Digital Input Port - SeaLevel PLC-16.PCI User Manual

Digital Input Port

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 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 (1K ohm) that is used to limit the input current to the opto-
isolator. A current path is required to 'turn-on' the sensor.
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 1K ohms. The opto-isolator requires approximately 3 mA to turn on.
The maximum input current is 60 mA. Adding additional resistors to the input current limiting resistor can increase
the voltage to be sensed. Two things to consider when selecting additional input resistance:
1. Turn on voltage for the circuit to sense.
2. The maximum input voltage. Maximum input voltage must not provide too much power to the input
resistor, and must not overdrive the opto-isolator input current specification. The following formulas apply:
Turn on current: 3 mA
Isolator diode drop: 1.1 V
Resistor power Max: 1 W
Turn on Voltage = diode drop + (turn on current) x (resistance)
Or:
1.1 + (.003) x R
Maximum voltage = square root of (resistor value)
The input circuits are not for monitoring 120-volt AC circuits.
Sealevel Systems PLC-16.PCI
+5VDC
1K
User Installable Capicator
Ground
Technical Description
Voltage input (5-24V)
1 K Ohm
1 Watt
Ground Reference
Page 5