Unquestionably, VXI heralded a new beginning in electronics technology. But no sooner did we have an instrument on a card than we wanted more–more than one instrument on a card which ultimately reduces cost, space and weight.
VXI manufacturers are successfully addressing these needs. But to fully appreciate today’s evolving VXI technology, it’s good to first remember how it all started.
The VXI standard was developed to satisfy the demand from the test and measurement industry for high-performance, downsized, reconfigurable instrumentation. When compared to existing standards, it achieved its results and drove all major manufacturers to quickly deliver VXI products to this growing marketplace by converting existing GPIB, VME, PC or CAMAC products to the VXI standard.
By converting existing designs, the amount of available VXI products grew to more than 1,000 in just a few years. As a result, VXI had a relatively deep product offering early in its existence, which aided in early acceptance of the standard by system designers.
In the process of bringing their own products to market and to aid in quickly converting products and designs to the VXI platform, many manufacturers introduced VXI development tools. Companies such as Racal Instruments, Hewlett-Packard, Tektronix and VXI Technology offered breadboarding products and interface cards. Others, such as Interface Technology, developed single-chip VXI interfaces.
These products allowed system designers to build custom circuitry on the bus. They also permitted manufacturers with less knowledge of the bus to adapt their existing products to the VXI format quickly, again providing more products for the VXI market.
As instrument manufacturers became more knowledgeable and the VXI standard continued to mature and gain market share, more sophisticated products began to appear. Instruments were being designed specifically for VXI. Products began to use more features of the bus, such as shared memory, dynamic configuration and fast handshake.
Most importantly, instrument density began to increase. A major driving force allowing densities to increase was the continuing reduction in the size of electronic components. The PCMCIA market prompted component manufacturers to reduce IC and passive component sizes by substantial factors.
The maturing field-programmable gate array market also increased the amount of conventional logic which could be swept up into a compact IC package. This opportunity, coupled with creative designs, allowed new VXI products to be designed with phenomenal densities and performance. Some examples are Talon’s SR192 and Interface Technology’s SR5000 series of digital products and VXI Technology’s VXI Modular Instrumentation Platform (VMIP).
Compact But Not Portable
It became apparent early in its existence that VXI-based test systems could be made smaller than their predecessors. Instead of a full rack of individual instruments connected via a GPIB interconnect scheme, a full complement of resources could be assembled in a 13-slot VXI chassis. System designers quickly recognized that it might be possible to configure portable test solutions which could allow the test set to be brought to the UUT, rather than the other way around.
Although VXI cards promised portable test, in reality, many of the final test solutions were still too large or heavy to be considered portable. In addition, the moment a test solution required a 13th instrument slot (Slot 0 being reserved for the controller), a second chassis had to be added. Even if a single full-size chassis were adequate, the weight of the test set could easily exceed 120 lb, surpassing the specified or practical weight limit for such equipment.
The only practical portable test sets that have been built use a five- or six-slot chassis. These chassis allow four or five instruments to be combined with a controller and provide some limited level of test functionality. If more complex testing must be done in a portable environment, you must either reduce the weight of a full-size VXI chassis and the equipment installed in it, or house more instrument functions within each VXI instrument module.
The high-density VXI products definitely help make test stations portable, but they do not overcome the problem of single functionality per card slot. The only way to resolve this is for manufacturers to develop multifunction VXI cards.
Certain manufacturers have responded to this market demand by designing application-specific multifunction cards, such as HP’s combination card for Automotive testing.1 To satisfy the growing market demand for multiple functionality per VXI card slot, several companies are introducing new generation VXI products. Some examples are:
Racal Instruments has a family of cards that provides both A/D and D/A functions on a single card.
C&H Technologies and TASCO provide mezzanine carriers for industry-pack modules.
Talon’s SR192 Digital Tester allows the addition of some other instrument functions.
VXI Technology’s VMIP allows three independent VXI modules to be accommodated in a single C-size card slot.
These new, higher-density modular products move the system designer one step closer to a true portable test stand and reduce the number of instrument modules in a nonportable system.
Each one of these new-generation VXI products makes VXI test solutions denser and more cost-effective, but all use different means. Some are adapting existing technologies to the VXI standard, similar to the early CDS 53A/VXI adapter implementation where a mezzanine carrier is used to interface to the VXI backplane.
