Since more than a dozen companies now provide VXIbus mainframes, selecting the right mainframe–especially one that has the right features and is most cost-effective for your specific project–may not be a simple task.
It starts easily enough. The basic mainframe requirements are stipulated in the VXIbus specifications (presently Revision 1.4) authored by the VXI Consortium. But the details are more difficult because you must consider structural details that may affect ease-of-use and maintainability, check whether the power supply is adequate, examine air flow and cooling requirements, evaluate backplane capabilities and determine compliance with your monitoring needs.
Mechanical Considerations
The application determines the required ruggedness and structure. Consider how and where it will be used. Will the mainframe be located on a desk or in a 19-inch rack? Will you reuse it in either configuration? Will you use it in a lab, in a factory or in the field?
Many suppliers furnish rack-mounting kits to make reconfiguring easier. Low-cost standard mainframes are available for benign lab or factory environments, and ruggedized ones meet military requirements. If the system must be portable, weight and size will influence your choices.
Next, look at the internal structural details, such as assembly and mounting provisions. For instance, mainframes contain card guides to facilitate insertion of instruments-on-a-card. These guides must be nonconducting per the VXI Specification, and are usually plastic molded clip-ins so they can be easily removed to accommodate insertion of box-style instruments.
“A locking mechanism for these pop-in guides must be provided because they may just as easily pop out during transit or in a harsh environment,” said George Ross, Marketing Manager at Vero Electronics. “Our KM6-11 mainframe enclosure system provides a screw-down feature on each guide for positive retention. On the other hand, our HD167 uses a locking ladder system which secures all guides with one locking screw.”
The number of instruments to be housed determines whether a 6-slot or a 13-slot rack is required. The selected instruments also determine whether you need a B-, C- or D-size mainframe.
“Given the overwhelming number of modules available in C size and the fact that B-size modules fit into C mainframes, most customers select a C-size mainframe,” said Tom Wultich, Product Marketing Manager at Tektronix. “Even if the appropriate B-size module exists, there is a good chance that future applications will need the performance and selection choices afforded by C-size instruments.”
Often, cables must be run from the front panels of VXI instruments to the rear connectors of other instruments located in the same rack. A cable tray simplifies routing of these cables. Since the cable tray height is determined by the number and the diameters of the cables to be accommodated, suppliers often provide a variety of tray sizes.
“For extra flexibility, the Tektronix VX1410 cable tray attaches in three ways to give three depths,” said Mr. Wultich. “This allows the mainframe to grow with the customer’s cable needs.”
When the VXIbus-based system serves as an ATE, an efficient interface to the UUT must be provided. Interface adapters that are semi-permanently wired to the VXI instruments and offer a means to quickly connect or disconnect fulfill that function.
In the past, mainframes accommodated interface adapters from a variety of suppliers, each calling for different mounting provisions. But now, most VXI mainframes conform to the VXIplug&play VPP-8 Specification, which standardizes the mounting of a modular interface system to the mainframe, reported Mike Richards, Vice President Sales and Marketing at Mac Panel.
Usable and Available Power
To conform to VXI specification requirements, suppliers must state the dynamic and the peak currents their instrument draws from any one or more of the seven allowed voltage rails. The mainframe power supply, in turn, should be adequate to furnish the sum of all dynamic currents as well as the sum of all peak currents drawn by all modules contained in the mainframe.
Peak currents, by their nature, exist only for short time periods; therefore, peak power consumption is almost always lower than dynamic power consumption. Most supplies are peak-current limited. Their peak power rating is usually referred to as usable power, while the dynamic power rating is commonly called available power.
Vendors always specify available power, but not necessarily usable power. To determine whether a particular power supply is adequate for your application, you should not only consider the available power rating but also check whether it can simultaneously furnish the sum of all the peak currents of your modules.
While some mainframe suppliers design and manufacture the power supplies they install in the VXI chassis, others purchase them. But there are few power supplies on the market that fully comply with the very strict VXI requirements, warned Mr. Ross.
