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Portable data acquisition instrument manufacturers are taking full advantage of the notebook PC's capabilities of operating remotely, enabling users to configure, acquire, monitor, and analyze data with the same software used in desktop PC data acquisition systems.
An array of portable data acquisition instrumentation exists for use with notebook PCs. Most portable data acquisition modules are based on one of three I/O interfaces: serial port (RS-232), parallel port (standard and enhanced), and PC Card™, commonly referred to as Personal Computer Memory Card Industry Association (PCMCIA) card.
The important features examined here for each I/O interface are form factor and price, data acquisition capabilities, power requirements, and software support.
Form Factor and Price. Most notebook PCs feature at least one standard 9-pin RS-232 serial port where data acquisition modules can be installed using up to 50 feet of cable. Compact RS-232 modules, small enough to be held in the palm of the hand, are now more prevalent than the older, less portable, data loggers. They are priced from $50 for basic modules to as much as $500 for modules with greater functionality.
Data Acquisition Capabilities. Typically, the serial port's maximum data transfer rate is 11.5 Kbytes/s at a maximum speed of 115,200 bits per second (bps). However, inexpensive RS-232-based data acquisition modules commonly transfer data at a rate up to 2 Kbytes/s at a speed of up to 19,200 bps. The sampling rate is one-half the raw byte transfer rate because each reading requires two (or more) data bytes. Therefore, RS-232-based modules characteristically have a maximum sampling rate of 1 to 2K sample/s, which is very slow compared to other I/O architectures. Typically, inexpensive RS-232-based data acquisition modules provide a maximum of 12-bit resolution, and have limited analog and digital I/O. For example, they may offer up to eleven channels of single-ended analog inputs, three digital outputs, and three digital inputs. Signal conditioning and expansion options can also be restricted.
Power Consumption. Portable RS-232-based data acquisition modules may consume as little as 5 mA at 6 to 18 VDC, excluding the power required for sensors such as RTDs or strain gages. Battery run time, when using an internal battery, is generally under two hours.
Software Support. Ordinarily, these modules are software programmable and allow data to be imported into DOS and Windows programs via a data logging utility. General-purpose RS-232 communications software is widely available and may be used to control these modules. However, Windows-based data acquisition software choices are often limited. Support on other operating systems, such as NT and UNIX, is minimal.
The parallel port architecture has many variations, but the two most common are the original IBM PC standard parallel port (SPP) and the IEEE 1284 enhanced parallel port (EPP). SPP is generally found in lower to mid-range notebook PCs, while EPP is featured in almost all the higher-end notebook PC models. Soon, EPP will most likely displace SPP as the parallel port standard for both desktop and notebook PCs.
Form Factor and Price. Typically, notebook PCs have at least one standard 25-pin parallel port, where an external SPP-based data acquisition module, comparable in size to a notebook PC, can be installed via a cable. To provide reliable data transfer at the fastest possible rate, SPP cabling length is usually limited to a few feet. These modules generally cost around $1000.
Data Acquisition Capabilities. SPP-based data acquisition modules' maximum data transfer rate varies widely, depending on how SPP is implemented in various notebook PCs. For example, some parallel ports can transfer data up to 150 Kbytes/s or 75K samples/s; for others, the data transfer rate drops to 60 Kbytes/s or fewer than 30K samples/s. Typically, inexpensively priced SPP-based modules provide a maximum 12-bit resolution.
SPP-based modules frequently accommodate a maximum of 256 channels of analog input, 2 channels of analog output, 4 digital inputs, and 4 digital outputs. Superior modules provide programmable gains, filtering, A/D conversion, and additional analog signal conditioning for inputs such as thermocouple, RTDs, and strain gages. These products also enable users to expand as their application requires.
Power. Excluding the power consumption of external sensors such as RTDs, strain gages, and other signal-conditioning options, SPP-based modules can consume less than 500 mA at 12 VDC. In addition to an internal battery pack, some SPP-based data acquisition modules are available with a compact stand-alone battery supply that can provide power for additional signal conditioning options. The battery run time of SPP-based data acquisition modules is comparable to that of a notebook PC (2 to 4 hours).
