Keithley 194 Instruction Manual page 42

OPERATION
The Uprange key is also used with many other front panel
operations such as data entry and various XY modes.
These aspects are covered in the respective paragraphs.
3.3.3 Downrange
The Downrange key operates much like the Uprange key,
except, of course, for the fact that the instrument is moved
downrange. Once the lowest range is reached, this key has
no further effect. Pressing downrange, will also cancel
autoranging, if that mode was previously selected.
Like the Uprange key, Downrange is used with other
modes including data entry and various XY modes.
3.3.4 Range Selection Considerations
Generally, the lowest range that can be used without over-
ranging the instrument is the best one for most situations
(the instrument will display the "OFLO" error message
if an overrange signal is applied). Doing so will generally
result in the best overall accuracy. However, because the
instrument operates a little differently than an ordinary
DMM, there are some key points to keep in mind when
selecting a range.
When the instrument is measuring a signal, it is actually
taking a series of samples at pre-programmed intervals.
As these samples are taken, the resulting data arc stored
in memory as a complete measurement. If the signal
amplitude varies with time, it is possible that one or more
of these samples is an overrange value. Under these con-
ditions, the unit will still display the "OFLO" message even
if most samples are on range. If this condition exists, it is
still possible to go back and access the good samples by
using the RECALL button.
When the instrument is in the continuous trigger and
autorange mode, it continuously samples the signal at the
selected rate and attempts to choose the best range based
on the maximum buffer sample amplitude. However, there
could be some situations where an occasional overrange
reading occurs because the instrument was unable to move
uprange fast enough to keep up with the applied signal.
While autoranging, the instrument will take a complete
measurement and, if necessary, change range; it will repeat
measurements until a satisfactory range is found.
3.4 DATA ENTRY
Many front panel modes such as zero value, delay, trigger
level, rate, and samples require that numeric data bc
entered from the front panel. Model 194 data is cntrred
by using the ENTER, CANCEL, FREQITIME, 0 through
9, +/-, ., MHz, kHz, and IIz keys. The operation of these
kevs or kev uoutx is discussed in the follm\Gg
p&graphs.' " '
DATA ENTRY KEYS
3.4.1 Data Keys
The data keys, 0 through 9, +/-, ., MHz (,,s), kHz (ms,
mV), and Hz (s,V) are used to enter a numeric quantity
into those modes requiring data entry. These keys are
operational only when the instrument has been placed into
the data entry mode by pressing another mode button re-
quiring data entry. For example, pressing RATF will enter
this mode and allow you to program a rate.
During the data entry process, a flashing digit or segment
(the cursor) on the display will indicate which digit \vill
be affected by a numeric key press. The 0 through 9, ., or
+/-- keys can be pressed at the appropriate times to enter
the desired data. During the numeric entry pnxcss, the
cursor can be moved to the right or left by pressing
Uprange or Downrange respectively, and '1 single-digit
change can be made at that point.
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