Measurement Computing CIO-DAS16/M1 - Manual (Page 33)

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not bother to 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 a 'high Z' or high impedance state. If a 82C55 is connected to another input
chip (the device being controlled), the inputs of that chip float whenever the 82C55 is in input mode.
If the inputs of the device being controlled are left to float, they may float up or down. Which way they
float is dependent 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 the controlled device is turned on.
This is why pull-up or pull-down resistors are needed. A pull-up resistor provides a fixed reference to
+5V while its value of 2.2K ohms allows less than 2.3mA to flow through the circuit. If the 82C55 is reset
and enters a high impedance input mode, the line is pulled high. At that point, the 82C55 will sense a high
signal.
If the 82C55 is in output mode, the 82C55 has ample power (2.5mA) to override the pull-up resistor's
high signal and drive the line to 0 volts. If the 82C55 asserts a high signal, the pull up resistor guaranties
that the line goes to +5V.
A pull-down resistor accomplishes a similar function except that the line is pulled low when the 82C55 is
reset. The 82C55 has enough power to drive the line high.
A 2.2K ohm, 8-resistor SIP is made of eight, 2.2K resistors. All resistors are connected on one end to a
single common point. The other end of each resistor tie to a pin on the SIP. The common point is marked
with a dot and is at one end of the SIP.
The SIP can be installed as pull-up or pull-down. At each port location, A, B, and C there are 10 holes in
a line. One end of the line is marked HI, the other end LO. The eight holes in the middle connect to the
eight lines of a port, A, B, or C. To pull an input high on power-up or reset, install the SIP with the
marked end toward the “HI” label. To pull an input low on power-up or reset, install the SIP with the
marked end toward the “LO” label.
UNCONNECTED INPUTS FLOAT
Keep in mind that unconnected inputs float. If you are using the board for input, and have unconnected
inputs, ignore the data from those lines.
In other words, if you connect bit A0 and not bit A1, do not be surprised if A1 stays low, stays high or
tracks A0. It is unconnected and so unspecified. The 82C55 is not malfunctioning. In the absence of a
pull-up/down, any input which is unconnected, is unspecified!
You do not have to tie input lines, and unconnected lines will not affect the performance of connected
lines. Just make sure that you mask out any unconnected bits in software!
6.5 TTL TO SOLID STATE RELAYS
Many applications require digital outputs to switch AC and DC voltage motors on and off and to monitor
AC and DC voltages. These AC and high DC voltages cannot be controlled or read directly by the TTL
digital lines.
Solid State Relays, such as those available from Computer Boards, Inc. allow control and monitoring of
AC and high DC voltages and provide 750V isolation. Solid State Relays (SSRs) are the recommended
method of interfacing to AC and high DC signals.