Measurement Computing PCI-DIO48H/CTR15 - Manual (Page 22)

Of course, a pull-down resistor accomplishes the same task except that the line is
pulled low when the board is reset and the output has more than enough power to
drive the line high.
The PCI-DIO48H/CTR15 is equipped with positions for pull-up/down resistor Single
Inline Packages (SIPs). The positions are marked PORT0 A, B and C and PORT1 A,
B and C and are located beside the I/O connector.
A 2.2K, eight-resistor SIP is made of eight, 2.2K resistors all connected to a single
common point and the other pins protruding from the SIP. The common pin at one
end of the SIP is marked with a dot.
The SIP can be installed to pull-up or pull-down. At each location, A, B & C on the
PCI-DIO48H/CTR15 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. The eight holes in
between are connected to the eight lines of the port, A, B, or C.
To implement a pull-up, insert the SIP with the common end in the HI hole. To do an
eight-input pull-down, insert the SIP’s common pin in the LO hole.
A resistor value of 2.2K is recommended. Use other values only if you have
calculated the necessity of doing so.
REMEMBER, UNCONNECTED INPUTS FLOAT!
Keep in mind that unconnected inputs typically float high. 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, is unspecified.
You do not have to pull up or pull down input lines since unconnected lines will not
affect the performance of connected lines. Just make sure to mask out any
unconnected bits in software.
7.2 TTL TO SOLID STATE RELAYS
Many applications require digital outputs to switch high AC and DC voltages on and
off and to monitor high AC and DC voltages. Obviously, these high voltages cannot
be controlled or read directly by the TTL digital lines of a PCI-DIO48H/CTR15.
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.
The most convenient way to use solid state relays and a PCI-DIO48H/CTR15 board
is to purchase a Solid State Relay Rack. A SSR Rack is a circuit board with output
buffer chips which are powerful enough to switch the SSR and sockets to plug SSRs
into. SSR Racks are available from Computer Boards and most manufacturers of
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