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70 L4400 User’s Guide
3 Operating and Programming
• Each time you start a new scan, the instrument clears all readings
(including alarm data) stored in reading memory from the previous scan.
Therefore, the contents of memory are always from the most recent scan.
• The Analog Bus relays are automatically opened and closed as required
during the scan to place the signal on the analog bus. For example, with
the L4421A, all 2- wire measurements use the ABus1 (MEAS) relays; for
4-wire measurements, the ABus2 (SENS) relays are used in addition to the
ABus1 relays.
• When the scan is initiated, the instrument will open all channels in banks
that contain one or more channels in the scan list.
• In order to guarantee that no signals are connected to the Analog Buses
prior to the scan, the instrument will open all ABus1 relays (applies to all
banks in all slots). In banks that contain channels in the scan list, the
instrument will also open all ABus2 relays (regardless of whether 4- wire
measurements are involved). If no channels configured for 4- wire
measurements are included in the scan list, the state of the ABus2 relays
in the non- scanned banks is not
altered.
• The state of the ABus3 and ABus4 relays is not altered and these relays
remain available for use during the scan. However, be sure to use
CAUTION when closing these relays on banks involved in the scan. While
the scan is running, any signals present on ABus3 and/or ABus4 will be
joined with the scanned measurement on ABus1 and ABus2.
• While the scan is running, the instrument prevents use of all channels in
banks that contain one or more channels in the specified scan list (these
channels are dedicated to the scan). In addition, the instrument prevents
use of all ABus1 and ABus2 relays on banks containing channels in the
scan list. If one or more channels configured for 4- wire measurements are
included in the scan list, then the rules for ABus2 relay operations are
extended to the non- scanned banks as well.
• If the ABus1 relay used for current measurements (channel 931 on
L4421A only) is not closed prior to the initiation of the scan, the four
current channels (channels 41 through 44) are not affected by the scan.
However, if the ABus1 relay is closed, the instrument will open the ABus1
relay as well as the four associated current channels in a
make- before- break fashion.
• When you add a digital read (digital modules) to a scan list, the
corresponding channel is dedicated to the scan. The instrument issues a
Card Reset to make that channel an input channel (the other channel is
not affected).
• While the scan is running, you can perform low-level control operations
on any channels on the digital modules that are not in the scan.
For example, you can output a DAC voltage or write to a digital channel
(even if the totalizer is part of the scan list). However, you cannot change
any parameters that affect the scan (channel configuration,
scan interval, Card Reset, etc.) while a scan is running.