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Inexpensive battery-operated digital multimeters with built-in serial ports are now available from a number of suppliers and can, in principle, be interfaced easily to any microcomputer or personal digital assistant. Instructions on connecting a Metex multimeter to a PC or Macintosh microcomputer to implement a versatile data logger that can continuously store any electrical parameter measured by the multimeter are discussed.
R. Viswanathan and G. Lisensky Department of Chemistry Beloit College Beloit, WI 53511
D. A. Dobson Department of Physics and Astronomy Beloit College Beloit, WI 53511
Inexpensive (<$150) battery-operated digital multimeters with built-in serial ports are now available from a number of suppliers and can, in principle, be interfaced easily to any microcomputer or personal digital assistant (PDA) that has a serial port. We have examined two multimeters with nearly identical operational features, namely the 4-1/2 digit Metex(1) Model M4650CR and the 3-1/2 digit Radio Shack(2) Model 22-168. Operating manuals for both of these multimeters provide only minimal information on the serial port interface. DOS/Windows-based microcomputer (PC) software supplied with the multimeters is of limited utility because no source code is included. In this note, we provide a detailed description of how the Metex multimeter can be connected to a PC or Macintosh microcomputer to implement a versatile data logger that can continuously store any electrical parameter (voltage, current, resistance, capacitance, frequency) measured by the multimeter. The implementation of a lightweight and portable battery-operated data logger package by interfacing a Metex multimeter to an Apple Newton PDA handheld computer is also described. The package is well suited for data acquisition in the field and allows for convenient transfer of the stored data to a computer workstation for further analysis. Other uses(3) that we have made of the multimeter connected to a microcomputer include display of the multimeter readout on a LCD projection system for live demonstrations in a classroom, and implementation of software for a PC equipped with a sound card to convert the multimeter readout to speech for use by visually impaired students.
The Metex multimeter has a five pin RS-232 compatible serial port. The non-standard port uses only the TD, DTR, and RTS-input signals and the RD-output signal in its emulation of an RS-232 Data Communication Equipment (DCE) modem device. An adapter cable with a five-pin connector at one end and a DB-25F connector at the other end is supplied with the multimeter. See Figure 1 for details. (All figures omitted) While a standard modem uses the RTS- and DTR-input signals to determine whether the Data Terminal Equipment (DTE) computer device it is communicating with is turned on and ready to receive data, the Metex apparently uses these signals (DTR positive voltage, RTS negative voltage) as voltage sources to drive the RD line while transmitting data. Furthermore, communications software using the serial(COM) port of a PC usually expects the peripheral device to provide positive CTS and DSR signals to indicate to the computer that it in turn is connected, switched on, and ready to communicate. Because the Metex serial port does not implement these output signals, CTS (pin 5) and DSR (pin 6) of the DB-25F connector of the Metex adapter cable should be connected permanently to DTR (pin 20) inside the hood of the connector. Even with this modification, the Metex multimeter cannot send data to a Macintosh computer because the Macintosh serial port does not provide the crucial negative RTS signal that is required by the multimeter. However, TD from any computer (DTE) is always in the marking state (negative voltage) when the computer is not transmitting data; i.e., when it is in its quiescent state or receiving data. Therefore, the Metex adapter cable can be modified to work with any PC or Macintosh computer by connecting the wires from both pin 2 (TD) and pin 3 (RTS) of the five pin connector to pin 2 (TD) of the DB-25F connector. Details of the modified universal Metex adapter cable are shown in Figure 2.
The bidirectional serial port of the Metex multimeter operates at 1200 baud and sends an ASCII string corresponding to the result of the latest A/D conversion, along with an indication of the type of data (Volt, Ohm, Hz, etc.) being acquired, to the microcomputer on receipt of any character transmitted by the microcomputer. Thus, it is very straightforward to implement a computer program in almost any programming language to receive continuously digital data from the multimeter, with practical update times being limited only by the multimeter A/D conversion time of approximately 0.5 s. We have written virtual "strip chart" recorder programs for the Macintosh and PC using True Basic and a serial I/O routine for the PC using Borland Turbo Pascal to serve as prototype general purpose data logger software.(4) Typical data acquired and displayed by the chart recorder program in a sensitive solution calorimetry experiment (multimeter connected to the output of a Wheatstone bridge that responds to changes in thermistor resistance) are shown in Figure 3. Note that the data points in Figure 3 represent voltage as a function of time, and that the full-scale expansion for the display (which can be changed dynamically as data are being acquired without loss of data, unlike on a analog strip chart recorder) is approximately 10 mV. The most sensitive DC voltage range on the Metex multimeter (200.00 mV) was used to acquire the data. The 4-1/2 digit resolution available on the multimeter roughly corresponds to a 14-bit A/D conversion.
We also have interfaced the Metex multimeter to an Apple Newton Message Pad Personal Digital Assistant (PDA),(5) which has a Macintosh DIN-8F serial port and uses pinouts and connections identical to those for the Macintosh, as shown in Figure 2. This resulted in a data logger package that is unique in the sense that it is extremely lightweight (total weight == 1 kg), has a small footprint and is truly portable and ideal for use in the field under normal operating conditions. In principle, any of a number of PDA's with built-in serial ports that are currently available (1) could have been used, but we selected the Apple Newton based on its cost, wide-spread availability, robustness, reliability, battery life (approximately 10-12 h under normal operating conditions), and comprehensive documentation (2). In addition, an object-oriented version(6) of the BASIC programming language, with support for user interactions via pen taps and standard widgets including scrolling windows, buttons, check boxes, and other visual features, recently has become available. Using this version of BASIC, we were able to develop quickly and implement a data logging program(4) that continuously stores data in the nonvolatile memory of the Newton. Large data sets can be stored without difficulty using PCMCIA memory cards with 1 to 4 MB of nonvolatile memory that are available for the Newton. Finally, the data can be transferred easily to a desktop computer via the serial port for analysis, visualization, etc., in a format (e.g., tab-delimited text) that allows for direct import into a number of popular spreadsheet/database/statistics programs.
One of the authors (RV) gratefully acknowledges support provided by the National Science Foundation through grant DUE 9354813.
1 Jameco Electronics, 1355 Shoreway Road, Belmont, CA 94002.
2 Radio Shack, a Division of Tandy Corporation, Fort Worth, TX 76102.
3 To be published elsewhere. Details may be obtained by writing to the authors. Internet email addresses are ramav(at)beloit.edu, lisensky(at)beloit.edu, and dobsond(at)beloit.edu.
4 Executable binaries, source code and documentation are available as shareware from the authors.
5 Message Pad 100 with updated 110 ROM, Message Pad 120. Apple Computer, Inc., 20525 Mariani Avenue, Cupertino, CA 95014.
6 NSBASIC Corporation, 77 Hill Crescent, Toronto, CANADA M1M 1J3.
Literature Cited
1. Nadeau, M. Byte 1995, 20, 147-156.
2. See, for example, McKeehan, J.; Rhodes, N. Programming for the Newton; Academic Press Professional: Cambridge, MA, 1994.
Copyright American Chemical Society Feb 1996