A host of VXI instruments is entering the market—instruments that are faster and more accurate, smarter and more feature-laden than their predecessors. For the first time, it is possible to configure VXI test systems that are truly equivalent in performance to GPIB instrument systems, even for the most exacting automated measurement tasks.
All this capability puts new demands on the VXI mainframe. In particular, more speed means more power, and mainframes designed just a few years ago may no longer be adequate to power a full complement of high-performance instruments.
Perhaps more importantly, conventional mainframes—crates—may be unable to keep valuable instruments safe from destructive overheating. Instrument failure carries a high price: not only the cost of repair or replacement, but also lost throughput due to downtime.
Less tangible but also critical are the costs of “fear of failure.” If measurement performance must be derated to make up for tolerances due to heat variations, then the quality of the test suffers as well.
The recent developments in VXI instrumentation have precipitated a significant rethinking of VXI mainframe architecture and the mainframe’s role in the system. No longer is the mainframe simply a crate with a power supply and some fans; it has evolved into an intelligent instrument that contributes to system reliability and performance. Mainframe designers have evaluated power, cooling, reliability, programmability and mean time to repair (MTTR) requirements, and redefined the VXI mainframe to meet the most demanding system application needs.
New Mainframes Pass the I.Q. Test
Intelligence is the foremost attribute of the new generation of VXI mainframes. It provides superior protection for VXI instruments and improved test-system reliability.
The mainframe itself can now be connected to the system host computer via the VXIbus, just like any other VXI instrument. With this connection to the VXIbus, minor problems can be detected and reported before they turn into larger problems that affect instrument or system reliability.
For example, mainframe power-supply voltages can now be monitored and reported in the form of messages over the VXIbus. Out-of-tolerance voltage levels are detected immediately.
Temperature monitoring and mean time between failures (MTBF) are also enhanced by mainframe intelligence. Conventional mainframes monitor average temperature rise across the whole mainframe. A hot slot, and therefore a hot instrument, can go undetected with this averaging approach.
Running any VXI instrument in an over-temperature condition for a long period of time can lower the instrument’s MTBF. The instrument is often blamed for frequent failures when, in fact, the mainframe is the culprit.
In contrast, some VXI mainframes are programmable via the VXIbus, allowing the user to specify the maximum permissible temperature rise from ambient in each slot. Again, the mainframe promptly reports out-of-tolerance conditions on a slot-by-slot basis. For the first time, the VXI mainframe takes an active, positive role in overall test-system reliability.
Keeping It Cool
In the intelligent mainframe, the cooling and the temperature monitoring systems make up the two halves of a closed loop. The cooling system automatically sets the cooling rate throughout the mainframe according to the needs of the hottest slot, as determined by the temperature monitoring system. If the cooling system cannot accommodate the rate of cooling required, a message sent over the VXIbus alerts the user.
A single fan failure diminishes the cooling capacity of the whole mainframe, which jeopardizes the instruments inside. An intelligent cooling system actually monitors its own cooling fan speeds and reports them over the VXIbus. If there is a failure, the user is notified immediately. In fact, it’s possible to program a prompt to occur when mismatched fan speeds are detected, alerting the user to impending fan failure.
Go With the Flow
New VXI mainframes offer more evenly distributed airflow across each slot in the mainframe. New cooling-system designs also have much higher total airflow, essentially doubling the cooling capacity of previous-generation mainframes.
Added “cooling insurance” comes in the form of shutters that block airflow to empty instrument slots in the mainframe. This automatic mechanism directs all airflow to the instruments installed in the system.
Because mainframe cooling is such a sensitive issue—and one that is prone to self-service interpretations—the VXI Consortium is proposing a new standard for VXI mainframe cooling measurements. Known as VXI-8, Cooling Methods, the proposed standard provides a means of comparing cooling capacity among VXI mainframes.
VXI-8 is the best safeguard against hasty, misleading “showroom” measurements. It prescribes the tools, test points and methods to make an exhaustive evaluation of any mainframe. If the specifications of a mainframe do not clearly state that cooling measurements were made in compliance with the VXI-8 standard, the validity of the specifications may be questionable.
Open Mainframes for Open Systems
The VXIplug&play Systems Alliance has created open system standards for instrument drivers and soft front panels. Because intelligent VXI mainframes communicate over the VXIbus as message-based instruments, they take advantage of VXIplug&play software standards and operate with VXIplug&play drivers and soft front panels. The programmable cooling, monitoring and user messages of the VXI mainframes are utilized within the user’s application program to ensure repeatable measurements and enhance system reliability.
The Quick Fix
The new VXI mainframes are easier to maintain because their local intelligence helps find and diagnose problems. For example, if a power supply or fan should fail, the problem is announced over the VXIbus.
The design of the mainframes also promotes excellent MTTR figures. With easy access for power supply and fan removal and replacement, VXI mainframes offer MTTRs measured in minutes rather than hours.
Summary
More manufacturers are putting VXI production test systems squarely in the middle of their revenue stream. Consequently, users not only need more measurement capability, but also more reliability from their VXI systems. Intelligent VXI mainframes promote reliability with a wealth of control, monitoring and mechanical features that ensure higher performance, higher uptime and lower MTTR.
About the Authors
Steve Jennings joined Tektronix in 1990 and today is the VXI Product Line Marketing Manager. He has more than a dozen years of sales, marketing and management experience in the test and measurement industry.
Alan Whiteside is a Marketing Communications Program Manager at Tektronix. He has been with the company for 18 years in manufacturing, engineering and marketing positions.
Tektronix, Inc., P.O. Box 500, Beaverton, OR 97077, (503) 627-6364.
Copyright 1995 Nelson Publishing Inc.
June 1995
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