Selecting I/O Units  Measurement Errors  We've reached the end
of the wire on the analog side. The last item on the wire is the
sensor, or effector, which is subject to errors as well. It's
important to mention these errors because they affect the value
the I/O unit reports or sends. It's a ``garbage in, garbage
out'' affair. I'll quickly review the possible measurement
errors.  Sensors and effectors are affected by the same accuracy
and linearity issues as analog-digital converters. Measurement
accuracy is typically a calibration issue. Calibration is a
procedure to verify that the measurement corresponds to the
reported value. Drift is long-term deviation that causes errors
in the measurement value. Drift may be caused by degradation of
a sensor's components or life-cycle fatigue of a system. Sensor
and effector performance also may be subject to environmental
factors such as temperature or humidity. It's important to read
the specifications for a sensor or effector and thoroughly
understand them.  I also l