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Signal Connections Chapter 3
PC-OPDIO-16 User Manual 3-8 © National Instruments Corporation
Reducing the Forward Current for 24 V Inputs
You can reduce the forward current, I
f
,
for 24 V input signals by adding a series resistance with
the 3.3 kΩ current-limiting resistor, as shown in Figure 3-5. The value of resistance should be
such that at least 1 mA flows through the LED. You can choose a value close to 20 kΩ for R
s
.
0.5 W
PC-OPDIO-16
Isolation
3.3 kΩ
Load
VIN
IGND
Supply
Isolated Ground
R
s
I
f
Figure 3-5. Reducing Input Current for 24 V Signals
Power-on Condition
At power up, the PC-OPDIO-16 will register a logic low if nothing is connected to the inputs.