Keithley PIO-24, Руководство пользователя

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1-4 Introduction
Product change notice
A component change was implemented in March 1999, reducing the input load
current of the PIO-24 digital I/O card for improved consistency. The input load cur-
rent on the newly designed board is now
≤
-2µA, which continues to meet the product
specification of
≤
-450µA. This change was made because some devices having low
drive current, such as the ERB-24, were accidentally being turned on when power
was first applied to the PIO-24. The specific change involved changing parts U3, U6,
and U9, the digital input latches. The older design used 74LS373 octal tri-state
latches, which had a higher input load current than the 74HCT373 parts that are now
being used. In existing designs using the PIO-24, this reduction in input load current
will generally cause no problems. However, if the existing design depends on this
input load current being near the -450µA limit, this change is important. The reduc-
tion in input load current reduces the possibility of the input state being driven high if
the input is floating. Figure 1-2 shows an application monitoring a contact switch that
would be a problem as a result of the new design, and Figure 1-3 shows the correct
implementation of this application that would work with either design.
Figure 1-2
Poor implementation of monitoring a digital input
In Figure 1-2, the digital input would probably float high with an older design
board because of the higher input load current. When the switch is closed, the input
will be pulled low. While this would work on an older design of the PIO-24, this is a
poor implementation because the input load current can vary from one board to the
next. In the case of a new design board, this would not work because of the lower load
current.
PIO-24
<450uA
R=unknown
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