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EX1629 Multi-Instrument Operation
In addition, the Star topology may allow a larger number of instruments to be connected together,
through the repeater capability of the LXI Trigger Bus Hub/Switch. Signals passing through the
LXI Trigger Bus Hub/Switch are actively driven with transceivers, so the signal integrity issues
that limit the number of devices that may be connected via the Daisy-Chain topology are
mitigated.
In a “pure” Star topology, each instrument is directly connected to the LXI Trigger Bus
Hub/Switch. In a “hybrid” Star (aka Star/Daisy-Chain) topology, an LXI Trigger Bus Hub/Switch
is used to connect a number of Daisy-Chains together, allowing hundreds of EX1629 instruments
to be connected together. It should be noted that in these Star/Daisy-Chain topologies, it is not
possible to perfectly match propagation time, as in the Daisy-Chain configuration. For many
applications, though, the small variations in propagation delays are not important. For very large
acquisition systems, the sample rates are typically low (typically on the order of a few hundred
samples per second), so the small propagation delay induced phase errors are negligible.
Triggering
NOTE
The discussion of triggering below applies equally to arming as well.
There are several triggering modes that may be used in a multi-instrument configuration,
depending on the application’s needs. Fundamentally, all trigger and arm events are transmitted
across the EX1629’s internal LXI Trigger Bus to the actual acquisition hardware. The events may
come from the external LXI Trigger Bus, or from other trigger domains, such as DIO, Timer, and
software. Events from these other trigger domains may be transformed into LXI Trigger Bus
events via the “pattern” trigger source. This “pattern” source is useful in several situations,
including software triggering an ensemble of instruments.
The simplest triggering mode is to configure all instruments to accept an external LXI Trigger Bus
event, such as a positive (rising) edge on LXI Line 3. The Master and the Slave(s) instruments are
configured identically and some external device is used to generate an LXI Trigger Bus event.
The next simplest mode, which works well for a large number of data acquisition applications, is
to trigger the Master device under software control, the Master device being configured to trigger
the slaves, in turn. For this mode of operation, the Master device is configured with a trigger
source of “pattern” (vtex1629_set_trigger_source). No DIO, LXI, or timer events are configured
as part of the pattern; only the
lxiInput
and
lxiOutput
parameters need to be specified
(vtex1629_set_pattern_trig_configuration). Since the Software Trigger is always valid while in
pattern mode, it can be used to trigger the pattern. When the pattern is triggered, via the
vtex1629_soft_trig function, an edge is generated on the
lxiOutput
line configured in the pattern.
The Master device acquisition is triggered based on the
lxiInput
setting – it may be the same as
lxiOutput
, or different, depending on if an LXI Trigger Bus Hub/Switch is being used to route
signals. Slave devices are configured with a non-pattern trigger source equal to the LXI line
specified in the
lxiOutput
configuration of the Master. To trigger the ensemble of instruments in
this configuration, a vtex1629_soft_trig function is used on the Master instrument. All
instruments’ Trigger subsystems must be initialized (vtex1629_trig_init) prior to the software
trigger.
Another common configuration handles the situation when the trigger source is a TTL signal. For
simplicity, this signal can be routed to the Master instrument and the Master instrument can be
configured to generate an event on the LXI Trigger Bus to trigger itself and the Slave instruments.
This configuration is similar to the above, software-triggered configuration, except that the
Master’s trigger pattern is configured with a DIO event or level appropriate to the external TTL
event. The Master device, upon receiving the DIO event will generate a rising edge on the LXI
line specified in the pattern’s
lxiOutput
signal. Slaves are configured to trigger on a positive
(rising) edge.
Содержание EX1629
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