VXIbus Mezzanine Modules Maximize I/O and Function Choices

Add-in modules let you match VXIbus systems to specific test needs.

Jon Titus, Editorial Director -- Test & Measurement World, 8/1/2000

Mezzanine modules come in many sizes and provide many functions. But they break into two camps—those based on open-standards and those that employ proprietary buses. In the VXIbus market, both types play important roles.

The standardized mezzanine modules further divide into two popular subgroups that together encompass most of the market, IndustryPack modules and M/MA modules. In cooperation with the VMEbus International Trade Association (VITA), ANSI standardized both module types, actions that reassured buyers about the longevity of module markets. Approval of the standards also gave rise to many module suppliers (" Mezzanine-Module Resources").

You may wonder why anyone would go to the trouble of configuring a VXIbus system using mezzanine modules. After all, you can buy just about any functions you need on standard VXI-bus boards. The answer in a word: cost. Why buy expensive VXIbus boards only to use a few functions on each? Better to buy a bare VXIbus motherboard, or carrier board, and choose only the functions you need. Instead of purchasing a VXIbus chassis and populating it with six or seven full-blown VXIbus boards, you may need only two or three carrier boards loaded with small mezzanine modules.

The following example shows how a few mezzanine modules increased the capability of a tester. Agilent Technologies was supplying a VXIbus-based system that tested cell phones, one at a time. A single RS-232 port on a VXIbus card sent commands and program steps to a phone undergoing testing. Timing analysis showed engineers they had sufficient time in the test sequence to test three more phones. But that meant they would need three extra RS-232 VXIbus cards (plus some additional functions) that would greatly increase the test system’s cost. As an alternative, the engineers put together a lower-cost system using a carrier board and four independent RS-232 communication mezzanine modules. The modified tester now checks four phones in the time it took to test one.

IP Modules Offer Many Choices

IndustryPack (IP) mezzanine modules (Fig. 1) come in about 350 varieties from more than 130 manufacturers. You can find a complete list of suppliers on the VITA Web site (see "Mezzanine-Module Resources," p. 16). These modules provide analog and digital I/O functions, as well as serial I/O, network control, motor control, and other specialized functions. The IP modules measure 1.8x3.9 in. (46x99 mm)—slightly narrower than and slightly longer than a business card. Each module provides two connectors, one for the interface to the carrier board and one for I/O connections to the carrier board. IP modules also come in a double-wide size that measures 3.6x3.9 in. (91x99 mm). This larger module provides two interface connectors and two I/O connectors. All I/O signals go through the carrier board to external I/O connectors that mate with test equipment, test stands, probes, and so on.

Look at M Modules, Too
You also can choose modules from the M and MA family of mezzanine modules—collectively called M/MA modules—promoted by the Manufacturers and Users of M-Modules (MUMM), and standardized by VITA and ANSI. These modules are slightly larger than IP modules, and they also come in two sizes: 2.1x5.7 in. (53x145 mm) and 4.2x5.7 in. (107x145 mm).

The basic M modules and the expanded MA modules use the same size PCB, but the MA modules include an extra row of contacts in the motherboard connector. The MA modules operate with 24 address lines and 32 data lines; the M modules use a 16-bit data bus and a 16-bit address bus. Both the M and MA modules address 256 I/O locations. You can purchase more than 150 varieties of M/MA modules from 30 manufacturers. You’ll find lists of manufacturers on the VITA and MUMM Web sites. The M modules have a large following in Europe, where they got their start, and they continue to make inroads in North America.

Unlike the IP modules that route I/O connections through their carrier boards, the M/MA modules provide front-panel connectors. These connectors protrude through the front panel of a VXIbus carrier board and provide direct connection for I/O signals. A B-size VXIbus carrier can accommodate as many as four M/MA modules, and a C-size carrier can hold as many as six. The single-slot E2251A C-size VXIbus carrier from Agilent Technologies (Fig. 3) provides front-panel connections for four M modules. The remaining two M modules route their I/O signals through cables to external connectors.

If you don’t find the functions on IP or M/MA modules that fit your needs, several manufacturers supply prototyping boards based on the IP or M/MA standard. Thus, you can take advantage of the IP and M/MA connections to the motherboard and wire your specialized circuits on a prototype board. These modules provide bus-control circuits, power-supply connections, and I/O connectors just as would a normal IP or M/MA module.

