Keithley 194 Instruction Manual page 50

OPERATION
The TRIGGER button is also used to arm the remaining
trigger modes. For example, suppose you select input
signal triggering with the SOURCE key. You must then
press TRIGGER to start the AID converter so it can pro-
cess samples to determine when the input signal has
crossed the programmed trigger threshold. Once the trig-
ger occurs, the instrument will begin the measurement se-
quence in the usual manner.
NOTE
Pressing TRIGGER will alternately arm and
diliiey the A/D converter when in the continuous
Note that TRIGGER is operational in the dual-channel
display mode, as well as in the CHl-CH2 and CHlKH2
modes.
3.6.4 Trigger Delay
The DELAY key can be used to program the location of
the beginning of a measurement relative to the trigger
point. The delay value can be entered either in units of
time, or in number of samples. The limits for the delay
parameter are -65,536 to +lO' seconds or -65,536 to +lO'
samples. The input data is alternated between time and
sample units by pressing the FREQITIME key.
When negative values are entered, pre-triggering is in ef-
fect. Post-triggering is in effect when positive delay values
are entered. Both post- and pre-triggering arc available
with all trigger sources.
NOTE
If pre-triggering is selected, and the A/D converter
is armed, any previous stored measurements will
be overwritten even if the next measurement se-
quence has yet to be triggered.
The basic keystroke sequence to enter a trigger delay is:
DELAY, data, ENTER or time units. If an incorrect
parameter is entered, an appropriate error message will
be displayed. During the entry process, you can press
CANCEL to restore the display to the previous value.
When entering the delay parameter in time units, it is
possible that you might enter a value that is not an integer
multiple of the sampling interval. Under these conditions,
the instrument will automatically adjust the entered delay
to the nearest valid value.
Whether delay is entered in time or sample units, the ill-
strument will "remember" which units were previously
in effect. Thus, pressing DELAY will give you a display
of the delay parameter in previously programmed units.
Also, if you enter the delay in time units, and then change
the sampling rate, the instrument will maintain the same
delay time by recalculating the number of samples to delay.
However, no such recalculation is pcrformcd if you enter
the delay parameter as a number of samples.
Example 1: Program a 50msec positive delay.
1. Program a lms sample interval as follows. Press RAlX.
FREQ/TlME (if necessary to display time units), I, ms.
2. Press DELAY to cntw the delay entry mode. The instru-
ment will display the presently programned v~luc, ior
example: 0 DELAY.
3. If necessary, press FREQITIME to display the delay \.dlw
in time units.
4. Press: 5, 0, i-I- (only nccessall-y if ncgativc value is cur-
rently displayed), ms. The new trigger dclq period is
now programmed.
Example 2: Program a 25msec negative delay period.
1. Press DELAY to place the instrument in the delay cntr).
mode. The unit will display the previously pmgr.m~mrd
VdW.
2. Press: +/-, 2, 5, "IS. The new delay \,alue is n,,\\
programmed.
Example 3: Program an 80 sample positive delay.
1. Press DELAY to enter the delay entry mode.
2. Press FREY/TIME to change the displw tu sh(1\x
number of samples.
3. Press 8, 0, +/-, ENTER. The ne\\' value is nc>\\
programmed.
Example 4: Automatic recalculation of delay number of
SiW+?S.
If the sampling interval is changed after pwgramming the
delay, the instrument will automatically recalculate the
delay number of samples, as in the following rxamplc.
'I. Press DELAY to enter the delay entry mode.
2. Press FREQiTIME to display the delay in time units.
3. Program a 50msec positive delay as follows. Press S, (I,
+/- (only necessary if a negative value is displayed), ms.
4. Press DELAY then FREY/TIME and note that the delay
is programmed as 50 samples.
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