Choose the Right Bus for Electronic Test Systems 

DAN ROMANCHIK 

In the real world, a bus is something that moves people from one place to another. In the computer world, a bus is something that moves data from one place to another. In the real world, most buses are pretty much the same. They all have engines that belch black smoke, four or six wheels, and a driver. It doesn't really matter much which bus you choose as long as it is headed in the direction you want to go. 

In the computer world, however, there are many different types of buses: 

• Serial buses that transfer data one bit at a time 
• Parallel buses that transfer data in eight- or sixteen-bit chunks 
• Buses designed for computer systems, such as PC and PCI buses 
• Buses designed specifically for computer-controlled, electronic test-and-measurement applications, such as the IEEE 488 bus and the VXIbus.

Serial buses 
Serial buses, such as the EIA-232-E, EIA-422-A and EIA-423-A, are the oldest, and still some of the most widely used, types of data-communications buses. Each PC, for example, comes with at least one serial port. You normally would use this port to connect a modem to the computer, but you can connect nearly any kind of device to a computer with a serial bus. Some companies make test equipment that uses a serial port to transfer measurement data. 

Using serial buses is the simplest way to transfer data, and because of this, there are some applications where a serial bus is your best bet. Cables are widely available and inexpensive, and it is possible to locate the test equipment at some distance from the computer (approximately 80 to 100 m using EIA-232-E). Also, because nearly every PC has a serial port, it's easy to connect a computer to the test equipment. 

One of their biggest disadvantages is that serial buses are relatively slow. These devices transfer data one bit at a time, meaning it will take eight separate transfers to move just one byte of data. With EIA-232-E, the maximum data rate is also dependent on cable length. Longer cables have more capacitance, and because the signal is a voltage signal, rise times are longer, thereby limiting the maximum data rate. (EIA-422-A overcomes this limitation by using differential, instead of single-ended, signals. The use of differential signals also makes EIA-422-A a good choice for use in noisy environments.) 

Another disadvantage of using the EIA-232-E interface is that there are no control signals for test-and-measurement applications, such as trigger signals. Therefore, if you need sophisticated control, a bus designed specifically for test and measurement might be your best choice. 

IEEE 488: A true test bus 
To overcome the limitations of the serial bus in test-and-measurement applications, Hewlett Packard, Palo Alto, CA, invented the Hewlett Packard Interface Bus (HP-IB) in the 1960s. In addition to test-and-measurement equipment, Hewlett Packard used this bus to connect a wide variety of peripherals, including printers and magnetic-storage devices, to its computers. This bus quickly gained popularity among test engineers, and in 1978, the Institute of Electrical and Electronic Engineers (IEEE), New York, drafted a standard, IEEE 488 , which defined the mechanical and electrical characteristics of the bus, as well as its functional characteristics. The current revision of the standard is IEEE 488-1987, titled IEEE Standard Digital Interface for Programmable Instrumentation. 

Today, the bus, which is also known as the General Purpose Interface Bus (GPIB), is still in wide use in test-and-measurement applications. In 1987, the IEEE published an extension to the original standard, IEEE 488.2-1987, IEEE Standard Codes, Formats, Protocols, and Common Commands for Use with IEEE Std 488.1-1987. IEEE 488.2-1987 defines some common commands, command syntax, and data structures that test engineers can use when communicating with the IEEE 488 bus. 

The IEEE 488 bus is a parallel bus, (see "IEEE 488 bus lines," below), transferring eight data bits at a time. In addition to the eight data I/O bits, the bus has eight control signals, including three handshake lines and five bus-management lines. Also included in the bus are eight ground lines used for shielding and ground returns. The maximum data rate over the standard IEEE bus is 1 MB/sec., although there is a proposal making its way through the standards-approval process that would speed up the bus to 8 MB/sec. In either case, the actual throughput you will obtain from the system will be much lower than the maximum data rate.

To connect test equipment to a computer using the IEEE 488 bus, you need to install an interface card in the computer. These are available from several sources and are relatively inexpensive. This card, along with the software necessary to use it, turns the computer into the "system controller." The system controller can talk and listen to other devices on the bus, and control bus operation. 

With one interface card, you can connect as many as 14 other devices to the system controller. These can be pieces of test equipment, such as an oscilloscope or digital multimeter, or peripheral devices, such as a printer. You can connect these devices in a star configuration, which means that each device connects directly to the controller, or in a daisy-chain configuration, in which each successive device connects to the previous one. The maximum distance between devices is 2 m and the maximum allowable distance from the system controller to the last device on the daisy chain is 20 m, although you can purchase bus extenders that allow you to increase this distance if necessary. 

One big advantage to using the IEEE 488 bus is that it is widely supported. The standard has been around a long time, and you can find IEEE 488 controller boards for most popular computers , including PC-compatibles, Macintosh, and Sun workstations. There is also plenty of software to support IEEE 488 systems. Whether you're using DOS, Windows, OS/2, Mac, or UNIX, you will find software that will make developing your test system easier. The digitizers in IEEE 488 instruments are also among the highest in terms of sampling rates and resolutions (see "Digitizer technology capabilities," below). 

Another advantage is that most manufacturers of high-end test equipment support the IEEE 488 interface, building it into their equipment. There are thousands of instruments available with IEEE 488 interfaces. Most digital oscilloscopes, for example, now have a built-in IEEE 488 interface that allows users to control them and to gather measurement data. 