“Many VXI mainframes come with an open-frame power supply,” said Robert Varo, Director of Sales and Marketing at Ascor. “But a great deal of electronic noise can emanate from such an environment, negatively impacting measurement capabilities. We have gone to great lengths to identify the quietest switching power supplies available—those that offer high usable wattage at the same time—and our VXI mainframes include encapsulated and shielded supplies, designed specifically to minimize electronic noise within the VXI chassis, as standard features.”
Power supplies usually contain built-in overload protection. “But they also should include full global thermal overload shutdown capability,” said Mr. Richards. “The power supply should also be operable anywhere in the world.”
Cooling
To conform with VXIbus system requirements, mainframe specifications must include a plot of air-pressure drop vs flow rate for the worst-case slot in the chassis. Similarly, module specifications must identify a required operating point, identifying the air pressure (mm H2O) for the minimum required airflow (liters/s). If this point lies below the mainframe cooling curve, the module receives adequate cooling.
While the original VXIbus system specification suggested a simplified test setup that may be used to determine the cooling curve, there was room for misinterpretation. “In the past, each VXI mainframe vendor built its own test fixture and took several measurements in the worst slot to determine the cooling curve,” said Mr. Wultich. “Measurements were not even close to being repeatable from one vendor to the next.
“Recognizing that the customer was not being served, the VXI Consortium convened a subcommittee,” Mr. Wultich continued. “This subcommittee developed the VXI-8 Proposed Specification, which clearly detailed the design of the test fixture and the methodology to be used to produce consistent results.”
But meeting the VXIbus specification may not be enough. The availability of external air and the direction of the airflow must also be taken into account.
“Many mainframes are mounted in 19-inch equipment racks and may have other chassis mounted above or below the VXI mainframe,” said David Caserza, Director of Sales and Marketing at Dawn VME Products. “If this is the case, a bottom-to-top airflow system may not be appropriate because the mainframe will be unable to intake or exhaust air due to the other chassis blocking the grills. At best, it would take in warm air, which is also undesirable.
“Other considerations include the overall airflow in the equipment rack. Some have air-conditioned air brought in through the floor. In other cases, it may be best to take in ambient room air through the front of the chassis,” Mr. Caserza concluded.
Each mainframe manufacturer has its own method of providing cooling as well as locating air inlets and outlets. Available configurations include RR, RT, RST, SR and RS, where R stands for rear, S for side and T for top, and the first letter(s) indicates intake and the last denotes outlet.
Backplane Considerations
Most VXIbus chassis have multilayered backplanes that should satisfy many applications, commented Phil Dhillon, Executive Vice President of VXI Technology. “However, if the application involves high-frequency or low noise requirements, go with at least a 12-layer backplane with connector shields,” he said.
Early backplane designs required jumpers or mechanical switches to daisy-chain the VXIbus interrupt-acknowledge and bus grant signals across unoccupied VXIbus slots. But most VXIbus mainframe vendors now offer jumperless backplanes, employing solid-state switches to avoid the need to manually set backplane connections during integration.
Mr. Ross made several performance-related recommendations for selecting backplanes:
Use stripline construction to ensure precisely controlled impedances. This is achieved by burying all traces within the backplane and using outer layers only as power planes.
Use differential line-length equalization techniques to minimize signal skew on all high-speed lines.
Offer unpopulated decoupling capacitor positions in different sizes to permit integrators to fine-tune the mainframe to system parameters.
When intermodule timing is critical, avoid clock-signal delay differentials. “Previous backplanes ran the 10-MHz system clock along the backplane as a single-ended signal, and differential drivers were used at each slot,” said Mr. Wultich. “To minimize skew from slot to slot, the newer models incorporate full differential distribution of the clock along the entire backplane.”
Many VXI-based systems are designed for high-performance precision-demanding applications; and since all signals and power are distributed over the backplane, its characteristics can severely limit system performance. “To obtain the performance you need, carefully examine the power-distribution capabilities, the skew among signal groups and the vendor’s warranty policy,” said Mr. Caserza.
All VXIbus backplanes use a multilayer ground-plane design; however, there is still a lot of variation from one manufacturer to the next, added David Angelo, Electrical Engineer at Tracewell. To achieve the highest performance, Tracewell introduced several innovations.
