A Subsystem that Behaves as an LXI Instrument

A Subsystem that Behaves as an LXI Instrument
Brandon Thorpe
Military and Aerospace Division
Teradyne, Inc
North Reading, MA
This paper describes an intelligent subsystem that
integrates into a larger test system as if it were a
single complex instrument. Actually, the subsystem
contains a controlling embedded PC and various
instruments that conform to the PXIe test standard
working in tight coordination to perform complex
tasks. The test station computer communicates
with the subsystem with a high-level IVI driver via
LXI, or LAN Extensions for Instrumentation, is a set of
extensions that provide increased interoperability and
functionality of LAN enabled test and measurement
equipment. The LXI Consortium was founded in 2004, and the
LXI 1.0 specification was released in September 2005.
Instruments that are LXI compliant can be discovered,
configured, and controlled over Ethernet as opposed to GPIB,
VXI, or PXI. LXI is suitable for customers that don’t like
specialized cards and cables. LXI is also suitable for
customers who don’t like expensive slot 0 controllers. The
LXI consortium was formed after instruments with USB and
LAN interfaces became enormously popular. Without a
triggered backplane, LXI devices are triggered much like
traditional GPIB instruments, where users can send a trigger
over the Trigger Bus to start a measurement from an external
LXI is an attractive choice for integrating test
instruments into a larger test system largely due to
the interconnection simplicity provided by
Ethernet. LXI is typically used with individual
instruments, not a group of instruments controlled
with an embedded PC. While it is possible to
control all of the PXIe instruments directly from
the main test station computer, the task of
integrating the functions would need to be repeated
for each test system design. A novel solution is to
configure the subsystem as a single LXI compliant
device with extensive localized and reusable
The PXIe chassis that houses the subsystem
instruments is controlled with an embedded PC,
which hosts an LXI discovery service and IVI
remote driver. The services are implemented using
C, C# and .NET on an embedded 64-bit Windows 7
operating system. By developing LXI code that
runs on the embedded PC housed within the PXIe
chassis, we treat the entire subsystem as an LXI
compliant device.
Figure 1: High Speed Subsystem device. To the left end of the chassis is
the embedded controller, in the slots are Teradyne LVDS core
This paper shall discuss the high level API with
which the subsystem is controlled, as well as the
architecture of the services running on the
embedded controller that make the subsystem LXI
978-1-4244-9363-0/11/$26.00 ©2011 IEEE
The High Speed Subsystem is a subsystem which is controlled
by an embedded 64-bit Windows 7 computer. The computer
acts as the slot 1 controller for a PXIe chassis, housing multiple
sub-instruments. Each High Speed Subsystem has at least one
LVDS core sub-instrument and may have multiple expansion
instruments with unique test capabilities. Data acquired from a
unit under test can be processed using either the programmable
microprocessor on the core instrument, or the embedded
controller in the chassis. Our goal was to develop an interface
such that this entire enclosure could be seen by a Test Station
PC as a single LXI compliant device. Since the embedded
controller of the subsystem is a relatively high powered PC
running Windows 7, the technologies at our disposal for
conforming to LXI compliance, and our final API are in many
ways unique when compared to traditional LXI devices.
Figure 3: LXI 1.3 class based compliance hierarchy.
Due to the nature of the High Speed Subsystem, Class C is
befitting. LXI is used for communication between the Test
Station PC and the High Speed Subsystem device, this
communication does not involve triggering or high bandwidth
data transfer.
The conventional use of a PXIe chassis equipped with an
embedded slot 1 controller is to connect a monitor and other
peripherals in order to program subsystem instruments locally.
In a large scale test configuration, where many instruments are
controlled from a Tester PC, it is not realistic to host or
control the auxiliary instrument drivers on the embedded
controller. By creating an LXI interface for the PXIe chassis, it
is possible to treat the entire subsystem as any other device in
the test configuration controlled by the main Tester PC. Also,
it is possible to expose virtually all of the embedded
controller’s capabilities through a programmatic interface,
which is paramount for test developers interfacing with other
devices such as power supplies and auxiliary instrumentation.
Test Station PC
High Speed Subsystem
The High Speed Subsystem targets class C compliance within
the LXI specification revision 1.3 [7], which has major
disparities with LXI 1.4. LXI 1.4 has inherent support for IPv6
as well as the notion of extended versus core features as
opposed to a class based hierarchy.
Unit Under Test
Figure 1: This figure illustrates a simple network involving a test station
controlling an HSS device with LXI.
