Impulse 8012 - Manual (Page 9)

© Sealevel Systems, Inc. - 7 - DIO-16.LPCI User Manual
Input Ports
Port A is an 8 bit input port 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 is 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 3 mA to turn on. The
maximum input current is 60 mA. 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)
Or: 1.1 + (.003) x R
Maximum voltage = square root of (.25 (resistor value))
The following table shows four common input resistors and the ranges associated
with each.
Input Resistor
(Ohms)
Value Turn-On
(Volts)
Max Input Range
(Volts)
Max Current
(mA)
220 1.76 7 27
560 2.8 12 20
1K 4.1 16 15
2.2K 7.7 24 10
Increasing the input resistor accordingly can increase the maximum input voltage.
Because socketed DIP resistor networks 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.