Impulse 8207 User Manual - Manual (Page 8)

© Sealevel Systems, Inc. - 6 - SEALINK ISO-16 User Manual
Input Port
Ports A & B are eight 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 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 (2.7K ohm) 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 opto-isolator requires approximately 1mA 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 the ranges associated with the resistor.
Input Resistor Turn-On Input Range Max Input Max Current
2.7K
Ω
3.0V 5 – 30V 37V 13mA
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.