National Instruments PC-OPDIO-16 Manuale Utente

You can reduce the forward current, I

f

, for 24 V input signals by adding a series resistance with

the 3.3 k

Ω 

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 k

Ω

 for R

s

.

0.5 W

PC-OPDIO-16

Isolation

3.3 k

Ω

Load

VIN

IGND

Supply

Isolated Ground

R

s

I

f

Figure 3-5.  Reducing Input Current for 24 V Signals

At power up, the PC-OPDIO-16 will register a logic low if nothing is connected to the inputs.