Software Support. DOS and Windows are commonly supported, while NT and OS/2 drivers are not. Users are generally able to take advantage of the functionality found in widely available PC data acquisition software packages.
Form Factor and Price. Most current high-end notebook PCs have at least one 25-pin enhanced parallel port, to which an external EPP-based data acquisition module, the size of a notebook PC, can be installed via a cable. The EPP can be used with standard printers or SPP data acquisition modules, but during EPP operation the uses of the various signal lines are modified to achieve much higher performance. Modules start at $1000.
Data Acquisition Capabilities. EPP-based modules can transfer data up to 3 Mbytes/s, achieved with an interlocked data transfer in a single CPU I/O cycle. This data transfer rate is limited by the host PC's I/O capability; for most notebook PCs, this is currently around 800 Kbytes/s.
For applications requiring a higher data acquisition speed, some data acquisition module manufacturer's offer a PC Card option that enables users to increase the EPP data transfer rate by interfacing an EPP-based module through a PC Card slot or ISA-bus.
Superior EPP-based modules can accommodate a maximum of 256 channels of analog input, 2 channels of analog output, 4 digital inputs, 4 digital outputs and a range of features including programmable gains, filtering, A/D conversion, and other analog signal conditioning for inputs such as thermocouple, RTDs, and strain gages. Typically, EPP-based data acquisition modules offer a maximum of 16-bit resolution.
Power Requirements. Excluding the power consumption of external sensors such as RTDs or strain gages, EPP-based modules can consume as little as 500 mA at 12 VDC. The battery run time of EPP-based data acquisition modules is equal to or exceeds that of a notebook PC (2 to 4 hours).
Software Support. DOS and Windows are ordinarily supported, while NT and OS/2 drivers are not. Users can employ current PC data acquisition software for increased functionality.
PC cards are the hottest vehicle in today's portable data acquisition market. Many companies offer this data acquisition option.
Form Factor and Price. Most notebook PCs have at least one Type III (10.5 mm thickness) PC card slot or two Type II (5 mm thickness) PC Card slots. Many data acquisition PC cards available today are in the Type II form factor and plug into a notebook PC's PC card socket via a 68-pin connector. Many cards are available, most for under $1000.
Data Acquisition Capabilities. Data acquisition PC cards can transfer data at 5 Mbytes/s. However, the data transfer rate for PC cards is restricted by the host PC's I/O capability. Data acquisition PC cards can provide a sampling rate of 100 to 150K samples/s, and they characteristically offer a maximum 16-bit resolution.
Due to their small size, data acquisition PC cards have limited functionality. Typically, they have 8 differential or 16 single-ended analog input channels per card and a few channels of digital I/O. To offset the limited functionality, some data acquisition PC cards can support expansion channels via an external data acquisition module system or a more bulky, less portable ISA-bus-based data acquisition chassis.
Expansion and signal conditioning are generally very limited due to the card's small size, although the new PC card standard has extended the PC card length. Signal conditioning for analog inputs such as thermocouples, RTDs, or strain gages often must be external to a card, and most sensors and transducer inputs must be connected with wire sizes that are often inconvenient to use with a card.
Power. Cards can operate with as little as 160 mA at 5 VDC, excluding the power consumption of external sensors. Power is supplied via a host notebook PC's battery pack through the PC card slot. While this is a convenient source of power, it can significantly lower the notebook PC's battery backup operating time.
If sensors such as thermocouples and strain gages are required, power or isothermal connections must be supplied using external hardware, effectively negating the size advantage of the PC card.
Software Support. DOS, Windows, NT, and OS/2 drivers are available. One important advantage that the PC card software supports is hot--swapping the ability to insert, use, and remove cards without rebooting the computer.
Several I/O interfaces are receiving considerable interest in the PC market--Universal Serial Bus (USB) and IEEE P1394 (FireWire). These show promise for the portable data acquisition market because of the high data transfer speeds they can achieve. Until they are widely implemented in notebook PCs however, they remain data acquisition hopefuls.