The Benefits of USB SCXI Switching for Automated Test and Control Applications

Switch modules designed by National Instruments can be used in a vast array of applications ranging from RF to simple thermocouple multiplexing applications. They exist in two main platforms, PXI and SCXI, both of which have been pioneered by National Instruments. This document will briefly discuss the advantages and use cases of each platform. It will then focus on specific examples of how a USB-controlled SCXI solution compares in price and complexity with other proprietary switching solutions on the market such as the Agilent 3499 and Keithley 7000 series.

Choosing between SCXI and PXI

The PXI and SCXI platforms both offer advantages for specific switching applications. This section will discuss use cases for each platform in order to help users pick the best solution for their particular application or test system

PXI – Mixed Measurements and Switching

The PXI (PCI eXtensions for Instrumentation) platform is ideal for creating rugged, compact, and accurate test systems involving instruments such as digitizers, DMMs, RF analyzers and generators. Due to its flexible architecture and the availability of a vast number of instrument modules, the PXI platform’s prowess lies in building integrated and complex test applications in one chassis. To elaborate, consider building the following system in PXI:

Test System Requirements:

1. Measure 90 thermocouples
2. Generate an AC sine wave up to 100 MHz and 10 Vp-p and route it to 15 different locations
3. Measure and analyze RF signals up to 2 GHz bandwidth originating from 4 different antennas
4. Digitize signals from 20 pins of 10 different ICs at a rate of 200 MS/s and a resolution of 12-bits

PXI Solution:

1. The NI PXI-4071 FlexDMM offers 7½ digits of accuracy and can be combined with the NI PXI-2575 98-channel 2-wire multiplexer for precise thermocouple measurements. This module is designed for low voltage measurements (such as thermocouple measurements) and will provide an acceptable level of accuracy for switching microvolts to millivolts.
2. The NI PXI-5421 arbitrary waveform generator can be used to generate a 10 Vp-p sine wave at 100 MHz and can be combined with the NI PXI-2593 500 MHz multiplexer to route the signal to each of the 15 different locations
3. The NI PXI-5660 Vector Signal Analyzer can take measurements of signals up to 2.7 GHz and can be combined with the NI PXI-2595 5 GHz 4x1 multiplexer to measure the four desired RF signals.
4. The NI PXI-5122 100 MS/s 14-bit digitizer can be combined with two NI PXI-2593 500 MHz multiplexers to accomplish the digitization of IC pins.
5. All components of the test system can be placed in ONE PXI-1044 14-slot 3U chassis

The only other test platform that can accomplish a task as diverse as the one described above is VXI which is substantially larger and thus not suitable for space-constrained applications. The versatility of PXI is also unmatched by competitive switch offerings. This, along with its rugged exterior and the ability to be a stand alone system, have aided PXI in becoming an industry standard for automated test equipment (ATE). Today, several different vendors build PXI switch modules with various topologies making it the most diverse switching platform on the market (more than 80 different topologies made by National Instruments and hundreds more made by other vendors). PXI can be thought of as a mixed-signal measurements/switch platform where complex, multifaceted applications can be built in a rugged, compact, single-chassis form factor.

SCXI – Standalone, Plug-and-Play Switching

SCXI or Signal Conditioning Extension for Instrumentation is a high-performance signal conditioning and switching platform for measurement and automation. A typical SCXI system consists of a chassis with several slots for application-specific measurement or switch modules. Most SCXI modules also have designated terminal blocks which contain screw terminals for signal connections. The two main purposes of SCXI systems are:

1. Signal conditioning for data acquisition
2. Switching

This document discusses the switching aspect of SCXI and its most appropriate use cases.

While PXI succeeds at building integrated instrumentation/switching systems using an array of instruments within one chassis, using a PXI chassis purely for switching with existing GPIB, serial, or Ethernet instrumentation may not be as efficient in some cases as an SCXI solution. Consider the following scenario: an existing test system that was being used to measure the voltage between two points with a benchtop DMM now needs to be expanded to accommodate 500 test points. The benchtop instrument currently communicates with a host computer using a protocol such as GPIB or serial. In this case adding a PXI system might be an expensive option, as PXI requires the use of either an embedded controller or MXI connection to act as a bridge between the PXI chassis and a host computer. The same concept would also apply to a system that uses a PCI DMM such as the NI PCI-4070 6 ½ digit FlexDMM. A less expensive and more appropriate solution for such a test system would be a USB SCXI Switch Mainframe. Such a mainframe can accommodate anywhere from 4 to 12 switch modules that can plug into the host PC using a single USB port. The system is immediately auto-detected by the PC as soon as it is plugged in and requires no additional configuration. After this, a user can open the NI-SWITCH Soft Front Panel - an easy-to-use graphical user interface that comes with the NI-SWITCH driver software - and begin switching relays within minutes. Furthermore, due to the modular nature of the SCXI platform, a user can easily modify his or her switching application as needed by replacing a single module rather than replacing the entire switch box or system. To learn more about setting up a USB Switch Mainframe, watch the 3-minute demo.


