Abstract - The VXI-bus is the platform of choice for many military
and commercial test systems primarily because it offers the user a solution that
is open-platform based, computer and operating system independent, and highly modular.
The densities and performance offered are unmatched by any other platform, and multiple
vendor support has ensured long life-cycle management. The emergence of the LAN
extension for Instrumentation (LXI) Standard, leveraging all the advantages of Ethernet
while adding several critical components necessary for true test and measurement
compatibility, has offered a glimpse into the future of test system interfaces.
However, what options are available to the user to ensure existing system sustainability,
and can the LXI Standard truly solve all test applications currently addressed by
other open-architecture platforms?
Historical data indicates that test engineers are accustomed to integrating hardware
with different physical interfaces. For example, power supplies may only provide
an RS232 interface, a high frequency spectrum analyzer might utilize GPIB, and the
switching and general purpose source and measurement instrumentation are housed
in a VXI chassis with a MXI-2 parallel interface. This combination of interfaces
is required because the test engineer must select the best solution to address his
requirements; however, the approach inherently drives the cost of the system as
well as complexity.
One of the fundamental advantages of LXI-based instrumentation is that all devices
can reside on a common inexpensive interface that is extremely stable and that has
been evolving for the past forty years. The interface is platform and operating
system independent, and is integrated into nearly every computer available on the
market. Even with these advantages one must evaluate the various use case scenarios
in order to determine if LXI can address all applications and if not, what must
be done to ensure that engineers are still not faced with the dilemma of mixing
and matching interfaces.
A common use case likely to arise involves the need to tightly integrate a sequence
of control signals in a source, measure, and switch scenario where deterministic,
repeatable operation is ensured. Often this type of application also involves a
large number of test points, and may also include the need to switch high voltage
current levels. The VXI platform is an ideal solution for this type of application,
but standard interfaces available today will still result in the creation of an
interface-restricted subsystem. The key to resolving this issue is a bridge between
VXI chassis and the LXI network community.
The concept of a bridge device immediately expands the utility of the LXI beyond
that of just another new emerging standard. This implementation will enable a wide
range of current and future test systems to leverage the advantages of LXI, but
there are still fundamental issues that must be addressed before wide adoption is
possible. Several issues of critical importance include:
- Bridge Device Functionality
- Hardware Triggering
- Software Interface
- Test Sequencing
BRIDGE DEVICE FUNCTIONALITY
One key purpose of a bridge device is to provide the communications link between
the platform of interest and the LXI network. Essential characteristics of this
device include compliance to all LXI Class C requirements which define the network
and LAN functionality; device discovery, IP address allocation, and behavior in
the event of
network conflicts are just some of the requirements. However, how will a platform
that incorporates a host interface to control a variety of independent, highly synchronized
devices, such as VXI or PXI instruments and switches, function
within this environment?
An LXI-VXI slot-zero control bridge, for example, must perform all of the functions
expected of any VXI controller, in addition to providing the LXI interface functionality
for the external network. This includes providing a communications path for the
host computer, facilitating instrument and switch card discovery (within the chassis),
memory allocation, trigger distribution and generation, and error reporting. Individual
instruments within the chassis will continue to be identified utilizing the familiar
VISA resource name scheme:
TCPIP[board]::host address[::LAN device name][::INSTR]
Each of the instruments or switch cards within the chassis will function independent
of the LXI interface, and retain all of the exceptional timing and synchronization
characteristics that have driven wide industry acceptance of platforms such as VXI.
Internal backplane functionality is unaffected by the LXI bridge, and performance
characteristics such as data transfer rates, device triggering, high-speed local
bus, and power supply capabilities are retained. Furthermore, other devices (LXI,
LXI-VXI Controllers, or non-LXI) can easily interface with bridge devices, thanks
functionality described in the Hardware Trigger Section.
Another key attribute of any bridge device is the ability to seamlessly transition
between interfaces. A typical test system will have significant manhours invested
in software and test program development; therefore, the ability to minimize the
impact on currently fielded software is essential. Updating a traditional VXI-based
system should be as simple as removing the existing slot 0 interface, updating the
instrument drivers, and installing the new LXI-VXI slot-zero control bridge.
The transition to a LXI-VXI bridge will typically involve no user code modifications.
This is primarily because of the functionality provided by the VISA I/O libraries,
which are designed with utilities for configuring, programming, and troubleshooting
instrumentation systems. There are essentially two widely accepted VISA implementations
in the market, Agilent VISA and National Instruments (NI) VISA, and the upgrade
process varies slightly depending on the architecture that the manufacturer has
adopted. The process defined in Figure 1 assumes that the LXI-VXI slot-zero control
bridge implements the Agilent VISA interface, and illustrates the steps required
for upgrading the bridge in both a native Agilent and native NI environment.
One step of particular interest involves adding the interface using Agilent’s Connection
Expert. This step essentially maps the LXI compliant resource string to the VXI
resource string that would be in use in the existing system (See Figure 2). Once
this has been completed all existing code will execute without any additional modifications.
