Chapter 2
Digital I/O
2-8
ni.com
Trigger Change Detectors
The NI 8254R is capable of detecting edges on various trigger lines and
latching this information for future retrieval. This feature is useful for
high-precision hardware-monitoring of the presence of external events
without the need for software polling. You can arm for the detection of a
rising edge, falling edge, or both on a supported trigger input line.
Supported trigger input lines include TTL Input 0, TTL Input 1,
ISO Input 8, ISO Input 9, ISO Input 10, and TRIG 0.
Quadrature Encoder
The quadrature encoder uses ISO Input 6 for its Phase A input and
ISO Input 7 for its Phase B input. Encoder speed is limited by the speed of
the isolated inputs. Each isolated input can change at a maximum rate of
100 kHz, making the maximum encoder rate 400,000 counts/s.
The quadrature encoder can also be used as a timebase for the pulse
generation delay.
The following figure shows a rising edge trigger and a low pulse with a
quadrature encoder delay and a microsecond width.
Product Selection Port
The product selection port consists of a group of five isolated digital inputs
that the software running on the NI 8254R reads simultaneously. You can
program the NI 8254R to switch between up to 32 (2
5
) inspection
sequences for different parts on an assembly line.
Based on the input to the product selection port, you can configure the
application software to run the appropriate inspection sequence. For
Phase A
Phase B
Delay
Width
Trigger
Low Pulse
