Overcoming the Legacy Equipment Replacement Blues
by Tom Lecklider, Senior Technical Editor
Investing in custom test
systems is seldom a popular decision although it may be the best
choice among a number of unattractive alternatives.
Nevertheless, custom military/aerospace test systems come with a
large amount of risk and expense. The exact requirements may be
changing as the equipment to be tested is developed.
Custom spare parts must be provisioned
together with the basic systems, which increases costs. A big
investment must be made to develop, test, and verify the
software test program sets (TPSs) that actually perform the
automated testing. And, after a number of years, the likely
obsolescence of some commercial off-the-shelf (COTS) test
instrumentation may threaten the useful life of the entire test
system.
Dealing with COTS equipment obsolescence
generally is termed legacy instrument replacement. Legacy
hardware issues have attracted a great deal of attention,
especially from the personnel tasked with purchasing and
maintaining critical test systems as well as from the test
system suppliers. Among the many factors complicating legacy
instrument replacement is the investment already made in the
related TPSs, often several man-years of effort and many
millions of dollars.
It’s as though you needed to replace a broken
part on an old car that you used for daily transportation. The
part is no longer available, but you have invested $20,000 for a
custom paint job and leather upholstery. You might decide to use
a rebuilt part, explore other sources of parts, or modify a part
from some other car. If the car ran well except for the one
broken part, chances are that you wouldn’t buy a different car
and again pay for custom paint and upholstery.
This is the situation in which
military/aerospace equipment maintenance depots find themselves.
A substantial investment in TPSs has extended over several
years, TPS and hardware problems have been corrected, and the
systems generally work well. During the time the systems have
been in use, the equipment they are being used to test often has
been upgraded or refurbished to extend its life. The end result
is aging but working test systems, a new requirement for them to
remain in use much longer than originally planned, and a legacy
hardware issue that must be addressed.
There are several approaches being used
today. Perhaps the simplest solution is to replace the obsolete
equipment with a direct form, fit, and function (FFF)
equivalent. Some test and measurement instrumentation companies
make this kind of product, duplicating the performance of the
legacy instrument even down to known eccentricities and
undocumented features. With an FFF solution, you simply replace
the old instrument and carry on as before. No changes are
required to the TPSs.
Many COTS instrument models never were used
in large enough numbers for production of FFF equivalents to be
economical. In these cases, it may be possible to emulate the
older instrument through a combination of new hardware and
special software.
A few companies specialize in this kind of
TPS migration, distinguishing between this term and translation
because a level of performance verification is anticipated. As
with FFF hardware, some legacy emulation efforts are claimed to
be an FFF solution.
Instrument manufacturers provide command sets
compatible with legacy TPSs. This may be a simple solution in
specific cases but is not as straightforward as it appears.
A modern instrument indeed may perform a
superset of the functions associated with the legacy instrument.
However, will a 1-µs-wide pulse generated by both have the same
noise level? Will similar frequency square waves have the same
rise and fall times? Will it take the identical amount of time
for a command to be executed?
Unfortunately, some TPSs work correctly
because of these kinds of secondary instrument features. When a
new command-compatible instrument replaces the original
instrument, the TPSs may not work as well or may even create
errors. Unless a replacement instrument can be guaranteed to
exactly duplicate the original, additional time and money may be
required to verify that the TPS still operates as intended.
Nevertheless, TPS translation is yet another approach that can
be effective.
In the future, synthetic instruments (SIs)
may minimize the effect of instrument obsolescence. With a
system based on SIs, generic modules are configured and
reconfigured by software to perform the required instrument
functions. However, SIs are relatively new, few if any deployed
test systems are wholly SI-based, and experience will tell
whether this approach lives up to its claim.
FFF Instruments
Geotest-Marvin Test Systems makes a range of
direct replacement pulse, waveform, and function generators. A
typical unit is shown in Figure 1. Several models of
obsolete Wavetek, Tektronix, and Hewlett-Packard generators can
be replaced with these 100% compatible instruments.
Figure 1. FFF Model GP1552W
Pulse Generator
Courtesy of Geotest-Marvin Test Systems
David Manor, vice president of engineering,
explained, "The designs are based on newer technologies and can
operate faster or offer higher performance than the legacy
instruments. Because it’s an FFF replacement, however, these
features are not exposed to the user.
