Agilent Technologies L4400 - Manual (Page 82)

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).