“Our backplane is the first to use only surface-mount active components and we use a full stripline design for all signal paths,” said Mr. Angelo. “In addition, ground guard tracks on the signal layers greatly reduce both forward and reverse crosstalk. With the surface-mount design, these guard tracks extend between connector pins, further reducing noise and crosstalk. Characteristic impedance is tightly controlled and fully specified.”
Monitoring
Monitoring facilities help verify proper system operation and may aid troubleshooting. Monitoring may include voltage/current levels, power consumption, adequacy of cooling and various VXIbus signal events. The results may be conveyed by analog signals brought out through a separate connector, or by digital data activating VXIbus alarms or VXIbus/IEEE 488.2 status register warnings.1
The degree to which these functions are implemented depends on the manufacturer’s philosophy and the user’s requirements. “We believe you should have access to each power rail, the fan tachometers and the mainframe slot temperature rise,” said Mr. Wultich. “We provide this data on a rear-panel D connector so you can monitor these with a voltmeter or system A/D. As an option, the data may also be provided on a front-panel display, a VXI message-based interface and an RS-232 port.”
A cooling monitor can provide an early indication of a threat to system reliability, according Mr. Angelo. “However, monitoring fan speed alone gives no indications of the actual airflow over the electronics and it can’t detect air blockages such as clogged filters or partially blocked intakes. Temperature monitors are very accurate but give no advance warning. But an airflow monitor can detect a variety of threats,” he said.
Some companies, such as Vero and Ascor, place expanded monitoring circuitry and added intelligence on a separate module. “Our Guardian VXI Module accepts, controls and processes data from 24 measuring points that may be situated anywhere in the test station,” said Mr. Varo. “This monitor gives you the opportunity to program and control actions to be taken when something goes wrong within the entire test environment, such as a fan failure or air-circulation problem in the rack, and not just in the mainframe.”
Conclusion
There are two schools of thought when it comes to mainframe selection. “The first is to buy the best available, so that it is not quickly outgrown or can be reused for other applications,” said Mr. Dhillon. “The second is to buy a product that meets your needs at a competitive price.”
More work is obviously required in the second instance because you must carefully analyze the power and cooling requirements of all the modules intended to be housed in the mainframe as well as all backplane-related aspects. In either case, EMI/RFI, MTBF and MTTR requirements and capabilities must be considered along with the degree of monitoring that is appropriate for your application.
While there is an extensive range of off-the-shelf mainframes available, many suppliers also provide custom solutions and almost all of them are eager to provide you with guidance.
Reference:
1. “Fundamentals of Rack-Mount VXIbus Mainframes,” VXIbus Application Note, Racal Instruments.
VXI Test Products
VXIbus Chassis Features
Power/Cooling Monitor
The 1261B, a C-size chassis which complies with the latest revision of the VXIbus and VXIplug&play specifications, offers 2,000 W of usable power, up to 120 A of +5 VDC and 130 W-per-slot cooling. Special air-flow management minimizes the operating temperature of installed instruments. Cooling and power-supply supervision are provided with an intelligent monitoring system which reports status via the backplane or an RS-232 port. Virtually wireless chassis construction assures reduced MTTR and increased MTBF. Racal Instruments, (800) 722-2528.
VXI Microrack Offers
Screened Environment
The portable VXIbus Microrack accommodates up to six C-size VXI modules in an EMC-controlled and ready-to-run package. It measures 8″ x 17″ x 22″, weighs 33 lb and can be used in a desktop, tower or 19″ rack-mounted configuration. In addition to meeting the inter-module RFI screening requirements of the VXIbus specification, it provides an overall screened environment to help users meet EMC regulations. It includes a Revision 1.4-compliant backplane and a 400-W power supply. $3,300. Vero Electronics, (800) 642-VERO.
