Selecting Your Optimum LXI Feature Set
by Paul Schreier, Editor, LXI ConneXion
More than a year ago, a few suppliers of LXI
instruments were starting with LXI Class C to develop enhanced
instruments.1 Class C makes up the core LXI features
on which Class B and A instruments are built.2 Now
the LXI Consortium has formalized such expanded feature sets in
Version 1.3 of the specification, which recently was approved.
Integrating Additional Features to a Class C Instrument
Why is it desirable to add features to a
Class C LXI instrument in this way? Here’s the situation:
Suppose a supplier of a Class C instrument believes that its
customers would benefit greatly from more but not all features
that fall under the other classes. The capability to add
selected functions is a boon to both manufacturers and users.
Users don’t have to pay for features they
don’t necessarily need. This aspect will be especially important
for instruments at the lower end of the market such as portable
or other low-cost units that use microcontrollers with limited
resources to save cost or power consumption. Users also can
benefit from a broader selection of instrument products with
added features that can be brought to market sooner.
Manufacturers can use such a partial
implementation of Class A or B as a steppingstone to a higher
class instrument without holding back on the instrument’s market
launch. Designing a Class C instrument isn’t as major an
undertaking as developing a Class B or Class A instrument,
especially now that so many instruments already have Ethernet
ports.
However, and this is especially true for
companies new to the LXI market, it can take significant time to
develop a full Class A or B instrument and go through the
conformance testing. If a manufacturer can select only certain
features, it can get its products to market faster and at a
lower cost. For example, a user may find the LXI LAN messaging
feature of Class A and Class B is useful, but the addition of
IEEE 1588 and the triggering API is less useful and creates
trade-offs elsewhere.
There is nothing in the current or past LXI
specifications that prohibits LXI Consortium members from
introducing these additional features. In fact, some Class C
instruments have included the wired trigger bus (WTB) from Class
A for some time.
But because such an instrument doesn’t
include all Class A features, it’s not eligible for Class A
certification. And even though the LXI Consortium may have
tested the WTB implementation and declared it compliant to that
section of the specification, until now there has been no
official way to tell users about this fact.
Such certification is important for users to
gain full confidence in the interoperability of a non-Class A
instrument’s WTB in an LXI system. Interoperability testing is
one of the strengths of LXI. It has been very effective. Among
the LXI-certified instruments on the market, there have been no
interoperability problems, which is a great asset to anyone
assembling a test system.
With Version 1.3, vendors of Class C
instruments now can submit selected features from Class A/B for
testing: trigger bus, event messaging, clock synchronization
using IEEE 1588, timestamped data including clock
synchronization, and event logs including clock synchronization.
So how will users know that these extra features exist?
There are no plans for a special label on the
front of the instrument, but these extra capabilities can be
indicated on the instrument’s welcome page on the Web. Then a
product description will include a phrase such as Class C plus
LXI Trigger Bus. Those capabilities also can be listed on the
LXI Consortium website in the master list of products and in the
company’s marketing materials and corresponding help files and
manuals.
Only Certain Features Qualify
One aspect the LXI Consortium must deal with
in establishing added features to Class C instruments is
conformance testing, which ensures that these features meet all
the requirements in the spec. It is important to note that the
unit remains a Class C device.
The exception would be if all of the features
of another class were added to the device. For example, if a
manufacturer were to implement and test all five of the Class A
features in a Class C instrument, the result would be a Class A
instrument.
Most of the Version 1.3-added Class A/B
features are relatively self-explanatory. However, the
difference between timestamped data and event logs is worth
mentioning. With timestamped data, samples can be correlated
among many instruments so that you could, for example, see how a
change in a power supply was reflected in a UUT. An event log,
in contrast, records actions in an instrument such as trigger
received, alarm detected, measurements started, or error
occurred.
Adding WTB
A handful of vendors added extra features to
Class C instruments even before Version 1.3 allowed them to be
promoted as LXI approved. At the moment, these instruments fall
into two categories: those that have added the LXI trigger bus
and those that have added LXI event messaging.
Figure 1. Rohde & Schwarz FSL Spectrum Analyzer With Class C and LXI Trigger Bus
Rohde & Schwarz was the first to receive
conformance certification for adding the WTB to its FSL Spectrum
Analyzer (Figure 1). The box has a slot that accepts an
option card populated with an FPGA that implements the routing
of the trigger signals across the bus.
According to Jochen Wolle, head of software
R&D, external triggering is traditional in instruments such as
spectrum analyzers, signal generators, and scopes. Customers
know how to use it. But because it defines a standard connector,
the WTB eliminates problems with incompatible cable connector
types. Further, with the WTB there’s no longer a need to go
behind the instrument and reconfigure the wires on each setup
because the lines on the WTB are software configurable.
At the moment, the FSL is Rohde & Schwarz’s
only LXI instrument with the WTB. The LXI trigger bus can
connect to almost any other triggered instrument if users add
the Model 60-982 LXI Wired Trigger Bus Adapter from Pickering
Interfaces. This small external device measuring 30 x 84 x 100
mm converts low-voltage TTL (LVTTL) signals to the M-LVDS
signals on the eight-line WTB or vice versa, and it supports all
modes of WTB operation.
Two versions are available: the thru-line
model provides two WTB connectors along with 16 SMB connectors
for the LVTTL inputs and outputs from other instruments, and the
other includes just one WTB bus connector and a WTB termination
along with the 16 SMBs. The unit can be configured manually or
with digital I/O.
