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© Sealevel Systems, Inc.
8002H Manual | SL9021 9/2021
Programming the DIO-16.PCI, Continued
Each sensor input pair has a current limiting resistor that is used to limit the input current to the opto-
isolator. The opto-
isolator has two ‘back
-to-
back’ diodes internally. This allows AC or DC signals to be
sensed, regardless of polarity. When the applied voltage is high enough to cause the led in the opto-isolator
to turn-on, the output of the opto-isolator goes low (0 volts), and the signal is read as a low logic level (binary
0) by the PC. When the input signal is too low to turn on the opto-isolator, the output goes high, and the port
bit is read by the PC as a high logic level (binary 1).
The input impedance of each isolated input is approximately 3.3K ohms (factory default). The opto-isolator
requires approximately 3mA to turn on. The maximum input current is 50mA. There are two things to
consider when selecting the input resistor. The first is turn on voltage for the circuit to sense, and second
is the maximum input voltage. Maximum input voltage must not provide too much power to the input
resistor and must also not overdrive the opto-isolator input current specification. The following formulas
apply:
Turn on Voltage = diode drop + (turn on current) x (resistance) [Ex: 1.1 + (.003) x R]
Input Current = ((input voltage)-1.1V) / (resistor value)
Maximum voltage = 1.1 + square root of (.25(resistor value))
The following table shows common input resistors and the ranges associated with each.
Input Resistor
Turn-On
Input Range
Max Input
Max Current
220
1.8V
1.8
–
7.0V
8.5V
27mA
560
2.8V
2.8
–
10.6V
12.9V
20mA
1K
4.1V
4.1
–
13.8V
16.9V
15mA
2.2K
7.7V
7.7
–
20.0V
24.5V
10mA
3.3K
10.0V
10.0
–
24.0V
30.0V
9mA
4.7K
15.2V
15.2
–
28.0V
35.0V
7mA
The turn-off voltage for all resistors is less than 1V.
Increasing the input resistor accordingly can increase the maximum input voltage. Because socketed DIP
resistors are utilized, they can easily be replaced with a different value. Sealevel, if necessary, can do this.
The input circuits are not intended for monitoring 120-volt AC circuits. In addition to being too
high a voltage for the circuits, it is dangerous to have that high a voltage on the card.
