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The LED's on the output ports are driven through the current-
limiting resistor to +5 volts, so that when the output bit
is low the LED is on.
the 8212's have a greater ability to sink current than
they do to source current.
The strobe line into each 8212 input or output port is tied
through a IK resistor to +5 volts so that if the strobe line
is not used, it will remain high and noise will not trigger
signals on the input or output ports or the interrupt lines.
All of the input lines have a IK resistor to +5 volts, so
that when the lines are not connected they exist in a de
fined state.
8
4
This orientation was chosen because
PIO 4, Rev. 2
Theory of Operation
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