"It’s important that the new instruments can
emulate deficiencies or undocumented features of the legacy
equipment. These anomalies are mostly found in the host
controller interface. All of our FFF replacement instruments use
an FPGA-based architecture, which gives us the flexibility to
easily update functionality in the factory or in the field," he
said.
The Pulse Master Series of pulse/arbitrary
generators from Tabor Electronics also can emulate several
obsolete HP, Fluke, LeCroy, and Tabor generators. Although the
Tabor name may not be familiar to some readers, the company has
extensive test and measurement instrument experience, having
designed and brand-labeled pulse generators sold for many years
by leading instrument companies. Tabor products are marketed in
the United States by EADS North America Defense Test & Services.
Obsolete HP and Agilent Technologies
microwave signal generators can be emulated by the Giga-tronics
2500A Series. In addition, ASCOR, part of Giga-tronics, provides
FFF VXI modules that replace obsolete switching products from
Tektronix, Racal Instruments, VXI Technology, Datron, and
Interface Technology. A few FFF VXI modules not related to
switching, such as the Model 3000-4730 12-Channel D/A Module
that replaces the Tektronix VX4730, also are available.
Products developed as part of ASCOR’s
Tektronix VXI replacement card program have identical types and
locations of front-panel connectors, the same signal on the same
pin, equivalent or improved performance, hardware compatible
with the original Tektronix cards, and a plug-and-play driver
that handles a mixture of ASCOR and Tektronix modules.
An FFF instrument is the only drop-in
replacement that does not require extensive performance
verification. The instrument manufacturer has done that work as
part of the development, precisely because the product will be
used in legacy replacement applications. The bottom line is less
risk, lower cost, and shorter downtime.
Compatible Instruments
For Teradyne’s M9-Series Digital Test
Systems, the company’s modern Di-Series Digital Test Instruments
is a direct replacement. It can emulate obsolete instruments but
also includes new capabilities such as per-channel programmable
low-voltage differential signaling (LVDS) transitions. A TPS
used with L-Series Test Platform instrumentation is compatible
at the source-code level and requires conversion via the TPS
Converter Studio tool. The Di-Series instrument driver supports
legacy M9 binary code or translated L-Series source code as well
as new TPSs through a native C# interface for system integrators
and a C-code interface for TPS developers.
The Di-Series and the Ai-760 Analog Test
Instrument offer increased density and flexibility. A Di-Series
module can be configured as independent virtual instruments and
has per-pin capabilities. While some of these features may be
used for legacy instrument emulation, the Di-Series also
provides room for growth of TPS to address new requirements.
The Ai-760 maintains the earlier Ai-710
parallel test capability but really is aimed at system
integration. It replaces multiple conventional instruments such
as a 6.5-digit DMM, a timer/counter, an Arb, a digitizer, and a
VXI oscilloscope. Clearly, higher density is not necessary in an
FFF replacement instrument, but it would be useful were a test
system to be redeveloped to address further requirements.
Teradyne’s motivation in developing the Di
and Ai Series was to support the company’s own current as well
as legacy test systems. Nevertheless, both the Di and Ai Series
are considered core system instrumentation products and were
designed to replace other manufacturer’s legacy instruments such
as digital word generators.
Agilent makes a wide range of products that
can replace legacy instruments, usually older HP or Agilent
models. The company offers software that translates command sets
so that programs written to run on an older model will still run
on a new one. Of course, that doesn’t guarantee that any
undocumented features will be supported, just intentional ones.
A new product can be compatible with a TPS
but is not necessarily an FFF replacement for the instrument
originally controlled by the TPS. In some cases, making such a
replacement might cause no problems at all. Comprehensive TPS
validation is needed when a compatible instrument is introduced
into an existing test system unless that instrument is FFF
guaranteed.
TPS Translation/Migration
According to EADS Business Development
Manager Mike Rutledge, "Translation can be a low-risk process
given a disciplined approach coupled with a few critical
software tools. If the legacy system was well engineered, then
the replacement modern instrument can support the required TPS
functionality in a similar vein. In addition to preserving
previous investments, modern instrumentation and technologies
can provide a basis for planned technology insertion. This
ensures that customers have the capability to migrate to future
workloads without the need to reinvent the wheel."
EADS prefers the term migration rather than
translation because some amount of validation is anticipated.
