Data Acquisition Systems M&C Buyers Guide Fred R. Schraff, P.E., IOtech Inc. Continuing advances in electronic and computer technology have expanded the options for data acquisition. Some data acquisition systems require a host PC, while other are capable of stand-alone operation. PC-dependent equipment leverages the intelligence of the PC and must be continuously connected to a PC for setup, control and data transfer. Stand-alone devices, which may capture data for eventual analysis on a PC, do not require connection to a PC for acquisition. A PC may also be interfaced to general-purpose instruments for data acquisition. PC-Dependent Systems PC-dependent data-acquisition devices fall into two main categories: internal or external. The internal devices are primarily PC plug-in boards compatible with either the ISA or PCI bus. PCs using the ISA bus are becoming obsolete, as software designed for 32-bit operating systems forces PC vendors to adopt a faster bus. Consequently ISA-based boards are being replaced by boards designed for the 64-bit PCI bus. PCI-based plug-in boards are capable of much higher performance than ISA-based boards, although they are not yet as widely available. Plug-in boards are contained inside the PC and offer high acquisition speeds because of direct data-bus connections. Finite space on the board limits the number of input channels, although some systems can be expanded via external I/O chassis or card cages. The limited rear panel space on a plug-in card also necessitates some physical inconvenience in making the actual connections unless some sort of fan-out adapter or custom cable is used. In many cases, the number of connection pins on a sub-D connector is the bottleneck. Also, because plug-in cards reside inside the PC, they are subject to an electrically noisy environment that can affect the measurement of low-level signals. Portable PCs such as notebooks lack expansion slots, creating the need for external data-acquisition systems. These external devices may be grouped by the type of PC communication ports they use. Types of Data Acquisition TYPE MEDIUM FEATURES Internal Devices Plug-in Boards Fastest Speed, non-portable, inconvenient installation External Devices Parallel Port Fast speed, portable, easy channel expansion & signal connection Serial Port Slow speed, portable easy signal connection PC Cards (PCMCIA) Medium to fast speed, most portable, limited channels & signal connection USB Port Slow to medium speed, portable, plug & play, easy signal connection Stand Alone Data Loggers Slow speed, portable, easy signal connection, no PC needed at test site Interfaces PC to IEEE 488, etc. Slow transfer speed, capabilities depend on specific instrument interfaced Most of today's PCs have several ports for external devices. The traditional printer port has been almost universally upgraded to an EPP type with backward compatibility. The RS-232 serial port is still included on most machines. There will generally be at least one, and sometimes two, PC-card (PCMCIA) slots. The newest "standard" addition is the universal-serial-bus (USB) port. There are external data-acquisition devices for each type of port. Several parallel-port-based data-acquisition systems are available. These systems, some of which can accommodate several hundred I/O channels, are conveniently installed, can be easily moved to a new PC, use a frequently available PC port, and sometimes allow a printer to share the port. Devices are available for acquisition speeds up to 1 MHz with 12-bit and 16-bit resolution A/D converters. In addition to external box solutions, parallel-port adapters to IEEE-488 bus provide an interface to hundreds of existing measurement instruments. Serial-port acquisition devices are not as common as some other types because of speed limitations, but there are many long-distance and isolation options available with the serial port that are unavailable with other ports. External adapters can turn the generic RS-232 into longer-distance RS-422 or multi drop RS-485 or almost any-distance fiber-optic communication. Less exotic optical isolation devices can link localized devices to hazardous-voltage readouts without exposing the user or the computer to unnecessary risk. In addition, built-in modems and the associated telephone interface can be used for data acquisition, particularly across the Internet. For PCs with a PC-card (PCMCIA) slot, data-acquisition cards are available with 12 or 16-bit A/D resolution. PC cards are conveniently installed and portable, but PC-card systems generally have limited capability due to limited space. In some cases they can be expanded to fairly large systems by using external hardware. PC-card-based systems also suffer from a generally fragile cable connection. The newest PC port is the USB port. Although designed to serve traditional peripherals such as mice, keyboards and printers, USB is ideal for medium-speed data acquisition. With potential data rates as high as 10 MHz, real plug & play capability and a simple 4-wire connection that includes power, it is expected to gain significant popularity. USB-based devices are already available that are similarly priced to PC cards and plug-in boards with the added benefits of isolation, a quiet signal environment, and easy signal connection. Stand-Alone Systems Stand-alone devices, also called data loggers, actually proceeded the PC era. Included in this category are chart recorders and other devices that generate a real-time paper printout. Stand-alone data-acquisition devices can handle time-consuming data-gathering tasks without tying up your PC for days or weeks. In addition, a stand-alone system is ideal for remote applications because there is no chance that the PC can be stolen or damaged while in the field. Traditionally, stand-alone systems were slow and expensive. Their speed was limited by the prohibitively high cost of the onboard memory required for high-speed measurement. They were expensive because they included displays and control panels that added to their cost. Moreover, their often cryptic buttons and small displays provided an awkward user interface. There are new stand-alone devices that take advantage of lower-price PC technology and are capable of operating without a PC present. These systems offer the best of both worlds: independent operation plus low cost, high speed and large memory capacity. They are configured using a PC and a software interface. Configuration is transferred to the stand-alone device via a temporary serial or parallel link or saved to a common, low-cost PC card, which can be removed from the PC and inserted in the device. The system stores the acquired data in memory on the same PC card, which can be swapped periodically to allow remote analysis of recent data while the data-acquisition device remains in continuous service. Interfaces to General-Purpose Instruments A common approach is to interface the PC to general-purpose measurement instruments via the IEEE-488 bus. IEEE-488 interface controllers are available as plug-in boards or as parallel-or serial-port adapters. Although IEEE-488 is the most common, some instruments will also communicate using other serial buses. Choices There are many combinations of data-acquisition systems, with differing degrees of overall compatibility. The standardization brought about by a limited number of available computer interfaces does help to narrow the choices. If you select a single data-acquisition system that can handle all of the signal/sensor types you need, then the task of interfacing to the host computer will be simplified. Multiple devices using multiple interfaces complicate the system-integration task.