©
National Instruments Corp.
5
FP-DIO-550 and cFP-DIO-550
Digital Input
The [c]FP-DIO-550 has eight current-limited input channels with
optical isolators. Each channel has two input terminals, IN
+
and
IN
–
. When the voltage across the IN
+
and IN
–
terminals is at least
11 V, signal current flows through the input and turns on the optical
isolator, and the channel is ON. The current-limiting circuitry
limits the amount of current flow to 7 mA.
When the voltage across the IN
+
and IN
–
terminals is less than 5 V,
the channel is OFF. When the voltage is between 5 and 11 V, the
channel may be ON or OFF.
The input channels are
floating
, which means that they do not share
a common ground reference. Make sure that there is no more than
30 V difference between any two input terminals.
Figure 3 shows the input circuitry of a single channel.
Figure 3.
[c]FP-DIO-550 Input Circuit
The devices you connect to the [c]FP-DIO-550 should have
OFF-state leakage currents of less than 300
μ
A to ensure that
they do not cause false ON readings.
Digital Input Filter
Each input channel has a filter that can prevent electrical noise
from causing false readings. In Measurement & Automation
Explorer (MAX), you can select a value from 0 to 65,535
μ
s for the
digital input filter on each channel. The filter rejects pulses that are
1
μ
s or more shorter than the length you select. If you select 0
μ
s,
the filter is disabled.
IN+
IN–
Status
Indicator
Optical
Isolation
[c]FP-DIO-550
Current
Limiter
V
C
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