Mr. Rutledge commented that translations are never 100% perfect
because too many variables are involved. On the other hand, they
can approach 90% with the remainder of the task handled by sound
engineering practices in a structured environment. The company
has been doing this type of work for more than 10 years.
Geotest also is involved in translating TPSs,
but primarily those that use digital subsystems. Mr. Manor
explained that the host controller interface plays a less
significant role, making translation practical. The critical
issue is having a set of tools that can accurately translate
test vectors and sequences to the target digital subsystem. In
Mr. Manor’s experience, translating or porting digital test
vectors to a new digital subsystem requires less effort and has
less risk than emulating a host controller interface with its
undocumented features.
WinSoft® makes a command processor
that can emulate a range of instruments. The processor is
presented with the unmodified TPS, and it then drives new
instruments to emulate the performance of the legacy equipment.
Up to 13 instruments can be emulated by one WinSoft
Instrumentation System Emulator (WISE™) box, a number set by
GPIB limitations. Agilent has worked with WinSoft on WISE
projects to refresh system life for many customers using old HP
equipment (Figure 2).
Figure 2. WISE Emulator Module
Courtesy of WinSoft
Ehud Shany, WinSoft’s president/CEO, said
that several of his company’s customers in the
military/aerospace markets had not had success with TPS
translation. "The time and money involved with the translation
are about 50 times more than with the WISE approach. And, the
translation cannot be done in those cases where the source code
is not available or if the ATE is classified. In addition,
revalidation of the software can be costly, as much as the
translation process itself."
SI Systems
A DoD initiative created a working group for
RF SIs, initially known as the Synthetic Instruments Working
Group (SIWG). This group completed specifications for synthetic
DACs, up-converters, down-converters, and ADCs. The IVI
Foundation has taken over with the intention to create IVI
classes for these modules as well as switching, signal
processing, and software-algorithm or signal-conditioning SI
blocks.
SIs attracted a lot of attention after their
successful integration in the Agile Rapid Global Combat Support
(ARGCS) technology demonstration exercise. In this case, a
common core of SI hardware and software replaced legacy RF and
microwave test equipment from older test systems. The project
proved that SIs could replace legacy instruments and be
interoperable. The same SI configuration could perform
equivalent measurements that once ran on four types of testers
using different hardware.
Test systems based on SIs make sense in many
ways. They save space, weight, power, and money and have an
upgrade path for future requirements. On the other hand, there
are limits to what is practical. For example, a group of
functional modules with similar frequency ranges typically is
packaged together and programmed to perform a number of
measurement jobs. It might be less advantageous to combine
precision DC instrumentation with a high-frequency digitizer and
RF generator.
Within well-defined functional limits,
SI-based test solutions are becoming available from several
suppliers. A good example of an SI-based test system that from
the outside appears to be a single instrument is Teradyne’s
Bi-41x Bus Test Instrument. Using the same hardware, it can
handle RS/IEA-, ARINC-, and MIL-STD-1553-style buses. These
types of serial buses are considered relatively low speed but
otherwise quite distinct and, in the past, required separate
hardware instruments.
The Bi-41x also is compatible with variations
on standard buses intentionally or unintentionally designed into
a weapons system. Without the flexibility of an SI-based
solution, past test systems have required additional
test-specific instruments to address nonstandard buses.
Despite the attractiveness of SI systems,
Charles Greenberg, EADS senior product marketing manager and an
active participant in the SIWG for two years, cautioned, "The
unique capabilities of a legacy instrument may not be duplicated
by a generic, purely synthetic approach. Care must be taken to
match the measurement modes very closely to avoid changes to the
TPS necessary for the replacement SI.
"We sometimes take a hybrid synthetic
approach
to legacy replacement by assembling COTS synthetic building
blocks with custom signal conditioning circuits," he continued.
"In this way we can create the combinations needed to reproduce
the required legacy signals and measurements."
Giga-tronics has combined its signal
generation, up/down conversion, and switching expertise in
flexible and reconfigurable SI systems. On the other hand, much
of the IP developed as instrument firmware in stand-alone
products now must be provided in the SI control software. This
makes system development more difficult and the resulting
software more complex.
Agilent’s Application Note 1465-24, Using
Synthetic Instruments in Your Test System, covers many
aspects of SI-based systems. Comparisons are made between the SI
approach and the VXI, PXI, or GPIB architectures. Very broadly,
SI-based systems shift the emphasis from hardware to software (Figure
3).