No matter which type of mezzanine module you buy or build, you may be in for some software work. Because the module suppliers traditionally worked with VMEbus users, they’re used to supplying only C-language drivers. But as more VXIbus users have begun to design systems using IP and M/MA modules, suppliers have started to provide easier-to-use drivers. You may have to wait a bit for fully compatible VXIbus plug-and-play drivers, though. To most IP and M/MA suppliers, the VXIbus market is still a small part of their business.

The IP and M/MA modules aim mainly at industrial-control applications, so although you can choose from a wealth of functions, not all suit T&M applications. You may need to turn to proprietary mezzanine modules that also play an important role in VXIbus systems. Bustec (Shannon, Ireland), VXI Technology (Irvine, CA), and Agilent Technologies (Palo Alto, CA) all offer VXIbus cards that incorporate a proprietary bus. But why would someone buy a product with a proprietary bus when IP and M/MA modules offer so many choices? The answer is special features, ease of use, and ease of configuration.

Using a proprietary bus doesn’t lock a manufacturer to specific module sizes, connector types, or signal groups, so the manufacturer can add features such as extra triggering signals to its own mezzanine module. A manufacturer also could enhance power connections and add control circuits not found on standard modules. Due to limited card size and circuit restrictions, you won’t find a 61/2-digit DVM or a 50-MHz ARB on an IP module. But you can buy such functions on proprietary mezzanine modules.

Because manufacturers of proprietary modules don’t use IP or M/MA interface circuits, they can make their modules operate more like standard VXIbus cards, thus they’re easier to use and require less work to set up. And proprietary modules designed specifically for T&M applications fill specific functional test, signal-switching, and signal-conditioning needs.

Finally, because proprietary add-in modules come from one source, you don’t have to worry about getting a VXIbus system to properly work with low-level module drivers for different modules from different suppliers. Because the proprietary-module vendors aim their modules squarely at VXIbus applications, they also write drivers expressly for VXIbus systems, not for general-purpose use. Thus, the drivers are easy to configure. In fact, you can forget about a proprietary bus as a design issue. Just accept it as a means by which suppliers increase the flexibility of their VXIbus boards.

Look at an Example
Bustec’s line of ProDAQ boards provides an example of how a manufacturer takes advantage of a proprietary bus to configure VXIbus cards for customers. The company’s 3150 high-performance motherboard can pack as many as eight modules in two VXIbus card slots ( Fig. 4). The company’s modules include a high-speed transient recorder, a counter-timer, an ADC, and an analog-comparator interrupt controller, among others. You choose the modules you want and the manufacturer will put them on the carrier for you.

In the Bustec configuration, all the mezzanine modules act like standard VXIbus cards. And because the Bustec cards were designed specifically for data-acquisition and T&M applications, they provide features such as self-calibration and on-module memory. And the modules all provide I/O connections at the VXIbus carrier’s front panel.

VXI Technology takes a similar approach with its line of VXIbus cards. The cards all accept proprietary modules, although the company’s VXI Modular Instrumentation Platform (VMIP) configuration is available to the public. No other company yet offers products that use the VMIP configuration, but several VXI Technology customers develop VMIP cards for use in systems they sell.

Some VXIbus mezzanine modules are meant to be hidden from users. Agilent Technologies produces two VXIbus boards, the E1419A multifunction measurement card and the E1413C 64-channel scanning ADC board, both of which accept proprietary mezzanine modules. The modules—called signal-conditioning plug-ons (SCPs)—let buyers specify the types of signal conditioning they want on input channels.

The SCP family includes direct inputs, filters, attenuators, strain-gauge excitation circuits, current sources, and other types of signal conditioning. By choosing the SCP functions you need, you can configure a multichannel data-acquisition system that uses a variety of sensor types. You no longer need one VXIbus data-acquisition card per sensor type, nor do you need external signal-conditioning modules.

The use of mezzanine modules gives you flexibility in specifying a test system. Not only can you choose mezzanine boards from the IP and M/MA families, you can select proprietary modules from different manufacturers and use them together in one VXIbus system. The variety of choices lets you configure the test system you need, and you’ll do so using the fewest VXIbus slots. T&MW

You can contact Jon Titus at jontitus@cahners.com.