The potential benefits of SCXI apply to more than just traditional switch/DMM applications. Control applications that simply turn devices on or off and do not require instruments for measurements and RF applications that require routing high frequency signals between benchtop instruments are just some examples where a standalone switching solution is more cost effective. The NI SCXI platform is designed for applications that do not require instrumentation to be purchased with switching. A few switch mainframes on the market come with built in DMMs. Such solutions force customers to pay for the DMM whether or not they plan to use it. By separating instrumentation from switching, the SCXI platform relieves customers of this cost burden. Alternatively, solutions such as the Agilent 3499 series and Keithley 7000 series also offer standalone switching. Some of the attributes that differentiate SCXI from these platforms are:

1. Cost and size effectiveness – SCXI is a much less expensive solution than the Agilent and Keithley systems. Furthermore, due to the availability of high-density multiplexer and matrix modules in the SCXI form factor, applications that can be developed using a single USB Switch Mainframe sometimes require multiple Agilent 3499 and Keithley 7000 mainframes. While SCXI modules (4U) are not as small as their PXI (3U) counterparts, they are significantly more compact than VXI.
2. Module options – National Instruments currently offers more than 50 SCXI switch configurations in 13 modules. These modules can be used in a wide range of applications such as RF (up to 5 GHz bandwidth), high-density (up to 256 1-wire multiplexer channels or up to 256-crosspoint matrices), and high voltage (up to 300V, CAT II) test systems.
3. USB connectivity and speed – Using either the 4- or 12-Slot USB Switch Mainframes, customers can take advantage of the plug-and-play functionality and high speed of USB. Any of the 13 SCXI switch modules can be placed in the USB Switch Mainframe which can then be connected to a host PC via the High Voltage Analog Backplane (HVAB). Once plugged in, the SCXI system is automatically detected and configured by the PC.
4. Continued platform development – National Instruments continues to invest in SCXI switching – a fact that will yield more switch modules in the future catering to an even more diverse set of applications. The most recent of these releases include the NI SCXI-1194 and SCXI-1195 2.5 and 5 GHz multiplexers, respectively, that can be used to enhance communications test systems and route high frequency signals.

SCXI Switch Systems Comparisons

The SCXI-compatible USB Switch Mainframe is a modular and easily expandable standalone switch system. Here are a few of the many examples of applications where the USB Switch Mainframe is a powerful and cost-effective solution when compared to standalone switch systems from other test equipment vendors:

Note: all prices are in US dollars and are current as of May 15th, 2006 and taken from vendors’ price lists or online catalog listings.

Example 1: Building a 16x64 2-wire matrix to test several signals from a device under test (DUT).

High density matrix applications are often needed for testing of integrated circuits (ICs). A good example of such a test would be 4-wire measurements of resistances between any 2 points on a high-density IC. This example assumes that the user has already purchased the DMM.

NI SCXI Solution

Part Name	Description	Quantity	Price
SCXI-1000 w/ HVAB	4-Slot NI USB Switch Mainframe	1	$995
SCXI-1129	256 channel Mux/Matrix	4	$15,580
SCXI-1334	4x64 Terminal Block for SCXI-1129	4	$1780
TOTAL			$18,355

Agilent 3499A solution

Part Name	Description	Quantity	Price
3499A	5 Slot Switch Mainframe	7	$18,949
N2262A	4x8 Matrix Module	32	$38,944
TOTAL			$57,893

Keithley 7000 Series

Part Name	Description	Quantity	Price
7002-HD	2-slot High-Density Switch Mainframe	3	$7,500
7002-HD-MTX1	Differential 6x32 Matrix Card	6	$27,200
TOTAL			$34,700

Analysis of Solutions:

Besides being less than one third the price, the single-mainframe SCXI solution is far more compact than the Agilent 3499A system which requires seven mainframes to perform the same task. In addition to cost and space, the Agilent solution poses a much larger connectivity challenge requiring that signals travel between several switch mainframes. When compared with the Keithley 7000 series, the SCXI solution once again provides a much more compact solution using a single mainframe rather than four. This would help reduce total system cost by more than half.