Figure 3. Model 8200A Synthetic Instrument Platform
Courtesy of Agilent Technologies
In particular, software component
interchangeability is an area that SI vendors must address
through new software tools. Software module reuse is as
important as hardware reconfigurability in reducing SI system
development cost.
Agilent Product Manager Dan Pleasant
highlighted the software standardization that SIs support: "An
SI module by itself performs fewer functions than a classical
test instrument. This makes it easier to write standard IVI
drivers that cover all of a module’s functionality. It follows
that it also should be easier to write application software
using those drivers that works well with different underlying
test hardware."
Against this advantage, Mr. Pleasant noted
the increased complexity of SI control software. When SI modules
are used, a system designer must acquire and embed the necessary
measurement science previously resident in separate instruments.
NIST traceability will be difficult to ensure because no single
manufacturer has control. And, with no front panel, an SI-based
test system can be more difficult to debug.
A further implication of reconfigurable
SI-based system development is the need for a thorough review of
calibration and self-test capabilities. Geotest’s Mr. Manor
commented on replacing a legacy instrument with one based on
SIs. "For complete compatibility, it also is necessary to
accurately emulate the legacy instrument’s self-test function to
ensure that the system’s self-test functions still execute
correctly with the replacement SI. Using an SI provides
flexibility, but the implementation/verification/certification
effort can require significant time and effort."
Conclusion
A test depot manager’s main responsibility is
described succinctly: He must keep the test systems running.
Problems arise when an obsolete instrument has to be replaced.
If replacement coincides with a TPS upgrade, perhaps the
manager’s available options are greater. Some amount of TPS
revision might be accommodated given that the software is being
changed anyway.
If only the legacy instrument is to be
replaced, an FFF solution appears to attract the least risk and
perhaps is the lowest cost solution. It certainly is the easiest
form of replacement assuming that a suitable FFF instrument
exists. If one does not, the choice widens to include compatible
instruments and instrument emulation.
WinSoft’s WISE emulator is claimed to be an
FFF solution based on instruction translation and new,
compatible hardware. Mechanically, a 1U-high emulator must be
housed together with the new instrument, but this may be only a
small inconvenience. The WISE solution requires no TPS changes
so it generally is preferred to an approach that does involve
TPS modifications and verification.
Similar emulation solutions from EADS,
Teradyne, and Geotest also preserve the TPS investment, which
often is the primary objective. Depending on the instruments
involved, some degree of TPS change may be required.
Longer term, test system rationalization via
a common set of SI modules and software components may occur.
Certainly, SI-based test systems are becoming more prevalent as
are specific types of flexible instruments internally based on
SI-style architectures.
WinSoft’s Ehud Shany distinguished between
SI-based systems designed to minimize future obsolescence
problems and stand-alone replacement instruments that address
current obsolescence. Teradyne’s Peter Hansen, a product line
manager, emphasized the major impact SI-based systems can have
on life-cycle costs. Because the cost of adding instruments to a
test system often is underestimated, the flexibility of an
SI-based system can be very important.
To assist initial test system instrument specification as
well as later legacy instrument replacement, a standard way
to describe instrument capabilities using the Instrument
Description XML format is being developed. EADS’s Chris
Gorringe, co-chair of the ATML working group, explained, "ATML
allows the instrument performance specification and capability
to be captured in a common and reusable manner. This means that
users can compare and contrast different instruments, both for
requirement evaluation and as a legacy replacement exercise.
Obviously, the real benefit will come when all instrument
specification sheets are expressed online in this format."
| FOR MORE INFORMATION |
|
Click below |
| Agilent Technologies |
N8200A SI Platform and SI |
Click here |
| ATML Working Group |
ATML Instrument Description |
Click here |
| EADS North America Defense Test & Services |
Racal Instruments 3171 Pulse Generator |
Click here |
| Geotest-Marvin Test Systems |
GP1552W Pulse Generator |
Click here |
| Giga-tronics |
Tektronix VXI Replacement Card Program |
Click here |
| Tabor Electronics |
Pulse Master Series Pulse Generators |
Click here |
| Teradyne |
Bi-41x Bus Test Instrument |
Click here |
| WinSoft |
WISE Emulator |
Click here |