ICS Advent PCI-DIO32, User Manual

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3-2 PCI-DIO32 User’s Guide
Input Ports
Ports A and B are eight-bit input ports connected to optically isolated input
sensors. Each sensor can be used to interface a voltage input and sense
whether the voltage is on or off. Each sensor is isolated (with respect to a
common ground) from every other sensor, and each is isolated with respect to
the host PC ground. This means that signals such as low level AC line voltage,
motor servo voltage, and control relay signals can be sensed, or read, by the
PC without the risk of damage from ground loops or ground faults.
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”
internal diodes. 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 560 ohms
(factory default). The opto-isolator requires approximately 3 mA to turn on.
The maximum input current is 60 mA. There are two things to consider when
selecting the input resistor: (1) the turn-on voltage, sensed by the circuit; and
(2) the maximum input voltage. The maximum input voltage must not provide
too much power to the input resistor, and it also must not overdrive the
opto-isolator input current specification. The following formulas apply:
Turn-on current = 3 mA
Isolator diode drop = 1.1V
Maximum resistor power = 0.25W
Turn-on voltage = diode drop + (turn-on current) x ( resistance)
Or : 1.1 + (0.003) x R
Maximum voltage = square root of (0.25 x (resistor value))