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CHAPTER 2: Introduction
2.3.1 F
EATURES
• Selectable I/O port addressing from 100H through 3FFH.
• Two sets SPST relays (each set has eight relays).
• Two eight-bit input ports.
• DB37 male connector for relay outputs.
• DB37 female connector for optically isolated inputs.
• Highly reliable 10-VA DIP reed relays used.
• Multiple adapters can reside in same computer.
• All address, data, and control signals are TTL compatible.
2.3.2 I
NPUT
P
ORTS
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 it’s 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 power
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 power on the
opto-isolator, the output goes high and the port bit is read by the PC as a high
logic level (binary 1).