Dual VXI Mainframe
Is Flexible
The Model 3026 Dual-Chassis VXI Mainframe provides 26 interconnectable C-size slots. It combines the capabilities of two single-chassis VXI mainframes without requiring external wiring, and supports optional intermediate level switching for optimum utilization of VXI resources. It features an ultra-quiet 1,200-W or 1,500-W switching power supply and a fully shielded double chassis to ensure measurement integrity. A six-fan cooling system resides between the two levels. Any of the slots can be interconnected through hard wiring and software control. Ascor, (510) 490-8819.
Portable Mainframe Has
12-Layer Backplane
An aluminum chassis and a modular approach are employed to create a space-efficient and user-serviceable 6-slot C-size VXIbus transportable mainframe. A hinged front panel ensures that modules are easily removable and a Plexiglas™ window allows viewing of VXIbus instrument indicators. Modules are recessed 3″ and a cable port is provided. Special construction techniques minimize EMI. The cooling system allows for a minimum of 85 W/slot of heat dissipation, regardless of the number of modules installed or whether the front panel is open or shut. Dawn VME Products, (800) 258-DAWN.
Five-Slot VXIbus Chassis
Holds 12 Instruments
The CT-310 five-slot VXIbus chassis accommodates up to 12 instruments when used with the company’s VXIbus VMIP Modular Instrumentation Platforms. Each of the seven VXIbus voltage lines is fed from its own power supply module and monitored by separate facilities. The fan assembly comes in AC and DC versions. The multilayered VXIbus backplane meets all VXIbus Revision 1.4 requirements, is jumperless and uses semiconductor switches for improved MTBF. The chassis weighs 18 lb and has a 20″ x 7″ x 15″ footprint. VXI Technology, (714) 955-1894.
13-Slot VXI Mainframe
Includes Intelligence Features
The VXI410 IntelliFrame™ Mainframe provides 872 W usable power from 0° to 55° C. It keeps power rails within VXI specification limits for ripple, even when subjected to harsh dynamic loads. An adaptive closed-loop cooling system includes temperature monitoring above each module and automatic fan-speed adjustments. The auto-configure backplane uses electronic jumpering of the IACK and BUS GRANT signal lines. A sub-D connector allows monitoring of each voltage rail, ACFAIL*, SYSRESET*, fan speeds and maximum module temperature rise. Message-based or RS-232 monitor output is optional. Tektronix, (800) 426-2200.
Double-Wall Construction
Reduces EMI and Noise
The VXI 13-slot and 6-slot C-size mainframes are VXIplug&play compatible and meet or exceed VXIbus Revision 1.4 specifications. They feature >100 W cooling/slot and may be equipped with power supplies ranging from 500 W to 1,500 W which include independent voltage overload and short-circuit protection. The plenum air- cooling system increases heat dissipation, reduces noise levels and obviates baffling. Voltages and internal temperatures may be monitored and an over/under-voltage indicator and audible alarm warns if problems occur. Mac Panel, (910) 861-3100.
Rack-Mount/Benchtop Enclosure
Includes System Monitor Panel
The Vector-Pak® VXI Rack-Mount/Benchtop Enclosures are available in C- and D-size configurations. They are furnished with a 13-slot, 12-layer VXI backplane; an internally cooled 1,200-W power supply and a plenum-directed cooling system. The fan capacity varies with heat-dissipation requirements and is self-regulating. A front-mounted system monitor reports voltage variation, airflow and standby conditions via LEDs and audio alarm. Remote monitoring is attainable through a rear connector. A separate 5-V supply for standby power is included. Vector Electronic, (818) 365-9661.
Mainframe Provides High Power,
Operates up to 50°C Ambient
The E1401B C-Size VXI Mainframe employs pressurized cooling for even airflow distribution to all slots regardless of module loading. Available power is 1,890 W; usable power is 900 W. Outputs are protected from overtemperature, overvoltage, overcurrent and shorts. The 13-slot backplane features solid-state automatic daisy-chain jumpering for the VMEbus grant and interrupt acknowledge lines. A connector on the rear panel provides access to all power supply voltages, remote on/standby, ACFAIL, SYSRESET and +5-V STDBY. Hewlett-Packard, (800) 452-4844
Copyright 1996 Nelson Publishing Inc.
February 1996