Measurement Computing CIO-DI48 - Manual (Page 19)

7.0 INTERFACING TECHNIQUES
This brief introduction to the electronics most often needed by digital I/O board users
covers a few basic concepts.
7.1 UNCONNECTED INPUTS FLOAT
Keep in mind that unconnected inputs float. If you are using the CIO-DI## board with
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 input buffer is
not malfunctioning. In the absence of a pull-up or pull-down resistor, any input to a
CIO-DI## 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. T here are locations on the b oard for installation of
these resistors marked RN1 through RN6 on the DI48, through RN12 on the DI96 and
through RN24 of the DI192. 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 and so on.
A 10Kohm, eight-resistor SIP has all its resistors connected on one end to a sin gle
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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