Revisiting Data Acquisition Configurations

The last time EE examined physical configurations for data acquisition systems (DAS) was in early 1993, slightly more than four years ago.¹ At that time, we said that the then-popular configurations—the stand-alone DAS, the PC plug-in board and the PC add-on box—will continue to thrive, each capable of providing an optimum solution in its particular application niche.

As predicted, the three configurations are still being widely used but have been joined by a fourth. This addition brings signal conditioning and A/D conversion closer to the sensors and communicates with the PC via networked connections.

Previous shortcomings of the PC plug-in board implementation, such as noise interference innate to the PC environment as well as sensor lead connection limitations, also have been addressed. In fact, now there are so many interconnection and signal- conditioning approaches to choose from that an update on available solutions is in order.

As it was then and still is today, even the most knowledgeable DAS design engineers do not cherish resolving I/O address, IRQ level or DMA access assignment conflicts often encountered during the initial DAS integration. Even this task has been simplified, with more of it done by software (instead of by setting switches or jumpers) or in some cases by an automatically software-controlled setup via Windows 95 Plug and Play.

Interconnection Choices

Due to their physical size, PC plug-in boards can accommodate only a few connectors or terminal blocks, although each connector may accommodate dozens of I/O wires. This is not a problem for most high-speed boards which are required to handle data from only a few sources since they usually accept inputs via a few coax connections.

If the number of required I/Os is limited, wires may be directly connected to terminal blocks on the board. However, if a substantial number of leads must be connected to the board or the data acquisition circuitry is contained within a PCMCIA card, an external connection block, terminal panel or interface box must be provided.

Most analog input PC plug-in boards accept 16 single-ended or 8 differential signals accessible via a multipin connector. “In this case, the common interconnection method is to plug a shielded twisted-pair low-noise cable into this connector and terminate it at the other end at a screw-terminal breakout (STB) box,” said Scott Walsh, applications engineer at Analogic. “Sensor wires are then connected to the terminals housed in the STB.

“This method easily supports up to 16 analog input channels in addition to all other I/O connections,” Mr. Walsh continued. “To expand up to 256 analog input channels, this same remote screw-terminal method may be used; in this case, the screw terminals are mounted on high-speed multiplexer boards. Our multiplexer boards can also be fitted with ribbon-cable connectors in place of the screw terminals. Either way, connection to the host data acquisition board is done using the same low-noise cable.”

Digital and digital-plus-analog boards usually feature separate connectors for various signal groupings. “For instance, we provide a range of PC plug-in boards that accept up to 120 bits of parallel data, segregated into 24-bit groups with a 50-pin header provided for each group,” stated Rick Lagrand, vice president of sales and marketing at Acces I/O Products.

“We also provide ribbon cables to connect to the external data sources or destinations. If inputs come from various places, we furnish screw-terminal accessories to be used for I/O terminations and then ribbon cables to complete the connections to the I/O card,” Mr. Lagrand concluded.

When several hundred inputs must be accommodated, termination panels ordinarily are used. These may be rack-mounted assemblies or be an integral part of a free-standing PC-connected DAS.

“For data acquisition applications that require 75 to 1,000 inputs, it is best to use an external box configuration interfaced with the host PC through a single plug-in card, such as our 4000 Series I/O system,” said Don Di Rocco, president of American Data Acquisition (ADAC). “All A/D, D/A, I/O and signal conditioning functions are accomplished outside the PC in the external enclosure via a line of I/O cards. These cards are programmed by the host PC as though they were plugged into the PC’s ISA bus. Up to four enclosures, each handling up to 256 channels, can be daisy-chained so the PC accommodates more than 1,000 I/O channels through a single ISA slot.”

External box configurations, housing some or all data acquisition circuitry, are common and not only used for applications with a high number of inputs. They also may be interfaced to the PC using a bus extender card (as in the case of the 4000 Series) via IEEE 488 or RS-232, but most commonly through the parallel port. Many also interface with laptop or notebook computers through a PCMCIA adapter card.

