| LXI basics |
LXI basics
- a summary or tutorial about LXI, LAN eXtension
for Instrumentation, what it is and how it works, and how it can be used for
test instrumentation
Ethernet 802.3 is a technology that has gained almost
universal acceptance as a LAN technology, enabling widespread connectivity
to be achieved. It is therefore a natural step to adopt Ethernet as a
communications medium for linking test instruments. Although Ethernet has
been used in this role for some time, the problem is that the there is no
standard and the way instruments communicate over Ethernet varies from one
instrument to the next. It is this problem that LXI seeks to address. In
this way LXI will simplify the development of automated test programmes and
equipment that operate over LANs using Ethernet.
Background to LXI
For many years there has been a growing trend towards automated test systems
using bus technologies to enable a central controller to coordinate several
test instruments, initiate measurement and store and process the results.
With electronic equipment becoming more sophisticated, so too the need for
more comprehensive testing has developed. As already mentioned the GPIB was
developed in the late 1960s and early 1970s. This provided the first generic
standard that could be widely used. Realizing the size of many test systems,
VXI technology was developed. It used as its core the VME bus. Although
fast, and gaining wide acceptance it was nevertheless expensive. As a result
PXI and a number of other technologies came to the fore.
However with Ethernet appearing to offer an ideal medium
for fast, efficient and cheap communications there was a need to develop an
industry standard to address the problem. If no common standard were
developed, then different instruments would have different interfaces and
complete implementations of ATE systems using Ethernet communications would
not operate as well as would be possible using a standardised interface.
To overcome this problem a consortium was set up in 2004.
Known as LAN-based eXtensions for Instrumentation, LXI, its aim was to
ensure that there would be interoperability between local area network
(LAN)-based instruments. As such LXI builds on the Ethernet standard. It was
thought that a host of independent implementations for instrumentation over
Ethernet would result in poor interoperability and prevent this becoming a
viable successor to GPIB.
Ethernet has been in existence for many years. However it
is still at the forefront of technology as a result of the fact that it has
been continually developed. By adopting it as a standard, LXI technology is
likely to see a long life. This is good news for any company investing in
the technology either as a developer of equipment, or as a user buying
equipment and systems.
Basic concepts
The LXI standard uses an Ethernet backbone. Instruments are designed to be
rack mounted and contain their own power supplies. This gives a number of
advantages. The first is that it removes all the problems of heat
dissipation, crosstalk and the like that occurred in systems such as VXI.
Secondly, the fact that Ethernet can allow communication over a wide area
enables the creation of test systems with LXI that cover a wide area.
Conceivably using switches, routers, or hubs, an LXI test system could be
created to cover a whole production area using a single controller and in
this way provide a highly integrated system.
The standard
The LXI standard addresses both the mechanical and electrical aspects of the
equipment. In this way it means that equipment not only operates together,
but can also be mounted in the same way, allowing systems to be built more
easily.
The LXI standard defines that test equipment is rack
mounted, either one or two units high, and either full or half width. LXI
also defines that the equipment contains its own power supply, processor and
LAN connections. Although these may appear obvious, setting down a standard
enables full compatibility.
Additionally as the LXI instruments are controlled via a
LAN connection, they do not require controls of displays. Instead the user
can interact with them via web browsers and COM drivers. In this way
redundant controls are eliminated and the cost of the LXI instruments is
reduced. Additional mechanical constraints call for the LXI instruments to
have signal input / output on the front panel, and AC power and LAN
connections at the rear.
The electrical of functional elements of LXI are being
defined. A wide variety of issues are being defined so that the instruments
can operate together in a similar manner. To achieve this it is necessary to
address a variety of factors including triggering, interrupt handling,
discovery, synchronization of multiple devices, and software interfaces. Of
these timing, synchronisation and triggering are some of the most
challenging as the distances involved and delays in the system can introduce
small delays that may be unacceptable in some circumstances.
The triggering will use a Trigger Bus that will be
similar to that used on GPIB, and will also be compatible with VXI. It will
use a high speed differential LVDS interface.
Timing is another important issue, especially where
sub-microsecond synchronisation is needed. To enable the LXI to be able to
achieve the required levels of performance the IEEE 1588 Precision Timing
Protocol has been adopted. This allows enables users to simplify cabling by
triggering instruments over the LAN instead of using the Trigger Bus. It ahs
also been developed for use over LAN applications to provide accurate
synchronisation regardless of the separation of the elements.
Summary
With Ethernet usage growing, and the need to fast data communications to
link test and measurement instrumentation, LXI is bound to make a large
impression on the market place when it is fully defined and launched. The
specification is now in its advanced stages, before being fully adopted.
Even before its final ratification, it is likely that product will hit the
market in anticipation. When this happens it may sound the death knell for
GPIB which has given magnificent service for over 30 years.
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