Budget modules gather data

Letterle found what he needed for under $100: the Model U12 from LabJack, a data-acquisition module that communicates to a host computer over the Universal Serial Bus (USB). As a bonus, Letterle found that he can use the unit's analog outputs to control the tube's accelerator, which he used to manually control with a set of knobs. Now, he controls the counter and voltage outputs through a computer; he gets more repeatable settings and he didn't have to design his own control circuit.

To aid you in your search, we compiled a list of external data-acquisition modules and handheld dataloggers that are priced at $450 and less (see the product table ). The $450 cutoff is somewhat arbitrary; we chose a price range that is high enough to include products that are useful in an engineering environment, yet low enough to fall in the purchase-approval range of many engineers. The product table gives you a sample of each company's offerings, but you should check the Web sites for complete product listings.

Of course, your overall cost is likely to top $450. To operate an RS-485 I/O module through a PC's RS-232 serial port, you will need to purchase an RS-232-to-RS-485 converter, which can add around $100 to your bill. Also, if you want to add several USB modules to a PC, you'll need a USB hub, which typically costs less than $40. Likewise, if you need additional serial ports, you'll have to install a card into the host PC. Such cards typically cost between $200 and $300, although you may find less expensive offerings at computer retailers.

Figure 1. RS-485 data-acquisition modules require a converter to interface to a computer’s RS-232 port.

For bench applications, you should opt for modules that connect directly to a PC's communication ports. Modules that you control from a serial port include B&B Electronics' 232SDA12, which provides eleven 12-bit analog inputs plus three digital inputs and three digital outputs. If you need more digital I/O channels, look to the 232SDD16, which provides 16 channels that you can program as inputs or outputs. Other suppliers of RS-232-controlled instruments include Dataq Instruments and Ontrak Control Systems. When you need high resolution and digital outputs, look to the RAD242 from Acces I/O Products, which provides 24 bits of resolution and 13 bidirectional digital I/O channels.

Parallel ports and USB ports offer the advantage of higher data rates than serial ports. Although you can find numerous parallel-port data-acquisition systems on the market, most exceed our price limit. Yet, LPTek and Pico Technology both offer products in our price range.

LPTek's offering uses a motherboard that holds I/O cards. The system provides the highest sample rate in the product table, 75 ksamples/s. To keep within the price range of this article, we included the TMCPort-Basic motherboard with an LPT8AD12, an eight-channel, 12-bit input card. Other I/O cards are available, but they raise the price above $450.

The LPTek system comes with drivers that let you control the system, but you must write your own application program. According to LPTek, the drivers work with DOS, Windows 3.1, Windows 95, and Windows 98.

For applications that require faster sampling than you can get from serial-port modules, but at a lower cost than the LPTek system, offerings from Pico Technology may fit the bill. The company offers one module that samples at 15 ksamples/s and another that samples at 10 ksamples/s, and you don't have to write your own software to use them. The PicoScope application software lets you plot graphs and save data.

Over the last few years, the USB has brought the advantage of "plug-and-play" to PC peripherals. A PC can automatically detect and configure a USB peripheral, but not a serial-port peripheral. While many USB data-acquisition units cost more than $1000, we found several that sell for less than $450. To get the most number of analog inputs in a single USB module, look to Dataq's DI-700, which provides eight differential or 16 single-ended voltage inputs with 16-bit resolution.

When you need few channels but require high resolution, consider the Lawson Labs Model 301. At 50 samples/s, the unit resolves signals to 22 bits. Resolution drops to 20 bits at 1000 samples/s.

Figure 2. Some data-acquisition modules, such as the LabJack U12, come with data-analysis software.

For functionality and low price, LabJack's U12 covers all the bases. The unit comes with datalogging and scope applications (Figure 2), and it features eight single-ended or four differential 12-bit analog inputs, two 10-bit analog outputs, 20 digital I/O channels, and a 32-bit counter, all for $99.

Dataloggers

The serial-port, parallel-port, and USB instruments require constant connection to the host PC. Sometimes, though, you need to perform measurements remotely, then download, plot, and study the results later. This is where battery-operated dataloggers can help.

These modules, which often fit in the palm of your hand, use a PC for setup and data download but take measurements and store them without a PC connection. All dataloggers in the product table come with software that lets you program them and download their data.

When you use a datalogger, you may have to program it with your office PC before taking it to a remote location. You must, therefore, disregard any measurement the logger takes before you place it at the measurement site. Dataloggers timestamp each measurement, so if you know when you placed the logger at the measurement site, you know which data to keep. The same concept applies between the time you remove the logger from the measurement site and when you download the data to your PC. One datalogger, the OM-CP-TEMP100 from Omega Engineering, negates the need to disregard data because it lets you start and stop measurements with a pushbutton.

Dataloggers can connect to external sensors so you can use them to measure just about anything. But some loggers, such as the HOBO LCD Temp/RH datalogger from Onset Computer, contain internal sensors and can generate alarms if a measurement exceeds a limit.

For just a few hundred dollars, you can turn any PC into a data-acquisition system. With modules available for every standard PC port, you can probably find what you need without spending the time required to design your own hardware and software.