Signal-Conditioning Choices

Signal-conditioning circuits may be located on an assembly situated within a dedicated enclosure or in a self-contained DAS, reside on an A/D board or be placed on a separate PC-plug-in board. Optimum placement depends on the number of sensors to be accommodated by the DAS, sensor-to-DAS distance, sensor signal levels, the noise characteristics of the environment and, if the DAS is PC-based, the space available within the PC.

When less than 16 inputs require conditioning, using a single plug-in board that contains the A/D conversion function as well as the signal-conditioning circuitry is often advantageous. “This solution places the signal conditioning inside the PC and saves money because there is no need to buy external signal-conditioning accessories. Some DAS vendors claim that, due to the noisy PC environment, you must perform signal conditioning outside the PC. However, careful attention to shielding and board design proves this theory is completely false,” Mr. Di Rocco emphasized.

To provide flexibility, some companies place signal-conditioning circuits on daughterboards or similar replaceable modules. “For instance, the DCP5B modules interface to a specific sensor and are selected on a per-channel basis,” said Mr. Walsh. “Each module contains circuitry that addresses all three basic signal-conditioning requirements; that is, to amplify, filter and isolate.”

For many applications, minimizing the sensor-to-conditioner distance is still preferred, which normally requires an additional enclosure external to the DAS. However, the new networked configuration combines signal conditioning and the A/D conversion circuits into an assembly small enough so that it may be easily located adjacent to a group of sensors. The resulting digitized signals are transmitted to a PC or workstation through dedicated or, more often, through network facilities.

Intelligent Instrumentation, for instance, now offers the Ethernet data acquisition system consisting of one or several boxes that can be placed next to the sensors. “The signals are conditioned, converted and then transmitted to the PC over Ethernet,” explained Rob Winkler, product marketing engineer. “The Ethernet cable allows the DAS to be placed up to 100 meters away from the PC.”

Similarly, GW Instruments now offers the instruNet System that consists of a PC- or Mac-compatible network controller card and one or multiple daisy-chained network devices. Each network device accepts 16 analog inputs and digital I/O and contains signal-conditioning amplifiers for each channel as well as A/D conversion facilities.

One of the smallest implementations of this type is the Intelligent Link Module System from Keithley. It comes in various versions, each matching a particular sensor class, and provides a variety of network interface choices for its output.

“Signal conditioning in these units takes place internally after the multiplexer, which holds down costs by not requiring separate signal conditioners for each channel,” commented Frank Pereira, marketing manager at Keithley Instruments. “Some of the conditioning, such as scaling and filtering, takes place in the CPU after A/D conversion. Doing it there provides a high level of flexibility and performance while maintaining low cost.”

Interfacing Data Acquisition Boards With the PC

Interfacing network-based DAS modules with the PC does not entail any more of an effort than is encountered during any network setup. On the other hand, integrating conventional ISA-based PC plug-in boards with the PC usually involves setting switches and jumpers on the board, running board-specific setup programs and often resolving I/O address, IRQ or DMA conflicts.

Setup conflicts are certainly not unique to PC plug-in boards and this has prompted the development of the Windows 95 Plug and Play capability. Windows 95 Plug and Play software interrogates every one of the PC’s subentities. It then makes all necessary address, interface and memory allocations—as long as the interrogated hardware is plug-and-play-compatible.

Manual and Software-Aided Setup

Since the majority of today’s PC plug-in boards are not Windows 95 Plug and Play-compatible—and don’t necessarily have to be, board suppliers now provide simple-to-follow instructions and software to simplify the setup operation. “ADAC has long recognized the difficulties you may encounter when installing a data acquisition board in a PC,” said Mr. Di Rocco. “To minimize the installation and start-up effort, ADAC has developed a software package called Direct View for Windows (DVW).

“DVW is a tutorial software package that walks you through each of the board’s settings. Explanations of functions are complemented by hot links to appropriate locations in the user manual, a Windows help file. For boards with jumper or switch selectable options, DVW contains pictures of the board showing jumper positions for various configurations.

