PHYSC 3322 Experiment 1.6 13 February, 2016 Computer Interfacing & Data Acquisition Purpose This experiment will introduce you to the basic principles of computer interfacing and data acquisition. In particular, you will learn the RS-232 serial interface and use it to measure time-varying voltage and current in a RC circuit. Equipment HP 54603B oscilloscope, HP 34401A multimeter, DC power supply, and HP interface software (BenchLink Suite and I/O libraries). Background In many experiments, one needs to either monitor a signal over a long period of time or measure a quantity, such as voltage and current, which may vary with time very fast. In these cases, one may use a computer to control the experiment and take data. Many experimental instruments are now equipped with a digital output that can be connected directly to a computer. The connection between an instrument and a computer requires specific interfaces for “handshaking” and data transmission. These interfaces are either serial or parallel in nature. In the serial interface, digital signals (bits) are transmitted one after the other along a single electric wire. In the parallel interface, however, N bits are transmitted simultaneously along N electric wires. RS232 (serial) and IEEE-488 (parallel) are two most commonly used interfaces in research laboratories. In this experiment, we focus on the use of the RS-232 serial interface. Signals sent through RS-232 represent characters by a series of logic bits (1s and 0s). The RS-232 standard defines a voltage between +3 and +15 volts as logic 0 and voltage between –3 and –15 volts as logic 1. Because of the large voltage range, signals can be transmitted through RS-232 over a long distance up to 100 feet. The logic bits are transmitted through RS-232 at a particular frequency, which is known as the baud rate in units of bits per second. In most cases, characters are represented by the ASCII code (American Figure 1 Pinout of a standard RS-232 connector. 1 PHYSC 3322 Experiment 1.6 13 February, 2016 Standard Code for Information Interchange). This code represents each character by a 7-bit binary number. Whenever data are not being transmitted, the RS-232 line is in the logic 1 state; this is the idle state. When data are transmitted, each 7-bit character is proceeded by a start bit (logic 0) and followed by an optional parity bit and 1 or 2 stop bits (logic 1). The parity may be either even or odd and is determined by the number of logic 1 bits in the transmitted character. The parity bit serves to validate the data word. The standard connector used in the RS-232 connections is the 25-pin subminiature D connector. Figure 1 shows the pinout of the DB25 serial connector. While the connector has 25 pins, only a small number of pins are actually used for serial communication. In many applications, only 4 pins (2, 3, 7, and 20) are required. Because data transmitted by one device on pin 2 should be received by another device on pin 3, it is often necessary for RS-232 cables to cross the wires for pins 2 and 3. Procedure Select a capacitor (C=4.4 F) and a resistor (R=3k) to form a RC circuit as shown in Figure 2. Determine the value of each component using the Elenco multimeter and calculate the time constant of the circuit. Construct the 555 timer circuit shown in Figure 3 on your breadboard with R1=1k, R2=1M, and C1=0.1 F. The 555 timer integrated circuit provides a signal, which alternates between 0 and 5V at a frequency given by f 1 . ln 2R1 2R2 C 1 (1) Use the oscilloscope to display the waveform of the output voltage of the 555 timer and the voltage across the capacitor. Sketch these waveforms on your notebook and indicate the time scales. Verify that the measured frequency of the signal agrees with that given by Eq. (1). R A 555 C V Figure 2. The RC circuit . Figure 3. The 555 timer circuit. Connect the HP 34401A multimeter to a computer with a RS-232 cable. The multimeter has only one serial interface port but the computer may have several serial ports. Make sure that the RS-232 cable is connected to the COM2 serial interface port of the computer. Refer to the HP 34401A Multimeter User’s Guide, p. 162-165, to configure the remote interface of the multimeter. Select even parity, 7 bits, 9600 baud rate, and SCPI programming language. Start the interfacing program by clicking the BenchMeter icon. Select File |New Project from the Menu Bar, type a project name and set Project Type to be HP 34401A Multimeter. Select Project Window under Tool Bar. Select 2 PHYSC 3322 Experiment 1.6 13 February, 2016 Category: Function and set Function: DC Voltage, Range: Auto, Resolution Details: Digits-4 Fast, Autozero On, and Instrument Display Off. Change Category to Acquisition and set Trigger Source: Software, 00h00m0.01s; Completion Criteria: Readings, 512; and Multiple Readings/Trigger: 512. Now the computer is ready to take data. First use the multimeter to measure the output voltage of the 555 timer. Click the Run icon and select the Strip Chart icon to display your data. When the measurement is done, you may display the data in various ways by selecting the View menu. Compare the waveform displayed on the Strip Chart with that on the oscilloscope. If you are satisfied with your data, you can save the data into a PRN file by selecting File | Export Data, enter a file name, and set the file type as *.PRN. Repeat the above procedure to measure the voltage across the capacitor and save the data in a different PRN file. To measure the current passing through the RC circuit, you need to (1) connect the multimeter in series with the resistor and the capacitor, (2) switch the input leads of the multimeter for the current measurement, and (3) set Function: DC Current. Make sure that all the other settings in Function and Acquisition remain the same as those for the voltage measurements. If you cannot finish the three voltage and current measurements in one class period, you may save all the project information in a PRJ file by selecting File | Save Project. This file can be reopened next time when you start BenchMeter. Questions Plot the output voltage of the 555 timer, the voltage across the capacitor, and the current passing through the RC circuit as a function of time. The time stamps in your data files are not correct and you need to generate your own time axis. Assuming that all the data points are taken at an equal time interval, you can obtain the time axis by measuring the period of the output signal of the 555 timer using an oscilloscope. Explain the shapes of the voltage and current waveforms. Measure the rising time of the voltage across the capacitor using an oscilloscope and compare it with the time constant of the RC circuit. 3