All triggering and high bandwidth data acquisition tasks are
isolated to the region of communication between the High
Speed Subsystem, its PXIe based sub-instruments, and the
units under test. Data transfer between subsystem instruments
and the embedded controller uses PXIe, which is an extremely
fast and precise interface. The Test Station’s role is merely to
assert high level control of the High Speed Subsystem. Class C
compliance requires the device to support VXI-11 discovery,
host an advanced web interface, and be programmable with an
IVI compliant driver. LXI does not require any functionality in
the driver beyond what IVI requires. That being said, it is
possible to have an LXI compliant device that provides little to
no real remote programming capabilities. Our IVI-C driver
required additional capabilities beyond those specified by IVI
From that point, a user can add the instrument as a resource and
access it using VISA in their application. The capabilities
exposed through VISA may be very limited since LXI requires
that only VXI-11 identification be supported. In the case of the
High Speed Subsystem, the VXI-11 protocol was not an ideal
solution for many of the remote capabilities that the device
would expose. The alternative is discussed in the IVI
Compliant Driver section of this paper. The High Speed
Subsystem does support VXI-11 discovery for LXI compliance.
in order to access the devices file system and enumerate
subsystem instrumentation.
LXI compliant devices must be discoverable by a client using
the VXI-11 protocol. In theory, a user can open NI-MAX or
any LXI discovery tool and search for LAN connected or
TCP/IP instruments. If an instrument is LXI compliant, and on
the same subnet as the client machine, it will appear on the list
of available instruments.
VXI-11 was originally designed to mimic GPIB. It was added
to VISA specification in 2000 for LAN connected
instrumentation support. VXI-11 allows ASCII messages,
including IEEE 488.2 messages, and IEEE 488.1 instrument
control messages to be passed between a controller and a
device over a TCP/IP network built on ONC-RPC (Open
Network Computing Remote Procedure Calls).
Test Station PC
ONC-RPC is a technology traditionally seen on Unix systems,
though, there are Microsoft implementations available. With
ONC-RPC, a developer defines the client/server procedure call
interface in a definition (*.x) file, and then a generation tool
will create code stubs for the server implementation which will
be hosted on the LXI device.
LXI Device
LXI Device
LXI Device
LXI Device
Definition File
Figure 2: The figure above shows multiple LXI discoverable devices
residing on the same LAN as a test station PC.
Figure 3: The RPC generator tool converts an interface definition *.x file
into a code base shell for a simple VXI-11 server. This simple server is
used only for LXI discovery in the High Speed Subsystem.
The VXI-11 specific interface is defined in the VXI-11
specification and can be copied directly into a definition file for
the RPC generator tool.
In the High Speed Subsystem LXI Discovery Service,
create_link, destroy_link, device_lock, device_unlock,
device_read, and device_write are the only RPC procedures
supported and implemented. Link maitnence is required to
handle multiple requests for identification simultaneously.
Read and write are required for identification query responses.
After implementing the code stubs and deploying the server on
the device, clients can then perform reads and writes on the
server through client side VISA implementations that support
VXI-11 such as NI-VISA and Agilent VISA.
For LXI Class C compliance, only one remote command need
be supported. This is the identify command, which is
accomplished by performing an RPC device write of “*IDN?”
An LXI compliant device must reply with it’s identity upon
such a write. The implementation of these functions has some
important rules which can be found in VXI-11 specifications
[1] [2] [3].
Figure 7: Windows installer for the Teradyne High Speed Subsystem LXI
(IVI-C) driver installation.
An LXI compliant device must have an IVI-C or IVI-COM
compliant driver. High Speed Subsystem supports an IVI-C
compliant driver coined the Test Station Driver due to the fact
that it is designed to be installed and run on the Test Station
PC. An IVI-C driver must export functions and attributes that
comply with IVI-3.2 Inherent Capabilities Specification [4]
such as Session Init and Close functions, Attribute Set and Get
functions, Lock/Unlock, Self Test/Reset and Error handling
functions. In addition, an IVI-C driver can export functions
and attributes that are specific to the instrument. Again, there
are no LXI driver requirements beyond those stated in the IVI
Inherent Capabilities specification. If you can create a basic
IVI driver for your device, then you have done enough to
create an LXI compliant IVI driver. However, this bare
functionality is not sufficient for the High Speed Subsystem.
The Test Station Driver API exposes functionality for remote
file system control, remote application execution, and
subsystem instrument enumeration. The implementation of
this functionality was considerably simplified by the fact that
our LXI device is a subsystem controlled by an embedded
Windows based PC.
Figure 6: HSS device, hostname “springbok”, discovered on the LAN
with NI-MAX.
In the case of the High Speed Subsystem, this LXI discovery
server is built as a Windows service application that begins
running at device boot. Effectively this makes the device
discoverable as long as it is on and the operating system has
started. The service application queries the instrument model,
software, and serial number from the Windows registry, and
reports that information to clients when queried for
The most interesting aspect of the Test Station Driver API is
remote application execution. Actual test code which uses the
drivers of the subsystem instrumentation is transmitted from
the Tester PC to the embedded controller and executed on the
embedded controller. All core instrument use, data processing,
and in effect all “testing” done by the High Speed Subsystem is
done between the embedded controller and the PXIe
instrumentation. Test results are transmitted back to the Tester
PC after the heavy duty operations are completed. This is
important because it highlights the fact that LXI based
triggering is not a requirement for the High Speed Subsystem.
