and reads the most recent value latched into the
register. If ISO Input 5 is not used as a latch, it can be used as an extra bit
of data.
Note In Vision Builder AI, ISO Input 5 is always designated as a latch.
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 8254R 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 8254R with an
external event.
Table 2-3. NI 8254R 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