SeaLevel ISO-16.PCI User Manual page 9

PROGRAMMING THE ISO-16.PCI, CONTINUED
Input Ports
Ports A and B are 8-bit input ports connected to optically isolated input sensors. Each sensor can
be used to interface a voltage input and then sense whether the voltage is on or off. Each sensor
is isolated (with respect to a common ground) from every other sensor, and also 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 due to 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' 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 560 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:
© Sealevel Systems, Inc.
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))
8006H Manual | SL9023H 9/2021
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