Impulse DIO-104.OPTO (3730) - Manual (Page 10)

© Sealevel Systems, Inc. - 8 - DIO-104.OPTO User Manual
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 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 signal is high enough to cause the opto-isolator to turn-on, the output of the opto-
isolator goes low (0 volts) and the port bit 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:
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
Ω
1.8V 1.8 – 7.0V 8.5V 27mA
560
Ω
2.8V 2.8 – 10.6V 12.9V 20mA
1K
Ω
4.1V 4.1 – 13.8V 16.9V 15mA
2.2K
Ω
7.7V 7.7 – 20.0V 24.5V 10mA
3.3K
Ω
10.0V 10.0 – 24.0V 30.0V 9mA
4.7K
Ω
15.2V 15.2 – 28.0V 35.0V 7mA
Note: 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.
Important: 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.