The VXIbus standard incorporates the functionality of the VMEbus and provides additional capabilities primarily aimed at the needs of instrumentation. One of these additions is a bus structure, referred to as the local bus.
The VXI local bus serves as either a wideband analog bus or a high-speed digital bus. Its specific use and implementation are defined by the individual VXI manufacturer; its overall characteristics are defined by the VXIbus standard.1,2 But in spite of its flexibility and large bandwidth, the VXI local bus has seen relatively little application.
Local Bus Description
The VXI local bus has a daisy-chained architecture. This implies that modules using the local bus must be installed in adjacent slots to complete the chain. The local bus provides 24 pins (12 lines) on the P2 connector and, if the P3 connector is present, another 48 pins (24 lines) on P3. The local bus does not extend beyond slot 12.
The VXIbus specifications define several classes of local bus signals by the signal type and the levels allowed. Five classes of local bus signals are currently specified. There are two logic levels, TTL and ECL, and three analog levels that range from ± 5 V to ± 42 V (Figure 1). To accommodate the high-speed ECL signals, all of the local bus backplane lines are designed with a 50-W nominal characteristic impedance.
Since various VXIbus modules may support incompatible signal levels, it is necessary to prevent damage from occurring should they be plugged into adjacent slots. The VXIbus standard addresses this issue by supplying a mechanical lockout keying system that is used on the front of the modules to indicate the signal class that the module supports. These keys ensure that no damage will occur to adjacent local bus modules that are not of the same signal class.
VXI modules that support P2 operations are equipped with a P2 key located at the top of the module’s front panel. Modules that support P2 and P3 local bus operations have keys at the top and the bottom of the front panel.
Local Bus Testing
Most local bus devices can sense and source data to each of the slots adjacent to the UUT. As a result, a general-purpose test system for these modules must have system modules in each of the adjacent slots (Figure 2). These system modules must sense and source data on the local bus as well as communicate with the system controller.
A good example of local bus testing is the production test for the HP E1429 Digitizer and the HP E1488 Instrument Memory Module. Both of these products use the local bus for high-speed data transfers at ECL logic levels. Several data-transfer modes allow the devices to use and generate data, and to append, insert and pipeline data from other modules.
To verify these and several other operating modes, a pair of HP E1562 VXI disk modules is installed in the test system, one on each side of the UUT. This disk drive module communicates on the VXI backplane via the local bus and supports D08, D16, D32 and shared RAM data transfers.
A package of utility subroutines allows the test software to easily transfer test data files to and from each of the disk modules. The package also provides a series of standard test data patterns so you can define custom patterns if necessary.
A typical local bus test suite exercises the UUT in each of its local bus operating modes. In the generating mode, for example, the UUT is configured to output a known test vector to one of the disk drives. An external system controller can then access this data file via a command module and the D16 bus and verify its accuracy.
Alternatively, an embedded controller may be used instead of a command module and an external controller. There is a downside to this approach. If the power to the VXI cardcage is cycled, for the insertion and removal of the UUT for example, there will be a long time delay for the embedded controller to reboot. For this reason, an external controller with a command module is much more convenient for test-system applications.
Summary
The VXI local bus provides the VXI backplane with a great deal of high-performance flexibility, offering digital and analog data rates well up into the tens of megahertz range. But such performance also carries with it unique challenges for the test engineer. Fortunately, the growing number of available VXI products makes test- system design a much easier task.
References
1. Black, J., The System Engineer’s Handbook, Academic Press, 1992.
2. VMEbus Extensions for Instrumentation—VXIbus System Specification VXI-1, Revision: 1.4, April 21, 1992, VXIbus Consortium, La Mesa, CA.
About the Author
Bertram S. Kolts is a Manufacturing Test Engineer at Hewlett-Packard, a position he has held for 17 years. Before that, he spent eight years doing integrated circuit testing for HP. Mr. Kolts received a B.S.E.E. degree from Colorado State University. Hewlett-Packard, Loveland Manufacturing Center, 815 SW 14th St., Loveland, CO 80537, (970) 679-2988.
Copyright 1996 Nelson Publishing Inc.
October 1996