“For boards with software-selectable options, settings are done through DVW and initialization is performed automatically. All options such as DMA channel, interrupt level, input configuration (single-ended vs differential) and programmable gain are set via software in our new boards,” Mr. Di Rocco concluded.

Single-function boards usually do not require setting of jumpers or switches and are provided with setup and function-determining software. “For instance, our filter and amplifier cards have analog inputs and analog outputs and do not require interrupts,” said Leslie Brooks, marketing manager at Alligator Technologies. “The software programs the base address which uses four register locations settable between 200H and 3FEH. Key functions, such as the filter type, cutoff frequency and gain settings, are menu-selectable.”

Many of today’s PC plug-in boards require only a few parameters to be set in hardware with the majority activated via software. “Our ISA bus-compliant hardware, such as the DAS-1200 series of data acquisition boards up through the DAS-1800 series and the DAS-Scan, only requires setting the base address via a single bank of switches,” said Jeff Greenberg, marketing manager at Keithley MetraByte. “All other functions such as board type, DMA channels, IRQs, input configuration, gain or input range are handled by the configuration program.”

There are many situations where complete dependence on software control is neither desirable nor cost-effective, especially when multiple functional or operational options are being offered. “Most options on Datel boards are software selected; however, some must be hardware selected before installation,” said Larry Copeland, marketing manager at Datel.

“Users often don’t understand why this is. The reasons are many: Some features cannot be implemented at a reasonable cost, or within a small board area or with high quality and still be software selectable. We often provide extra hardware features which would have to be deleted or impossible to include if they were all in software. Certain options, however, easily lend themselves to software selection,” Mr. Copeland concluded.

Applicability of Plug and Play

While ISA boards may be plug-and-play-compliant, most of today’s ISA boards are not. All PCI-bus-based boards, however, must be plug-and-play-compliant as required by the PCI specification.

A PC may contain several PCI and plug-and-play-compliant ISA boards, and additional ones may be added without causing any conflicts. The PC’s operating system will interrogate the original and the added boards during boot-up and make new, mutually exclusive I/O address, IRQ or DMA level assignments.

This is not the case if compliant and noncompliant boards are intermixed. Conflicts are bound to arise. When the BIOS boots up, it cannot assign an address to the non-plug-and-play board, because its address has already been set by DIP switches or jumpers. Since the non-plug-and-play board does not respond to an address query, the BIOS is not aware of its address and may assign it to a plug-and-play-compliant board. The same is true for IRQ and other settings, requiring user intervention and conflict resolution.

Conflicts can even occur when only plug-and-play boards are used but the plug-and-play standards are not followed. “For example, a customer purchased a 12-bit 20-MS/s data acquisition card from us that conformed to all plug-and-play standards and used it in conjunction with a PCI bus frame grabber from another supplier. As soon as one of the boards was activated, the system froze,” reported Karen McCurry, assistant marketing manager at Gage Applied Sciences.

“The problem was that the frame-grabber designer had not followed the plug-and-play standards and had hard-wired its I/O addresses. But these types of problems are very rare and almost all PCI bus boards comply with the plug-and-play standards to avoid such conflicts.”

As Windows 95 is replacing Windows 3.1, more and more PC plug-in boards are becoming plug-and-play-compliant. “Our PCMCIA, PCI and plug-and-play ISA boards are completely jumperless so you do not have to set any jumpers at all when installing data acquisition, serial or VXI interface boards,” said Ray Almgren, test and measurement marketing manager at National Instruments.

“Upon installation of the software drivers, the Windows 95 operating system automatically configures the correct base I/O address, IRQ level and DMA channel to be used by the boards. These same boards can be used in Windows 3.1 and Windows NT; however, when you use these operating systems, you must confirm that no other devices are requesting the same address, IRQ or DMA lines,” he said.

References

1. Jacob, G., “Application Determines Best Physical Implementation,” Evaluation Engineering, February, 1993, pp. 20-27.

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Acknowledgments

These companies provided information for this article:

Acces I/O Products

(619) 693-9005

Alligator Technologies

(714) 850-9984

American Data Acquisition

(617) 935-3200

Analogic