Input Port - SeaLevel PLC-16.PCI User Manual

Input Port

Port A is an eight 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 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 1K 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)
Input Current = ((input voltage)-1.1V) / (resistor value)
Maximum voltage = 1.1 + square root of (.25(resistor value))
The following table shows the ranges associated with the resistor.
Input Resistor
Note: The turn-off voltage for all resistors is less than 1V.
Because through hole resistors are utilized, they cannot be easily replaced. If
modification is necessary, Sealevel can do this for an additional charge.
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.
© Sealevel Systems, Inc.
Turn-On
3.0V
1K
- 10 -
Input Range Max Input
5.0 – 24.0V
30V
PLC-16.PCI User Manual
[Ex: 1.1 + (.003) x R]
Max Current
29mA