Another early adopter of the WTB as an extra
feature was C&H Technologies. When it introduced the EM-405-8
LXI Ethernet M-Module Carrier/LXI Bridge two years ago, that
company added the WTB to this Class C instrument and has
received conformance certification for the WTB functionality (Figure
2). The box, housed in a 19"
rack-mount 1U-high enclosure, provides Ethernet connectivity for
up to eight ANSI/VITA 12-1996 single-wide M-modules or a
combination of double- and triple-wide modules. An IVI driver
supports control of all bridge functions. The -0001 version
implements the eight LVDS triggers required for the LXI trigger
bus.
Figure 2. C&H Technologies EM-405-8 LXI Bridge With Class C and LXI Trigger Bus
This capability was added because many C&H
customers are familiar with VXI and its high-speed backplane
triggers, explained Gary Guilbeaux, vice president and CTO.
Also, many of the firm’s products are M-modules with clocking
features. With this LXI bridge product, not only can eight
modules trigger among each other, but using a common clock they
also can trigger among multiple bridge boxes.
To make the EM-405-8 a Class A instrument,
C&H still needs to add IEEE 1588 clock synchronization,
timestamping, event log, and event messaging, but Mr. Guilbeaux
noted that doing so is more difficult to implement than the WTB.
Besides, he added, until now there has been limited customer
demand for 1588, but it is starting to pick up. The company does
plan to add the features necessary to allow this instrument to
jump directly to a Class A.
Figure 3. ZTEC Instruments ZT 42XX Series Oscilloscope With Class C and LXI Trigger Bus
More recently, ZTEC Instruments has combined
Class C with WTB capabilities in several of its digital storage
scopes, which are packaged in half-width 1U chassis in the LXI
versions (Figure 3). All of them digitize with 8 bits of
resolution, offer either two or four channels, and spec
interleaved sampling at rates to 1 GHz (ZT 4211/12) or 4 GHz (ZT
4611/12).
WTB enables users to configure multiple
instruments to trigger using either point-to-multipoint (driven
mode) or multipoint-to-multipoint (wired-OR mode). The beauty of
the WTB is that multiple LXI instruments can be daisy-chained
together to achieve very low timing uncertainty since there is
no software latency, according to Boyd Shaw, director of
marketing and product stragegy at ZTEC.
A very simple example would be where an LXI
oscilloscope triggers on a pattern of signals such as CH1 – H,
CH2 – H, and CH4 – L. Upon detecting this pattern of inputs, the
scope could use the WTB to trigger an LXI power supply to
decrease its output level and to trigger an LXI switch to change
its settings.
LAN Messaging
LAN messaging was a requirement that Kepco
found among its users, so that feature was built into the Class
C Model KLP-600-4-1.2K Hyperbolic Power Supply from the start (Figure
4). Now with Version 1.3, the only thing holding the company
back from getting conformance certification as a Class C and LXI
Event Messaging product is an LXIivisync interface, which the
company expects to have shortly.
Figure 4. Kepco KLP-600-4-1.2K Power Supply With
Class C and LXI Event Messaging
In this lab supply, the voltage/current
limits automatically are recalculated, forming a constant-power of up to 1,200 W
hyperbolic-shaped boundary between the voltage and current
modes. This curve, which replaces the single max-power operating
point of conventional power supplies, provides the user with a
greatly expanded choice of maximum-power
V-A combinations.
With LAN-based events and alarms, multiple
power supplies can intercommunicate without the need for a
central PC by multicasting messages to all LAN devices and
synchronizing the activities of multiple supplies within 100 µs.
The KLP can send LAN event messages that are alarms to other
devices. The alarms are triggered when a unit reaches its
current limit or has a fatal error such as missing sense
connections.
Upon receipt of such an alarm, another KLP
can take one of three actions: change its status from Off to On
after a time period from 10 ms to 1 min, change its status from
Output On to Output Off after a similar time span, or shut down
the unit within 5 ms in Immediate Output Off. Accordingly, if
one supply determines that it has an overcurrent condition, it
can send an alarm message to shut down any other supplies in a
large test system and prevent the UUT from getting damaged—and
again, without any PC intervention.
Attractive Feature Sets
Beyond WTB and LAN messaging, other features
such as adding 1588 clock synchronization from Class B will
likely turn out to be a big driver in sales. So believes David
Owen of Pickering Interfaces, who also is chairman of the LXI
Technical Committee. With 1588, users gain the ability to
synchronize events and record what has happened across a
distributed test system.
Adding 1588 initially was a major task for
most companies. Suddenly, though, with the emergence of 1588
chipsets, the barrier now is lower, and it becomes interesting
to suppliers of low-end equipment. A lightweight
data-acquisition box or process-control system could benefit
greatly from such synchronization. However, a vendor might not
want to add all Class B functionality because of additional
development expense and increased end-product price.
Certain combinations of feature sets will
likely prove very popular, added Mr. Owen. Market forces will
come to bear, and users will tell suppliers which combinations
they want. For instance, he sees that the combination of 1588
and event logging makes a lot of sense because it will be great
for debugging distributed systems. It will be very interesting
to follow developments along these lines and see what innovative
new instruments come on the market.
References
1. Schreier, P. G., "Climbing the LXI
Learning Curve," LXI ConneXion, September 2007, page 25.
2. Schreier, P. G., "The Killer Bs Are
Coming," LXI ConneXion, September 2008, page 50.
3. LXI Consortium,
www.lxistandard.org
About the Author
Paul G. Schreier is a technical journalist
and marketing consultant working in Zurich, Switzerland. He was
the founding editor of Personal Engineering & Instrumentation
News, served as chief editor of EDN Magazine, and has written
articles for countless technical magazines. Currently, he is the
editor for LXI ConneXion at EE-Evaluation Engineering. Mr.
Schreier earned a B.S.E.E. and a B.A. in humanities from the
University of Notre Dame and an M.S. in engineering management
from Northeastern University. e-mail:
paul@pspr.biz