Keithley 194 Instruction Manual page 54

OPERATION
When DC coupling is selected, a straight-through DC
signal path is established to the input section of the in-
strument. When AC coupling is in effect, the instrument
low-frequency response rolls off (see specifications). Thus,
DC coupling would be used when measuring DC signals
or very low frequency AC signals, while AC coupling is
useful for removing the DC component of the input signal.
When ground coupling is selected, the signal voltage is
disconnected from the input amplifier, and the input
amplifier terminals are shorted.
3.8 RECALLING DATA
The RECALL key allows you to recall any of the individual
samples that are stored within the measurement buffer.
Naturally, the number of buffer locations that contain valid
data will depend on the programmed number of samples.
RECALL KEY
SHIFT
ID El
RECALL
OD El
While in the recall mode, you can random access data 'It
any of the buffer locations by keying in the desired buffer
location and then pressing the ENTER key. If an incurrcct
location is entered, an ernx message will bc displayed.
hrr example, if you program a sample location higher than
the programmed number, a NMBR 'IWO LARGE message
will be displayed.
Alternately, you can increment or d~rernent buffer Ior+
tions by using the Uprange or Dwvnrangr keys, rcspc'c~
tively. If either of these buttons is held down for one se-
cond, the button will autorepeat.
Depending on whether the instrument is in the single or
continuous mode, the samples in the various buffer Iocd-
tions may or may not be changing as you access thenl. In
the single trigger mode, the samples will be updated ml>
when the instrument is triggered. In the continuous trig-
ger mode, however, buffer samples will be updated at a
rate dependent on the selected sa~~pling rate and intcr\.r~l.
When recalling data, the trigger point is indicated by the
letter "T" in the display as in the example below:
25.000 47x Tl
In this example, buffer location 478, which is the trigw
point for the measurement sequence, is bring displayed.
For quicker access to the trigger point you can use the
waveform mode, as discussed in paragraph 3.14.
To return the instrument to the normal display mode,
simply press the RECALL button a second time. The unit
will then return to the previous display mode.
When the recall mode is enabled, the RECALL indicator
will be on, and the display will show the data at the
selected location, the buffer location and the channel, as
in the example below.
25.000 0 1
In this example, a value of 25V is being read from the first
(0) buffer location of channel 1.
Example: Random and sequential buffer access.
Use the example below to demonstrate buffer .ICCC'SS.
Before using this example, program the instrument for a
lms sampling interval and 1000 samples with the RATE
and SAMPLES keys. Set the instrument for the single trig-
ger mode and apply a time-varying signal such as a 60Hz
sine wave to the VOLTAGE INPUT jack on the war pnnel.
Select AC coupling with the CI'LG key.
1. Press the RECALL button. The display will enter the
recall mode and display the sample at the selected buf-
fer location (0).
3-15