VXIbus - A Modular Standard For Test & Measurement VXIbus Objectives The goal of the VXIbus is to define a technically sound modular instrument standard based on the VMEbus (IEEE-STD-1014) that is open to all manufacturers and is compatible with present industry standards. VXIbus is an abbreviation for "VMEbus eXtensions for Instrumentation." The VXIbus specification details the technical requirements of VXIbus compatible components such as mainframes, backplanes, power supplies and modules. The specification also provides for interconnecting and operating different manufacturers' products within the same module mainframe (also known as a chassis). The VXIbus For many years test equipment and test system manufacturers have been trying to reduce the size of their products. Independent companies actually had some success, however, the products available could only be supplied from the original system manufacturer. Both commercial and military users of test equipment needed a means to standardize such downsizing in order to provide: 1. a wider choice of product capabilities 2. smaller size and weight 3. tighter timing coordination between instruments 4. longer system support through multi-vendor solutions. Five instrument companies formed the VXIbus Consortium and held its first meeting in July of 1987. These companies were Colorado Data Systems, Hewlett-Packard, Racal-Dana Instruments (now Racal Instruments, Inc.), Tektronix and Wavetek. A working document was rapidly drafted and complete system specification was issued less than one year later. As a result of the consortium's work, other companies chose to accept the specifications for development of new products. The IEEE Standards Committee, in its proposed standard P1155, adopted the VXIbus Consortium's specifications. The U.S. Air Force accepted the specifications as the basis for its Modular Automatic Test Equipment (MATE) Instrument-on-a-Card (IAC) standard. More than three quarters of the MATE document consisted of the VXIbus specification. The acceptance of the VXIbus in the commercial and miliary communities spurred the growth of companies involved with building products. During 1988 a number of other companies joined the executive committee of the consortium to further develop the specifications. Advantages of VXIbus Based-Systems The greatest advantage is that the VXIbus is closely tied with the VMEbus. VXIbus does not exclude VMEbus, in fact it contains specific provisions to ensure that standard VMEbus cards will operate properly with VXIbus systems. The VXIbus can accommodate almost any system structure. This means that a user is not locked into any particular type of microprocessor, operating system or interface to the host computer. The specification defines a hierarchy of device types with specific configuration protocols. The concept of shared resources such as power supplies, cooling and the common mainframe means the instrument manufacturers no longer need to include certain facilities within the modules. This results in fewer components being required in an instrument. One of the most significant advantages of having a number of instruments in a common, closely coupled environment is tighter time coordination. This has lead to higher levels of system performance than ever before possible. Traditional rack-and-stack instruments connected via cables do not match the tight control over signal characteristics and propagation delay that is possible in a defined and controlled backplane environment. This design enables TTL triggering at rates in excess of 10MHz while ECL trigger rates may exceed 50MHz. Using the Local Bus on P2 will allow data transfer rates approaching 100MB/second. Data rates between VXIbus modules can exceed 50MHz via the backplane's VXI Local Bus. Extending the VMEbus for Instrumentation A goal of the VXIbus Consortium was to allow for the broadest possible range of instrument performance and cost. Another goal was to make as much use of existing standards as possible. The A and B card sizes of VMEbus were included as part of the VXIbus standard. In fact, VXIbus retains the P1 connector and the center row of the P2 connector exactly as defined by the VMEbus. This includes the 5V and +12V power pins on P1. A major objective of VXIbus is to standardize instrumentation. However, the majority of high performance instruments will not physically fit in the smaller VMEbus areas. In order to overcome this, the Consortium added two additional card sizes for VXIbus. These include C-size, which is approximately 13 inches (33cm) deep and nine inches (23cm) high, and D-size, which is approximately 13 inches (33cm) deep and 14 inches (35.5cm) high. The C-size card has the same connectors as B-size VMEbus modules, however all pins on P2 are fully defined. The D-size card may have an additional connector, known as P3 which adds extra resources necessary for higher instrumentation performance. Figure 1. Module Sizes A VXIbus module may be a printed circuit board (PCB) or an enclosed assembly that contains several PCBs. If an instrument needs more than 1.2 inches (3cm), it may take up multiple slots in a VXIbus mainframe. The VXIbus specification requires manufacturers to publish module specifications relating to VXIbus, as well as those relating to the instrument's performance characteristics. These include cooling requirements (i.e. minimum airflow for maximum allowable temperature rise) and maximum power requirements. All modules must also meet EMC radiation and susceptibility criteria. These EMC requirements ensure that high performance instruments do not interfere with each other. Figure 2. Single Width Module Figure 3. Example of a D-size System A VXIbus system may have up to 256 devices, including one or more central timing and arbitration modules. These are referred to as the Resource Manager or Slot 0. Thirteen single-slot instrument modules conveniently fill a standard 19-inch cabinet when mounted vertically on 1.2 inch centers. Although there is a maximum of 13 single-slot modules in a VXIbus subsystem, there is no minimum number. For example, a subsystem may contain just a resource manager with two or three modules. A number of different size mainframe are available to suit the different card sizes. However, the most popular is C-size as this can handle the standard A and B-size VMEbus cards as well as the many available C-size instruments while maintaining a reasonable size. The VXIbus specification precisely defines the cooling, power and EMC tolerances of a VXIbus mainframe. Selection of chassis and modules is an interactive process as the chassis must be able to meet the total power and cooling requirements of the modules. The VXIbus specification expands upon the VME specification by defining