Measurement Computing PC104-DIO48 - Manual (Page 17)

The concept of voltage level of an 82C55 in input mode is meaningless.
Do not connect a volt meter to the floating input of an 82C55. It will
show you nothing of meaning. In input mode the 82C55 is in 'high Z' or
high impedance. If your 82C55 were connected to another input chip
(the device you were controlling), the inputs of that chip are left floating
whenever the 82C55 is in input mode.
If the inputs of the device you are controlling are left to float, they may
float up or down. The direction they float depends on the characteristics
of the circuit and is unpredictable! This is why it often appears that the
82C55 has gone high after power-up. The result can be that your
controlled device gets turned on! This is why you need pull up/pull
down resistors.
Figure 4-1 shows an 82C55 digital output with a pull-up resistor
attached.
The pull-up resistor
provides a reference to
+5V while its value of
2,200 ohms allows about
2.3 mA to flow through
the circuit.
If the 82C55 is reset and
enters high impedance
input mode, the line is
pulled high. At that point,
both the 82C55 AND the
device being controlled Figure 5-1. Output Being Pulled Up
will “see” a high signal.
If the 82C55 is in output mode, the 82C55 has enough power to override
the pull-up resistor's high signal and drive the line low. If the 82C55
asserts a high signal, the pull-up resistor guarantees that the line goes to
high (about +5V).
Of course, a pull-down resistor accomplishes the same task except that
the line is pulled low when the 82C55 is reset. The 82C55 has more
than enough power to drive the line high.
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