VTI Instruments Corp.
APPENDIX D
112
A
PPENDIX
E
M
ULTIPLE
C
HASSIS
S
YSTEM
O
VERVIEW
When a large number of measurement channels is required, or when a distance between
measurement points are large, instruments may need to be distributed among more than one chassis.
In order to achieve synchronized, simultaneous the data acquisition between instruments, both
sampling clock and the measurement state machine transition must be synchronized. When all
instruments are installed in a single PXIe chassis, they can share the same sampling clock and timing
signal using chassis’ backplane.
When more than one chassis are involved, these signals need to be somehow synchronized together.
One method to achieve this, is by physically connect them using dedicated cables. While this
approach gives the most consistent and best synchronization performance, it may not be practical
when chassis are separated by long distance. The other method is to synchronize them using LAN
messages over Ethernet. In this method, the sampling clock of each chassis are synchronized to a
grandmaster clock using IEEE1588-2008 protocol, and state machine transition are coordinated by
LAN messages between chassis as defined in LXI specification. See also
Multiple Cards, Segments
and Chassis
in Section 4 of this manual.
While the synchronization between chassis is automatically performed by instrument’s driver and it
is transparent to the user, it is important to understand the tradeoffs and limitations.
M
ASTER AND
S
LAVE
Regardless of synchronization method, one of the chassis becomes a master and others become
slaves. The trigger state machine transition is paced by a master chassis, and all slave chassis follows
it. Because of this, arm and trigger events are only detected by cards in a master chassis
2
. The analog
trigger signal, TTL external trigger, PXI or LAN arm or trigger event must be sent to a master
chassis.
When chassis are synchronized by clock and trigger cables, the master chassis must be the first
chassis of the resource string being used at the driver initialization. The trigger signal must be routed
from a backplane trigger line to EMX2500 front panel connector for a master chassis. For slave
chassis, the trigger signal needs to be routed from front panel connector to the backplane trigger
line.
When LAN synchronization is used, the first chassis in the resource string becomes a master by
default. However, when any analog channel or external trigger channel is assigned as an arm or
trigger source, the chassis contains the channel automatically becomes master. It is invalid
configuration to specify arm or trigger channels from more than one chassis.
2
When a master chassis has multiple trigger segments, only one of segments becomes a master.
