Keithley 2790, Reference Manual

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7-4 Model 2790 SourceMeter
®
Switch System Reference Manual
Scan process
Basic scan
— For functions that use 2-wire measurements, the basic scan process is to (1)
open any closed channel, (2) close a channel, and then (3) perform the measurement. This
3-step process is repeated for each channel in the scan. The last scanned channel opens.
Channel pair scan
— For the functions that use 4-wire measurements (
Ω
4 and 4-wire
RTD TEMP), the scan process uses paired channels. The scan process is to (1) open any
closed channels, (2) close the paired channels, and then (3) perform the 4-wire measure-
ment. The last scanned channel pair opens.
klqb
For the Model 7702 switching module, primary channels 1 through 20 are
paired to channels 21 through 40. Channel 1 is paired to channel 21, channel 2
is paired to channel 22, channel 3 is paired to channel 23, and so on.
Calculations using channel pairs
— Ratio and channel average performs measurements
on two channels and then calculates (and displays) the result. Therefore, these 2-channel
calculations also use paired channels. The scan process is to (1) open any closed channels,
(2) close the primary (displayed) channel and perform a measurement, (3) open the pri-
mary channel, (4) close the paired channel and perform a measurement, (5) calculate and
display the result, and finally (6) open the paired channel.
klqb
When scanning, the displayed channel number (i.e., 101) is not necessarily the
channel that is presently closed.
If both a reading AND a scan channel are displayed, the reading and annuncia-
tors pertains to that channel, but that channel is no longer closed. The next
channel in the scan list is the one that is now closed. Therefore, the reading (and
annunciators) pertains to the channel and does not necessarily indicate the
present state of the Model 2790.
If the display is blanked (-------), the displayed channel is closed and has not
been measured.
Trigger models
klqb
The following information on trigger model operations apply specifically to
front panel operation.
Block diagrams, known as trigger models, are used to show the two fundamental scan
functions: STEP or SCAN. These two scan functions are enabled by the STEP and SCAN
keys respectively. The trigger models for scanning are shown in
Figure 7-1 and Figure 7-2 .