Parallel-port data acquisition marks the beginning of the next evolution of the personal computer (PC) as an information-gathering tool. This new approach to data acquisition eliminates the limitations of the currently popular plug-in board technology, and can save you a great deal of time in the process.
The familiar ISA, NuBus and Microchannel bus architectures have extended the utility of a PC. With special-purpose plug-in boards, a PC can accomplish many tasks in addition to basic computing functions.
For example, with a SVGA board, the computer can display high-resolution color graphics. Or with an Ethernet board, the computer can become a node on a local area network. For data acquisition applications, you can convert a PC into an automatic data management device that collects measurements, displays them in real time, automatically responds with control outputs, logs data to the disk and analyzes the data in the context of the application.
Although the plug-in data acquisition board has provided much in the way of time savings over manually recording measurements, it also has some limitations:
Using a plug-in board requires that the PC be opened to install the board and again when adjustments are necessary or when the board is transferred to another computer.
Since other boards share the communication bus, there is the potential for hardware address conflicts.
Electrical noise inside the computer can degrade measurement accuracy.
Sometimes there are no slots available in the computer.
Open Computer
To perform data acquisition with a plug-in board, remove the computer cover. Set the board’s switches to the proper address, calibrate it by adjusting potentiometers (in some cases), and insert the board into the available bus slot. After you complete this process, connect sensors to an external termination panel which plugs into the edge connector of the board with a ribbon cable (Figure 1).
Before the system is ready to take data, you can spend more than an hour in preparation and configuration. If there are hardware conflicts, the process can be considerably longer.
Hardware Address Conflicts
Any board that is plugged into a PC bus slot must have a unique hardware address so the operating system and application software know where to obtain or transfer data. Data acquisition boards are no different in this respect.
When installing a new board into a PC, you don’t have any method of determining the address settings of boards currently residing on the bus. Consequently, you often must use an iterative process to determine if your address setting is unique or in conflict with other boards.
Intel is attempting to resolve this nuisance with its plug-and-play concept in which each board would assign its own address when plugged into the bus. For now, however, you must rely on a certain amount of luck when assigning board addresses and hope you don’t encounter any conflicts—which can take hours to resolve.
Electrical Noise
Electrical noise is one of the major causes of erroneous measurement of analog signals, especially with low-level signals such as those from thermocouples and strain gauges. Noise can attack the measurement signal from a number of locations.
One of the principal sources of noise is the electrical activity inside the PC caused by the power supply and other electrical components. Data acquisition board manufacturers attempt to minimize the effects of this noise by using careful grounding techniques and DC-to-DC converters. But the effects of electrical noise internal to the PC cannot be completely eliminated.
Another source of noise is the external environment, where power-line interference and other electromagnetic phenomena are ubiquitous in urban environments. This type of noise can be picked up by the ribbon cable that connects the terminal panel to the data acquisition board and by the sensor wires.
Spare Slots
Finally, there are times when the PC has no spare board slots. This can occur when the PC has boards performing other necessary functions, and most certainly with notebook computers which have no slots at all.
Parallel-Port Data Acquisition
A few manufacturers, including Strawberry Tree, Computer Boards, IOtech, and National Instruments, have recently introduced systems for data acquisition via the standard (Centronics) parallel port. With this technique, the potential exists to eliminate the limitations associated with plug-in board data acquisition.
With surface-mount circuit-board manufacturing technology, the A/D circuitry can now be integrated with the termination panel, resulting in only one piece of hardware that effortlessly connects via a standard PC connector, such as the parallel port (Figure 2). As a result, the PC does not need to be opened to accomplish a data acquisition task.
Advances in software technology, combined with the communications protocol of the parallel port, eliminate the possibility of hardware conflicts. You can now accomplish, in minutes, hardware configuration which could take hours with a plug-in board.
Parallel-port data acquisition eliminates two sources of electrical noise experienced with plug-in boards:
The analog-to-digital conversion is accomplished outside of the noisy environment of the PC.
The signal which is transmitted to the PC via a parallel cable is digital, so it is immune to noise for all practical purposes. This leaves the sensor wires as the sole component of the system vulnerable to noise.
With parallel-port implementation, you can use PCs without available slots for data acquisition tasks, greatly expanding the potential applications. For example, you can now perform process and environmental data gathering by carrying a notebook PC and a parallel data acquisition peripheral in the field. Because of the low voltage used by notebook PCs, medical technicians now can more easily use PC data acquisition for measurement of a patient’s vital statistics.
Parallel-Port Limitations
While parallel-port data acquisition devices overcome many limitations of a plug-in board solution, it is important to recognize that this solution, too, has limitations: data transfer speed and cable length. Many applications will not be hindered by these limitations, but an understanding of this will enable you to make better decisions.
As defined by communications protocols, the parallel port has a maximum output rate of roughly 500 kB/s and a maximum input rate of 125 kB/s. For data acquisition applications, this translates to a maximum throughput rate of about 50,000 S/s, or 50 kHz.
The maximum throughput rate is the maximum rate for one or more channels of analog input. The per-channel rate in a multichannel application is the maximum rate divided by the number of channels.
Most laboratory, field and factory applications can be accommodated by the speed of the parallel port. The speed limitation becomes a factor in applications such as shock and vibration analysis and audio, and in applications with a large number of channels. Know your throughput requirements for a specific application before making a decision about a data acquisition solution.
The maximum cable length of the parallel port is approximately 50 ft. This length is adequate for most applications. When distance is an issue, the most common solutions are a 4- to 20-mA current loop and RS-232/485, Ethernet, FieldBus and LON, a proprietary competitor of the FieldBus.
Future Evolution
So far, we have examined PC bus and I/O limitations as they pertain to data acquisition applications. Besides these limitations, there are various other constraints that present barriers for applications such as network communications, multimedia and 3-D imaging.
Because many such applications have mass-market forces behind them, we will likely see new bus and I/O technologies emerge in the PC industry. In fact, new technologies in this area have already emerged and others are on the way.
LocalBus, for example, which is now built into a number of PCs, is a new high-speed bus architecture especially suited for video applications. Although LocalBus is no threat to replace ISA or other general-purpose PC buses, another technology, QuickRing, is a threat to the ISA standard and is receiving much attention in the PC industry.
Such change, albeit speculative at this point, will surely bring added benefits to the data acquisition industry. For example, if a new bus technology, such as QuickRing, were to be widely adopted, PCs could be used to perform real-time display and analysis of ultra high-speed data acquisition, perhaps at 10 MHz or more. Or a single PC could be used to manage a factory control application with hundreds of input and output channels.
Today, parallel-port data acquisition is gaining momentum and brings you many benefits, such as greater ease-of-use, better noise performance and notebook compatibility. In the future, new technologies will likely yield even more benefits for PC data acquisition.
About the Author
Mark Caragio joined Strawberry Tree in 1990 and today is the leader of the product development team for the company’s DATAshuttle parallel-port A/D devices. Previously, he was a freelance technical writer for several companies, including MIPS and Raynet. Mr. Caragio received a B.S. degree in economics from California State University and a master’s degree in information systems from Santa Clara University. Strawberry Tree, Inc., 160 S. Wolfe Rd., Sunnyvale, CA 94086, (408) 736-8800.
Copyright 1995 Nelson Publishing Inc.
July 1995
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