VTI Instruments EX1401, User Manual

Page: 110 / 114

VTI Instruments Corp.
110 EX1401 Onboard Memory
A
PPENDIX
A
M
ULTIPLE
I
NSTRUMENT
S
YSTEM
O
VERVIEW
When a large number of measurement channels is required, multiple EX1401 instruments can be
used synchronously. In order to achieve synchronized, simultaneous the data acquisition between
instruments, both sampling clock and the measurement state machine transition must be
synchronized. The EX1401 can utilize IEEE 1588-2008 and LXI LAN Event messages to
accomplish this.
While the synchronization between instruments is automatically performed by instruments driver
and 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 instruments becomes a master and others become
slaves. The trigger state machine transition is paced by a master instrument, and all slave instrumetns
follow it. Because of this, arm and trigger events are only detected by the master instrument. The
analog trigger signal, External trigger, DIO or LAN arm or trigger event must be sent to a master
chassis.
The first chassis in the resource string becomes a master by default. However, when any analog
channel or external trigger is enabled as an arm or trigger source, the instrument that contains the
channel automatically becomes master. It is invalid configuration to enable arm or trigger channels
from more than one instrument at the same time.
S
YNCHRONIZATION
P
ERFORMANCE
The variance of instrument-to-instrument phase matching largely depends on the network topology.
The quality of network switches, amount of network traffic, and the stability of master clock each
greatly affect the actual synchronization performance, or phase error. A dedicated network for the
test system with a stable GPS Grandmaster clock (PTP v2), connected with boundary/transparent
clock switches
1
provides the best and the most consistent performance. With a fully IEEE 1588-
2008 aware infrastructure, it is possible to achieve typical (2σ) variance of less than
±
20 ns. A typical
non-IEEE 1588-2008-aware network could have phase variance of
±
500 ns or more. It is also
important to note, however, that because the worst-case internal channel-to-channel phase matching
of the EX1401 is ±1500 ns, this additional error will cause an offset that may dominate the low
variance of IEEE 1500-2008 aware networks.
1
e.g., Meinberg LANTIME™, Hirschmann MACH1000™