SeaLevel ISO-16.PCI User Manual page 11

PROGRAMMING THE ISO-32.PCIE, 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 isolated with respect to the host PC ground.
This means that signals such as low-level AC line voltage, e.g., 12 or 24VAC, 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). While this is true at the hardware level, the Microsoft
Windows driver gives the application the ability to use Positive or Negative Logic. This card defaults to
Positive Logic and therefore the driver inverts the logic level BEFORE the information is returned to the
application. Therefore, a voltage high enough to cause the LED to turn on will be read by the application as
a logic high level.
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:
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))
© Sealevel Systems, Inc. 8006e Manual | SL9236 9/2021
11