Adapted from an article that appeared in NASA Tech Briefs, January 2000

Advancements in personal computer technology continue to push the world of PC-based data acquisition to new capabilities. Many of the analog measurements that were once made with digital multi-meters, data loggers, and chart recorders are now made with PC plug-in data acquisition boards. In addition to measuring analog signals, these boards also have the ability to acquire digital, counter, and frequency input signals, and to output analog and digital signals.

Historically, plug-in boards have done an excellent job of providing time-synchronized analog measurements, where the time relationship between each measurement, even on multiple channels, was precise and predictable. This is particularly important when waveform measurements are made, since the time relationship between multiple channels is almost always necessary to accurately correlate the acquired data.

Take an example where an analog-to-digital (A/D) board uses a 100-kHz A/D converter, and two channels are to be measured at exactly 50 kHz per channel. Nearly all of today’s boards are capable of measuring first one and then the other of the two channels at precisely 50 kHz per channel, with a mere 10m sec (1/100-kHz) time skew between each measurement. Since the time relationship between the sampling of each channel is precise and deterministic, the acquired waveforms can be analyzed relative to one another with precision.

The problem with many of today’s A/D boards is that most or all of their other I/O capabilities cannot be time-correlated to the analog measurements or to each other, making it nearly impossible to correlate analog measurements to digital, counter or frequency measurements. Consider a test example where a board would be used to test a rotating motor in a piece of machinery. It may be desirable to measure various supply voltages and currents to the motor, the motor’s RPM, the status of control switches, temperature, and perhaps other parameters. Thus, not only is it necessary to acquire analog data, but digital input data and counter/frequency data as well. In addition, it may be desirable to output an analog waveform from the board’s D/A converters to create a dynamic load to the motor, or to output a control signal from the board to apply an impulse load to the motor.

Nearly every A/D board available today would have difficulty performing this test, because the analog output, digital I/O, and counter/frequency input is updated or measured asynchronously to the analog input scanning, making it nearly impossible to time-correlate the various I/O functions. The reason is that, while most boards operate their A/D converters and multiplexers from precise clock-driven electronics, the other I/O is usually controlled via software in the PC. And since all PC’s operate at different speeds, the rate at which they can access the hardware is highly dependent on what other tasks are being performed at the time, the timing of the other I/O is almost entirely non-deterministic.

Notable exceptions are some higher-end boards with on-board intelligence, which have their own processors that are dedicated exclusively to managing the I/O functions. These boards are typically two to ten times more expensive than the typical multifunction A/D boards, and often require the use of a unique programming environment in order to accomplish the time-correlated multifunction I/O.

Now that PCs are faster than ever before, and with the advent of the PCI bus as the backbone for all new PCs, the future is bright for using low-cost plug-in boards to accomplish time-critical multifunction I/O. Besides the cost advantage over processor-based plug-in boards, using low-cost non-processor boards for multifunction I/O means that they can be programmed with popular, and low-cost standard languages such as C++ and Visual Basic. (Most of the multifunction boards with on-board intelligence require the purchase of a special programming environment, often costing thousands of dollars in addition to the cost of the board).

Two technology advancements are making the use of low-cost plug-in boards for time-critical multifunction I/O possible. First, the PCI bus, now ubiquitous on today’s PCs, has the bandwidth necessary to acquire data from multiple sources on a single low-cost plug-in board, as well as to generate multiple channels of output to the same plug-in board. The 15-year-old ISA bus, which has only recently been replaced by PCI, simply couldn’t deliver this bandwidth.

The second advancement is the way in which multifunction A/D boards are designed. In the past, standard digital I/O devices, such as the 8255 chip, were used for digital I/O on nearly all boards. And the 9513 device was used to provide the counter/timer capability on most boards. Although these devices made it easy for board designers and manufacturers, their architecture made them incapable of being time-synchronized with one another or the analog input.

Today’s new board designs are incorporating all of the digital I/O and counter/timer capability on more powerful and customizable ASICs (Application-Specific Integrated Circuits) or high-density FPGAs (Fully Programmable Gate Arrays), making it possible to time-correlate their I/O capability with the board’s analog input and output capability. The motor test described earlier could be easily implemented with the new generation of low-cost data acquisition plug-in boards that are becoming available today.

For under $500, these boards are capable of synchronously measuring analog inputs, generating analog outputs, acquiring digital inputs, controlling digital outputs, measuring counter inputs, and generating digital patterns – all in synchronicity with one another. Although most multifunction boards are yet to offer this capability, it’s only a matter of time before these capabilities are considered standard features for low-cost plug-in boards.

About the Author
At the time this article was published, Tom DeSantis was the president of IOtech, Inc.