The most accurate synchronization mechanism between multiple devices, regardless
of the platform, involves the implementation of a hardware trigger interface. Most
functional test applications follow a relatively straight forward approach that
involves defining a signal path, applying a stimulus to the unit under test (UUT)
and then measuring the results. The key to generating accurate results is often
linked to the timing associated with the test sequence, and this is where the trigger
interface comes into play. As a result of this requirement a high-performance trigger
interface, the LXI TriggerBus has been implemented in LXI Class A devices and provides
the link between all devices in the test system for both triggering and clock signal
Deterministic trigger generation and propagation between multiple devices is accomplished
with an eight-channel, multipoint, low-voltage, differential signal (LVDS) interface.
This architecture permits individual lines to be configured as a source and/or receiver
and supports external, time-based or software-generated triggering as well as clock
distribution. Common topologies are supported including star, daisy-chain, and hybrid
configurations, providing the flexibility to distribute the trigger lines as dictated
by the application requirements. Additional flexibility is realized with the addition
of a star hub; this device permits very tight trigger tolerances to be maintained
throughout a large distribution network (See Figure 3 and 4)
Additionally, the TriggerBus is automatically extended into the VXI platform with
a LXI-VXI slot zero control bridge, providing a mechanism to link a VXI chassis
and all other LXI hardware. The LXI-VXI slot-zero control bridge will provide a
direct extension of the eight VXI trigger lines to any external device, providing
the ability to individually control specific instruments and switch devices with
the VXI chassis. This type of flexibility will provide the user the ability to integrate
stand-alone instruments, such as spectrum analyzers and power sources, into a homogeneous
test environment leveraging the strengths of each subsystem. Required functionality,
such as a high-density, highperformance switch subsystem, uniquely inherent in VXI-based
devices, can function transparently with LXI-based synthetic instruments without
additional integration activities (See Figure 5).
LAN synchronization, incorporating the IEEE-1588 Precision Time Protocol (PTP),
highlights another fundamental advantage of LXI Class B devices. PTP defines a precision
clock synchronization protocol for networked measurement and control
systems. The protocol is designed to enable the synchronization of systems that
include clocks of different precision, resolution and stability. Submicrosecond
accuracy can be achieved with minimal network and local clock computing resources,
and with little administrative attention from the user.
There are several ways in which PTP can be implemented ranging from user level software
control, to kernel-level driver modifications, to hardware implementations utilizing
dedicated FPGA devices. The highest level of precision is obtained when hardware
implementations assist in the time stamping of incoming and outgoing
network packets or frames; delay fluctuations can be in the nanosecond range with
this approach. PTP provides multiple device synchronization while eliminating the
need for external cabling between devices. Utilization of this approach is less
accurate than hardware triggering; however, Giga-bit Ethernet can provide synchronization
times in the hundreds of nanoseconds range.
Synchronization of hybrid test systems, including standalone instruments as well
as VXI-based subsystems, can easily be accomplished with a test approach that utilizes
time based execution. The background PTP functionality ensures that each device
is synchronized to within a high degree of certainty of one another. Test sequences
can then be initiated at specific time slots based upon the relative PTP time. An
LXI-VXI slot-zero control bridge can respond to a PTP based event in a number of
different ways ranging from initiating a specific control source/measure function
on a discrete instrument to generating an entire sub-system test sequence involving
multiple source, measure, and switch devices.
The application space that a solution addresses will clearly dictate the approach
required for control, data transfer, and software and hardware handshaking. Many
data acquisition applications only involve gathering large amounts of data and
then transferring large quantities of data, typically referred to as block mode
transfers. Conversely, many functional test applications require a significant number
of single measurement related commands and transfers based upon events that
occur during the test process. Again, a common test sequence involves setting the
test path through switching, enabling source devices, and then measuring the results.
A certain amount of delay is built into any test sequence involving relays due to
settling and debounce times (3-5 milli-second typical) inherent in mechanical devices;
however, not all test processes involve a continual step-sequence such as this.
A number of signal paths may be defined followed by analog source level changes
or digital pattern modifications that involve extremely fast setup times. The hardware
TriggerBus interface provides a deterministic control path for updating the state
of a device or acquiring a result, and the LXI-VXI Bridge extends this capability
to all devices on the backplane without additional hardware.
A seamless link between existing openarchitecture hardware platforms and LXI devices
is essential if wide industry acceptance of the interface is to emerge. The adoption
of any new standard is typically an incremental process that involves combining
various devices in a hybrid configuration that enables the end user to
evaluate these technologies prior to wide acceptance. Therefore it is essential
manufacturers provide the mechanism to facilitate this transition. Furthermore,
there will always be applications that demand the performance of open-platform,
chassis-based modular systems such as VXI, and providing bridge interfaces completes
a logical link into these systems. Engineers seldom appreciate having their design
path dictated, and welcome the freedom to choose the solution that best fits the
need. Bridge interfaces and transition devices will ensure this freedom of choice
and foster the adoption of the LXI Standard in a well thought out, logical manner.