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If the board is in output mode, the board has enough power to override the
pull-up/down resistor's high signal and drive the line to 0 volts. If the output circuit
asserts a high signal, the pull-up resistor guaranties that the line goes to +5 V.
Of course, a pull-down resistor accomplishes the same task except that the line is
pulled low when the board is reset. The board has enough power to drive the line
high.
The PCI-DIO96 series boards are equipped with positions for pull-up/down resistors
Single Inline Packages (SIPs). The positions, marked PORT#A, B and C, are located
adjacent to the I/O connectors.
A 2.2K, 8-resistor SIP is made of eight 2.2K resistors all connected with one side to a
single common point, the other side of each to a pin protruding from the SIP. The
common line to which all resistors are connected also protrudes from the SIP. The
common line is marked with a dot and is at one end of the SIP.
The SIP may be installed as pull-up or pull-down. At each location, PORT#A, B &
C on the PCI-DIO96 series boards, there are 10 holes in a line. One end of the line is
+5V, the other end is GND. They are marked HI and LO respectively. The eight
holes in the middle are connected to the eight lines of a port 1 through 4, A, B, or C.
A resistor value of 2.2K is recommended. Use other values only if you have
calculated the necessity of doing so.
UNCONNECTED INPUTS FLOAT
Keep in mind that unconnected inputs float (typically, but not reliably, high). If you
are using the PCI-DIO96 board for input, and have unconnected inputs, ignore the
data from those lines.
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!
7.2 TTL TO SOLID STATE RELAYS
Many applications require digital outputs to switch AC and DC voltage motors on
and off, or to monitor AC and DC voltages. These AC and high DC voltages cannot
be controlled or read directly by the TTL digital lines of a PCI-DIO96.
Solid State Relays, such as those available from OMEGA Engineering Inc.
allow control and monitoring of AC and high DC voltages and provide up to
4000VAC isolation. Solid State Relays (SSRs) are the recommended method of
interfacing to AC and high DC signals.
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