Adapters allow system integrators to use products found in the VME/PC market (industry packs) in VXI test stations. This approach becomes powerful in instances when required functionality might not be available on a VXI card, such as stepper motor control.
A mezzanine implementation option from C&H allows up to four industry packs to be accommodated on one VXI card. This provides multiple functionality in a single card slot.
Adapter and mezzanine approaches do not necessarily support all features defined and provided by the VXI standard, such as word serial protocol support, Standard Command for Programmable Instruments (SCPI) command sets, VXIplug&play drivers, TTL/ECL trigger-line support and Local Bus access. Power consumption may also become a problem because the industry-pack instruments are designed for VME and only draw current from the 5-V and +12-V supply lines.
Racal, HP and Talon offer units that are full VXI devices and provide the benefits of interoperability and ease of use, such as word serial protocol, instrument drivers and complete VXI support. Although these cards have multiple functionality, some may still be regarded as single instruments and require unique command sets.
This could mitigate the advantages provided by SCPI, which aims to make instrument command sets consistent from one manufacturer to another. The ultimate intent of SCPI is to allow system designers to change suppliers or upgrade functionality with minimal software effort.
To avoid unique command sets for multiple function VXI modules, each instrument function should have its own logical address and its own command set. This allows a function to be added or deleted from a module without rewriting or changing its embedded firmware.
The Slot 0 resource manager software could recognize if a specific test resource is available in a given system configuration. With a single logical address configuration, determining which resources are available is much more difficult, since multiple function modules may have to be queried in a nonstandard fashion.
Fortunately, the VXI specification foresaw the need to incorporate more than one instrument in a physical slot. The standard provides specific details about how modules with multiple instruments and logical addresses should be designed.
Each instrument must have the required VXI registers for the specific type of instrument, such as message-based, register-based or memory device, but each instrument would respond to the single MODID line routed to that module. This allows the Slot 0 controller to recognize that there is more than one instrument in the slot. Also, when multiple instruments are in a single module, their logical addresses must be sequential and may share decoder logic.
With regard to the Offset Register which controls the logical address of the instruments which support dynamic configuration, the VXI specification, revision 1.4, states in section F.2.3 Offset Register:
“Multiple device modules may share address-decoding hardware. This can result in a significant hardware reduction. In such a case, the devices will share one or more Logical Address bits. A set of such devices is defined to be address-blocked. These devices will be configured to a block of contiguous Logical Addresses. The Offset Register is used to indicate the number of devices sharing the addressing hardware.”3
Because the VXI standard defines a consistent way to share decoder logic, multi-instrument modules can enjoy reduced logic content and allow the more densely packaged logic boards to make multiple instrument VXI modules a viable alternative to their full-sized counterparts. For instance, VXI Technology’s VMIP product family utilizes shared decoder logic and allows three independent VXI devices that support all aspects of the VXI standard to be housed in a single C-size card.
By not limiting the user to one instrument per card slot, the original VXI concepts of virtual instrumentation and modularity can now become a very powerful tool for developing a test solution in a very small footprint. A 12-instrument solution will no longer need a 13-slot chassis, and 36 instruments could be housed in a 13-slot chassis, providing densities greater than VME (21 card slots per 19″ rack).
By sharing VXI card-slot space between different instrument functions, the cost of the VXI solution decreases considerably. Commercial applications where cost is a driving factor can now start to be addressed economically with VXI solutions. These new-generation devices are the first step to take the VXI standard into newer markets.
1. Bode Newsletter, Version 6.8, June 1995.
2. Bode Newsletter Version 7.2, October 1995.
3. VXI Specifications, Revision 1.4.
About the Author
Paul Dhillon, who has been involved in VXI test and measurement since the introduction of the VXI standard in 1987, is Executive Vice President of VXI Technology. Previously, he was employed at Racal Instruments. Mr. Dhillon holds a bachelor’s degree from Kingston University, U.K., and has authored numerous technical papers on VXI. VXI Technology, 17912 Mitchell, Irvine, CA 92714, (714) 955-1894.
Copyright 1996 Nelson Publishing Inc.