Specifications

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Programmable logic controllers (PLCs)
Analog output functions
momentary cable interruption, but
that is not necessarily the case. The
mechanical cable interruption will
tend to last longer than the noise
pulse. The noise pulse is typically
caused by capacitive or inductive
coupling of a large change in current
in a second cable running close to
the communications cable. This
noise interruption can occur when
a large motor is turned on or off,
and the transition (rise or fall time)
of the change in current is seen as a
differentiated pulse of short duration
on the communications cable.
Consequently, waiting for a short
time (a fraction of a second) allows the
fault detector to distinguish between
a real cable intermittent fault and a
noise pulse. The detection period
must be long enough to avoid false
error reports caused by fast transients
that are part of the harsh environ-
ment, and yet short enough to catch
short mechanical cable errors.
Extra safety is provided if more
conditions than just cable health are
monitored. Chip temperature, and
thus the environment over tempera-
ture, is one important example. The
field or factory can be spread over
several acres, so monitoring power-
supply voltage drops or brownout is
also important for system reliability.
Managing an output fault
If an output fault occurs, errors
must be latched and presented to a
hardware interrupt pin. This gives the
system microprocessor time to react
to short cable outages. By definition,
intermittent cable faults will be asyn-
chronous and many will occur while
the processor is busy. The interrupt is
generated so the processor can then
poll the output device registers for the
exact condition and clear the interrupt.
The output to the field or factory
needs to be protected against
common wiring errors and shorts.
Some faults cannot be tolerated,
such as a direct lightning hit.
However, the outputs should with-
stand reasonable fault voltages. The
most common errors are shorts to
ground or the 24V power supply, and
these errors should be tolerated with-
out the need to replace components.
Managing system functions
Some sensors require excitation
to function, and the output module
supplies such signals. Typical examples
are an AC signal for capacitive and
variable reluctance sensors or a DC
signal for a simple LED in a back-
lighted switch.
The analog output can also provide
other system-management functions
that include monitoring the local
isolated power supply, board
temperature, and calibration.