All triggering is done between code executed on the embedded
controller and the subsystem instrumentation via the PXIe
The High Speed Subsystem uses the Microsoft Windows IIS
(Internet Information Services) web server, and hosts active
ASP.NET pages to provide the required functionality.
ASP.NET is Microsoft’s .NET version of Active Server Pages,
which allows C# code to run behind dynamic HTML pages on
the server. This enables the High Speed Subsystem to have a
sophisticated web interface without many of the limitations
normally associated with hosting a web server on an
To expose these capabilities of the High Speed Subsystem
device remotely to a Test Station over Ethernet, the driver uses
Microsoft’s .NET Remoting technology. In some cases, ONCRPC, the same technology used for VXI-11 discovery, is used
to accomplish these remote driver operations on LXI devices.
However, since the High Speed Subsystem is running
Windows, .NET Remoting was a somewhat simpler and more
elegant solution. The High Speed Subsystem Test Station
Driver is an IVI-C native library. All of the .NET Remoting
functionality is packaged into a managed C++ library which is
invoked by the native driver.
Test Station Driver (Native)
Managed C++ .NET
Remoting Library
Subsystem .NET Remote
Figure 9: HSS device web interface as seen through NI-MAX.
Remotely Spawned
Subsystem Applications
Embedded Controller File
Subsystem Instrument
The web interface must display general instrument
information on the welcome page. This information includes
the instrument model, manufacturer, serial number,
description, etc. In the case of the High Speed Subsystem, this
information is stored in the Windows registry on the device.
Enumeration of subsystem instruments within the PXIe
chassis is accomplished by performing Teradyne driver calls
and 3rd party instrument driver calls at run time with
Figure 8: Subsystem instrument enumeration and embedded PC
capabilities exposed remotely to Test Station PC via Subsystem Server
and Test Station Driver.
On the device side, a .NET Remoting server is running
alongside the LXI discovery service. This server, which also
begins running during device boot, provides all the remote
capabilities accessed by the Test Station driver.
An LXI compliant device must host a web-server with W3C
compliant web pages that embody an interface for configuring
the device on the LAN. There are several broad requirements
as well as some detailed requirements that can be found in the
LXI specification [7]. The general requirements are listed
NIC Control
Backend Library
Web pages shall be W3C compliant
Protocol shall be HTTP
Connections shall be accepted on port 80
Mandatory welcome page display items
LAN Configuration web page
Security mechanism
Visible LXI logo
Control Backend
IIS 7.0
Figure 10: Subsystem instrument enumeration and device NIC properties
exposed via ASP.NET web interface.
The device’s web interface must also provide access to the
NIC (Network Interface Card) in order to configure the LAN
settings. The LAN settings that must be configurable are listed
Device Description on NIC
TCP/IP Configuration Mode
IP Addresses
Subnet Mask
Default Gateway
DNS Servers
In conclusion, providing an LXI interface to a subsystem is
convenient to the end user in that it gives them a standardized
way to locate, configure and program the subsystem. By using
a high powered Windows based embedded PC as the chassis
slot 1 controller, features were provided in addition to the
standard LXI requirements that would otherwise be
exceedingly difficult to implement.
The High Speed Subsystem web server contains a backend
library that uses the Win32_NetworkAdapterConfiguration
management class. The web pages interface with the library
using ASP.NET C# code behind the HTML. The
Win32_NetworkAdapterConfiguration management class
provides all the functionality required for LXI network
configuration on a Windows based device.
An LXI Class C compliant device’s web interface must also
provide a security mechanism so that NIC settings can’t be
maliciously tampered with. This is accomplished by using
Windows Authentication functions to allow ASP.NET code to
run as an impersonated user once a password was provided
through the web interface.
VMEbus Extensions for Instrumentation: TCP/IP-VXIbus Interface
Specification, VXI-11.1, Revision 1.0.
VMEbus Extensions for Instrumentation: TCP/IP-IEEE 488.1 Interface
Specification, VXI-11.2, Revision 1.0.
VMEbus Extensions for Instrumentation: TCP/IP-IEEE 488.1 Interface
Specification, VXI-11.3, Revision 1.0.
IVI Interchangeable Virtual Instruments: IVI-3.2 Inherent Capabilities
Specification, June 9, 2010 Edition, Revision 2.0.
NI-VISA Programmer Reference Manual, March 2003 Edition.
ICS Electronics: VXI-11 Tutorial and RPC Programming Guide.
LAN Extenstions for Instrumentation: LXI Standard, October 30, 2008
Edition, Revision 1.3.
Advanced .NET Remoting 2cnd Edition, Ingo Rammer and Mario
Szpuszta, 2005.