all pins on the P2 backplane and adding P3. The VXIbus P2 adds a 10MHz clock, ECL and analog supply voltages, ECL and TTL trigger lines, an analog sumbus, a module identification line, and a daisy chain structure called the Local Bus. P3 provides more of the above and adds a 100MHz clock and a star bus. Figure 4. VXIbus Defines all Pins on Connectors P1, P2 & P3 Clock Bus The clock bus provides two clocks and a clock synchronization signal. A 10MHz clock (CLK 10) is located on P2 (for C-size cards) and a 100MHz clock (CLK 100) with a synchronization signal (SYNC100) are both on P3 (for D-size cards). All three signals are differential ECL. Both clocks and synchronization signal are sourced from the Resource Manager and individually buffered through the backplane to each module. This ensures minimal loading on the reference and keeps the signal free of jitter. Previously in rack-and-stack systems, distribution of a clock signal in such a manner would have been handled by many feet of cable introducing synchronization and buffering problems. Figure 5. Clock Bus Trigger Bus The trigger bus consists of eight TTL trigger lines and two ECL trigger lines, all of which are located on P2. Four additional ECL trigger lines are for inter-module communication, and with a worst case delay along the backplane of 2ns ensures tight time coordination between modules. Figure 6. Trigger Bus Local Bus A 12-line local bus on P2 provides a private module-to-adjacent module communication bus. The purpose of the local bus is to decrease the need for ribbon cable jumpers between modules. P3 provides an additional 24 lines for use as a local bus. Figure 7. Local Bus Analog Sumbus The Sumbus is an analog summing node that runs the length of the chassis backplane and terminates into 50ê. It is used to sum outputs from sources in order to build up complex waveforms that can act as a stimulus for another module or be output to a device under test. Figure 8. Analog Sumbus Module Identification Lines The MODID lines allow a logical device module to be identified with a particular physical location or slot. Using the MODID lines the resource manager can detect the presence of a module in a slot, even if it is not operational. It can also identify the slot occupied by a specific module. Each module connects an 825ê resistor between its MODID pin and ground. By sensing the resistance, the Resource Manager can determine whether or not a module is installed in each of the 12 slots. Other information such as manufacturer, model number and last date of calibration can be obtained via the resource manager's control of the configuration registers. Figure 9. Module Identification Lines Power Distribution The power distribution bus can provide up to 268 watts of power to a single module that has P1, P2 and P3. The power is delivered to the backplane as seven different regulated voltages, selected to meet most instrumentation needs. Figure 10. Power Distribution Star Bus The star bus is located only on P3. It is composed of two lines--STARX and STARY--connected between each module slot and Slot 0. Slot 0 may be viewed as the center of a starfish with twelve legs--each module being situated at the end of an equal-length leg. Figure 11. Star Bus Figure 12. Commander-Servant System Structure System Control--Resource Manager This module is defined to deliver the CLK10 and access the MODID pins of the modules (also CLK100 and SYNC100 on P3-equipped systems). The Resource Manager should be selected to meet the requirements for all selected instrument modules. The Resource Manager is known as the Slot 0 Controller. It is a common resource system module containing the VMEbus Resource Manager and VMEbus System Controller. Many Slot 0 modules include other functionality for example, data interfaces (e.g. IEEE-488.1) and system intelligence. The VXIbus specification defines two common implementations for instruments: register-based devices and message-based devices. Typically, register-based devices are simple modules without embedded intelligence to respond to register reads and writes over the backplane. Examples of register-based devices include digital I/O cards and simple ADCs and DACs. Message-based devices follow the VXIbus word serial communication protocol and are typically devices with embedded microprocessors that receive and execute ASCII commands. The most sophisticated instrument modules are usually message-based devices. A register-based module requires a message-based module to control it if the user wants to program it using a high level ASCII command language such as IEEE-488.2 or SCPI. The manufacturer of the register-based module specifies the required message-based module. This can be combined with an interface or Slot 0 module, or supplied as an additional module. Message-based modules may have the capability of translating for another message-based module. For example, translation of SCPI messages into the "native" IEEE 488.2 language of the message-based module. System Cooling Requirements The VXIbus specification fully defines the cooling requirements for both mainframes and modules. Cooling requirements are specified in a way that accommodates a wide range of system applications. Each manufacturer of chassis and/or modules, is required to publish the product's cooling capacity and/or requirements. Modules should specify necessary rates of air flow and resulting pressure drop across the module. Mainframe manufacturers must publish a curve of air volume as a function of the pressure drop from the top to bottom of the card cage for the worst slot position in the mainframe. These two specifications will allow the user to determine that a module is compatible with a mainframe. Direction of airflow is also specified from P3 towards P1. This allows the module manufacturers to design the module with airflow direction to the hottest components or by placement of heat-sensitive components towards the bottom of the card. Methods of testing the cooling requirements are also defined. Electromagnetic Radiation and Susceptibility Considerations Radiated emissions and susceptibility are also quantitatively defined. Test methods are recommended to ensure that all modules and mainframes meet the VXIbus specifications in terms of EMC. Conclusion The VXIbus represents a step forward for instrumentation--offering both users and manufacturers a clear set of guidelines for a modular approach to meeting systems needs. Racal Instruments believes that the VXIbus represents the future for instrumentation. As a founding member of the VXIbus Consortium, Racal Instruments recognizes the importance of the VXIbus instruments and users. Figure 13. Radiated EMC Requirements TOC 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13