It is also important to note that SCXI is capable of building much larger systems than a single 16x64 2-wire matrix. A 12-slot USB Switch Mainframe would allow for more than 3000 2-wire matrix crosspoints in a single mainframe.

Example 2: Creating a switch system to turn on/off components in a control room. Since the system is simply controlling components, no instrumentation is required. The switch system is required to have the following:

1. 15 SPST switches that can switch up to 8A
2. 200 SPST switches that can switch up to 100V

Control applications such as the one illustrated above often demand high power switching to turn devices on or off. Such applications do not require any measurements to be taken. Hence stand alone switch systems tend to be the most cost effective solutions.

NI SCXI Solution

Part Name	Description	Quantity	Price
SCXI-1000 w/ HVAB	4-Slot NI USB Switch Mainframe	1	$995
SCXI-1161	8 Ch, 8A, 250V SPDT	2	$990
SCXI-1169	100 Ch, 1A, 100V SPST	2	$2,990
TOTAL			$4,975

Agilent 3499 solution

Part Name	Description	Quantity	Price
3499C	9 Slot Switch Mainframe	1	$4,273
N2267A	8 Channel General Purpose Relay	2	$2,358
N2261A	40 Channel General Purpose Relay	5	$8,955
TOTAL			$15,586

Keithley 7000 Series solution

Part Name	Description	Quantity	Price
7002	400-Channel 10 Slot Switch Mainframe	N/A	N/A
N/A	No High Current SPST Switch Available	N/A	N/A
7037-D	30 Channel Single-Pole Relay DIO Card	7	$10,115
TOTAL			$20,943

Analysis of Solutions:

Once again, the Agilent 3499 system is roughly three times as expensive as the USB SCXI solution. Also, the 9-slot 3499C chassis is considerably larger than a 4 slot, 4U SCXI mainframe. The Keithley 7000 solution did not have a solution for 8A SPST switching but was more expensive than SCXI even without adding the cost for this requirement. To obtain 200 SPST channels for switching up to 100V would cost $10,115 if done using the Keithley 7000 series mainframe while the same task can be accomplished for $2,990 in SCXI. In addition to being less cost-effective, using the Keithley solution would entail complex wiring and the need for a larger rack enclosure.

Example 3: Building an RF switching system that can route signals in the 500 MHz bandwidth range from 100 antennas to one channel on a 3rd party benchtop RF vector analyzer.

RF test applications usually require the routing of high bandwidth signals to and from spectrum analyzers and generators. Many of these analyzers and generators are benchtop instruments. Since such applications do not require an assortment of instruments to reside under the same chassis, a stand alone switch solution such as SCXI would be most cost-effective.

NI SCXI Solution

Part Name	Description	Quantity	Price
SCXI-1000 w/ HVAB	4-Slot NI USB Switch Mainframe	1	$995
SCXI-1193	500 MHz, 32-Ch Multiplexer	4	$11,580
TOTAL			$12,575

Agilent 3499 solution

Part Name	Description	Quantity	Price
3499C	9 Slot Switch Mainframe	1	$4,273
3499A	5 Slot Switch Mainframe	1	$2,707
N2272A	1x9 1 GHz Multiplexer Module	12	$18,336
TOTAL			$25,316

Keithley 7000 Series solution

Part Name	Description	Quantity	Price
7002	400-Channel 10 Slot Switch Mainframe	2	$8,190
7016A	2 GHz, Dual 1x4, 50Ω Multiplexer	13	$18,187
TOTAL			$26,377

Analysis of Solutions:

The NI SCXI platform offers several RF switching modules that range from 500 MHz to 5 GHz. In the above example, the NI SCXI solution is less than half the price of Agilent and Keithley’s. In addition, the SCXI solution is less space intensive due to its ability to fit all channels inside on chassis. Agilent and Keithley would both need two separate mainframes to achieve the same results.

Conclusion

For systems requiring a complete test system involving RF, digital multimeters (DMMs), digitizers, or any other instrument along with switching, PXI is the most cost effective and compact solution. However, for systems that require switching to be integrated in to a pre-existing system, SCXI can help eliminate the cost associated with purchasing an embedded controller needed for a separate platform. Furthermore, the larger form factor of SCXI allows it to have higher channel count in some cases than its equivalent PXI switch module. This can help save space and reduce chassis count. Just as the benefits of using PXI switching when building complete applications in a single chassis are compelling, the advantages of using a USB SCXI system as a standalone switch box can ease system integration for pre-existing or hybrid systems.
Related Links:
Switch Product Selection Guide
USB Switch Solutions

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