You can use this capability to simplify your test programs. For example, you can program some digital oscilloscopes to find important waveform parameters, such as a peak value, or perform a Fourier transform. This approach simplifies the test program, making the test instrument, rather than the test program, do all the work. Perhaps the biggest disadvantages to using the IEEE 488 interface are cost and size, and in some applications, performance. In an IEEE 488 system, the test equipment connects externally to the system controller, meaning that the controller and test equipment are separate units that connect through the IEEE 488 cable. This adds to both the cost and the size of a system because the controller and all of the test equipment are in separate boxes, each with its own displays and controls. If you're using a computer to perform most of the data analysis and display, these functions are wasted on the test equipment. 

Measurement functions in the computer 
One way to reduce the cost and size of a test system is to move the measurement functions inside the system controller. You can do this by purchasing add-in cards that plug directly into the computer. PC add-in boards are available from several manufacturers and perform a wide variety of functions, including analog input and output, digital input and output, filtering, and signal conditioning. Most analog input boards offer multiple channels and allow you to configure the inputs as single-ended inputs or differential inputs. 

These boards are also available for a wide variety of computers. You can, for example, purchase boards for PC-compatible computers that use the Industry Standard Architecture (ISA) bus, the Extended Industry Standard Architecture (EISA) bus, and the newest type of bus, the PCI bus. Boards are also available for portable computers that use a PC card (PCMCIA) bus and for Macintosh computers that use the NuBus. 

When manufacturers first introduced these boards a little more than ten years ago, performance was poor, resolution was low, and they were often noisy and not very accurate. Ten years of development have really paid off, however. Manufacturers have improved both the analog and digital designs of these boards, and if your application requires it, you can find boards with instrumentation-grade amplifiers that can make high-quality measurements. Performance is much improved, too. Boards using the PCI bus are capable of transferring data at more than 100 MB/sec. 

If you've decided to design a test system using PC add-in cards, which bus should you choose? For most applications, ISA boards or EISA boards will do the job quite nicely. Data transfer over the ISA bus is typically 1 MB/sec., which should suffice for most applications. If, however, your test system will be taking measurements from many different channels at high rates, then you may have to go with PCI bus cards. The PCI bus can theoretically transfer data at 132 MB/sec. (transferring four bytes at a time). At these speeds, though, you begin to run into PC-hardware limitations. 

If you are making measurements with a portable computer, then your choice should be a PC card. This option is popular, for example, with auto manufacturers and automotive suppliers making measurements in moving vehicles. The use of a portable computer and PC cards allows them to not only gather data, but also to analyze the data immediately. 

VXIbus: high performance, compact package 
While PC-based test systems offer cost and size advantages over test systems using the IEEE 488 bus, they can't achieve the high performance of IEEE 488 systems. For systems that require very high performance as well as a compact size, a good choice is the VXIbus. 

The VXIbus is another bus designed specifically for computerized test-and-measurement applications. Several manufacturers introduced VXIbus modules and systems in 1987, and an IEEE standard, IEEE 1155-1992, IEEE Standard VMEbus Extensions for Instrumentation: VXIbus, was published in 1992. Today, there are more than 1,500 commercially available VXIbus products, including computer controllers and test instruments, such as voltmeters and oscilloscopes. Also available are basic instrumentation building blocks, such as digitizers and filters. With these modules--and a good software package--you can build what are known as "virtual-instrumentation" systems. 

As its name implies, it's based on the VMEbus, a computer bus popular for industrial-computing applications. The VMEbus is a 32-bit bus that can transfer data at rates up to 40 MB/sec. To this 32-bit bus, VXIbus designers have added signals for precision timing, synchronization between modules on the bus, and resource management. In addition to specifying the electrical characteristics of the bus, the standard specifies the mechanical, power, cooling, and electromagnetic-interference characteristics of VXIbus modules. These specifications help ensure that modules from different manufacturers will work together when used in a system. 

VXIbus systems can be configured in an almost limitless number of ways. VXIbus instrumentation modules reside in a VXIbus chassis that may contain up to 13 modules. The computer controlling the system may reside in the chassis, or it may be a stand-alone computer connected to the chassis with an IEEE 488 interface or a higher-speed interface called the MXI bus. If you are designing a very complex system and need more slots than one VXIbus chassis can hold, you can use a second chassis, connecting it to the main system via an IEEE 488 interface or with the MXIbus. 

One disadvantage of using VXIbus is increased cost. VXIbus modules generally cost more than PC add-in cards. VXIbus systems are also sometimes more difficult to integrate than PC-based systems, which adds to the cost of developing a test system. To address the integration problem, some manufacturers formed the VXI Plug and Play Alliance in 1993. This alliance addressed many of the system issues developers face when putting together a VXIbus system, and has come up with standard software interfaces that help ensure that modules will work together at both the system and hardware levels. 

The hybrid solution 
While you could theoretically put together a system based solely on a single bus type, there's really no need to limit yourself to just one bus. If it makes sense to mix and match, do it. For example, your test system may be almost completely PC-based, but for a specific test, you may need an instrument function that you can most easily implement with an IEEE 488 instrument. 

When designing a hybrid system, it's important to consider what kind of software you will use. If you're going to use a commercially available test software package, make sure that the software package will support the different types of interfaces in your system. This will save you a lot of time in writing software to control these interfaces. 

Still more bus choices 

Another possibility is the Enhanced Parallel Port. EPPs are available on desktop PCs and some notebook computers, and some test-equipment manufacturers make data-acquisition equipment using this interface. Data transfers up to 2 MB/sec. are possible with the EPP, which is twice the data rate of the IEEE 488 bus. Test equipment using the EPP allows users to connect more channels to a notebook computer than they could connect to a PC data-acquisition card. 

A third possibility is the IEEE 1394 bus. The IEEE 1394 bus, also sometimes called "Firewire", is a serial bus that may soon become standard equipment on personal computers. It is capable of transferring data at 100 MB/sec. and, unlike the EIA-232-E interface, you can connect multiple devices to a computer. 

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