Measurement Computing PC104-DI48 - Manual (Page 12)

7.0 ELECTRONICS AND INTERFACING
This short introduction to the electronics, most often needed by digital I/O board
users, covers a few key concepts.
7.1 UNCONNECTED INPUTS FLOAT
Unconnected inputs will float. If you are using the PC104-DI48 board for input, and
have unconnected inputs, ignore the data from those lines.
For example, 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. The input
buffer is not malfunctioning. In the absence of a pull-up/pull-down resistor, any input
to a PC104-DI48 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.
An alternative to masking inputs is to define the state of unused inputs by using
pull-up or pull-down resistors. There are locations on the board for installation of
these resistors marked RN1 through RN6. The location associated with FIRST PORT
A (the port at Base +0) is RN1. The location for FIRST PORT B (the port at Base
+1) is RN2. FIRST PORT C (Base +2) is RN3; SECOND PORT A (Base +4) is
RN4; SECOND PORT B (Base +5) is RN5; SECOND PORT C (Base +6) is RN6.
A 10Kohm, eight-resistor SIP has all its resistors connected on one end to a single
common pin. The common pin is marked with a dot and is at one end of the SIP. The
other ends connect to eight in-line pins.
The SIP can be installed to pull-up or pull-down. At each location 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 are connected to the eight lines of a port, A, B, or C.
To pull-up lines, orient the SIP with the common pin (dot) in toward the HI end; to
pull-down, install the resistor with the common pin in the LO hole.
Carefully solder the SIP in place.
A resistor value of 10K is recommended. Use other values only if you have
determined the necessity for doing so.
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