General Electronics

Horowitz, P. & Hill, W., The Art of Electronics, Cambridge Univ. Press, New York, 1980.

Higgins, R., Electronics with Digital and Analog Integrated Circuits, Prentice Hall, Englewood Cliffs, 1983.

Millman, J., Microelectronics, McGraw-Hill, New York, 1979.

Digital Electronics

Greenfield, J., Practical Digital Design Using IC's, Wiley, New York, 1977.

Wagner, D., Digital Electronics, Harcourt Brace Jovanovich, San Diego, 1988.

Analog Electronics

Faulkenberry, L.,An Introduction to Operational Amplifiers, Wiley, New York, 1977.

Computer Interfacing and Programming

Titus, J., Larsen, D. & Titus, C, Apple Interfacing, Blacksburg Group, Blacksburg, 1981.

Robbins, J., Understanding Microsoft QuickBASIC, Sams, Indianapolis, 1988.

Sensors, Transducers and Computers in the Laboratory

Snider, J. & Priest, J., Electronics for Physics Experiments Using the Apple II Computer, Addison-Wesley, Reading, 1989.

Kuckes, A. & Thompson, B., Apple II in the Laboratory, Cambridge Univ. Press, New York, 1987.

Gates, S. & Becker, J., Laboratory Automation Using the IBM PC, Prentice Hall, Englewood Cliffs, 1989.

Ratzlaff, K., Introduction to Computer-Assisted Experimentation, Wiley, New York, 1987.

Warring, R. & Gibilisco, S.,Fundamentals of Transducers, Tab, Blue Ridge Summit, 1985.

Tien Lang, T., Electronics of Measuring Systems, Wiley, New York, 1987.

Horn, D., How to Use Special-Purpose ICs, Tab, Blue Ridge Summit, 1986.

General Reference Manuals

The TTL Data Book for Design Engineers, Texas Instruments Incorporated, P. O. Box 5012, Dallas, TX, 75222.

Linear Databook, Vol. 1, 2, 3, National Semiconductor Corporation, 2900 Semiconductor Drive, Santa Clara, CA, 95052.

Specific Reference Manuals

Apple IIe Reference Manual, Apple Computer.

Technical Reference for the Personal Computer AT, IBM Corporation.

Data-Acquisition Databook, Vol. 1, Analog Devices, P.O. Box 280, Norwood, MA, 02062.

The Analog Devices AD573 10-bit, 20μsec analog-to-digital converter was selected for the data acquisition systems in this book. In the PDC plan, two of the IC's comprise the ADC module which interfaces to whatever application circuits need voltage measurements. Reasons for picking this particular device are in Chapter 4.

Obviously, more than two ADC's are necessary for conceivable applications. And cases may arise where faster and/or more accurate measurements are required. On the other hand, slower, less accurate and less expensive converters might be desirable when many voltages are measured. Because the devices are in a separate module, additional AD573's or some number of different ADC's can be substituted with minimal changes in the rest of the system.

In addition to hosting conversion devices, the ADC module receives power supply voltages and sends +5 and ground to the rest of the PDC (as shown in Figure 7.1). The scheme allows the -12 volts required for AD573's to go only to the module where it's needed.

Circuit

The AD573 is a 20-pin IC. Ten connections are Data outputs (D0…D9). Three are control inputs: CV (ConVert), HE* (High byte Enable) and LE* (Low byte Enable).

Section 3.2 explains the design of a pair of 8-bit input/output ports for IBM PC/XT/AT and compatible computers. Figure 3.11 is the complete circuit. This appendix is a guide to building the ports. Section D.1 lists components and presents a layout. Section D.2 suggests an interfacing cable and connectors. Section D.3 gives wiring instructions.And Section D.4 outlines testing and troubleshooting. The objective is a complete presentation of one construction approach. Details such as connectors and cables are somewhat arbitrary and other schemes may be substituted.

Components and Layout

The circuit in Figure 3.11 requires the following components.

1. (1) A Vector Model 4613-1 or 4617-1 wire wrap board for the IBM PC/XT or PC-AT expansion slot.

2. (2) Four 74LS373 latch/buffers.

3. (3) Two 7425 4-input NOR IC's.

4. (4) Two 74LS04 INVERTER chips.

5. (5) One 74LS138 decoder.

6. (6) Nine IC sockets: four 20-pen, one 16-pin, and four 14-pin.

7. (7) A bag of Vector T44 wire wrap terminals.

8. (8) Six .01 μf disc capacitors.

9. (9) A wire wrap tool and an assortment of wire.

Vendors for these components are in Appendix B.

Figure D.1 suggests a layout for the circuit on a PC-AT expansion board. The view is from the components side with the rear of the computer on the right. (The regular PC board differs primarily in the absence of the second plug on the bottom left.)

Along the bottom, just above the right plug, are holes for wire wrap terminals. When inserted, the terminals allow expansion slot signals to be connected to the ports circuit.

The approach to data acquisition described in this book utilizes three categories of large scale integrated circuits: Analog-to-Digital Converters (ADC's), Random Access Memories (RAM's), and Programmable Interval/Timers (PIT's). The specific IC's are the Analog Devices AD573 10-bit ADC, the 6264 8K by 8-bit RAM and the Intel 8253 PIT. The purpose of this and the following two chapters is to describe these integrated circuits in detail, to show how they can be interfaced to the parallel ports of Chapter 3 and to present all hardware and software for example applications. While these tutorials focus on particular devices, the ideas and principles can be applied to other ADC's, memories and PIT's, as well as to other types of devices (including voltage-to-frequency converters, digital-to-analog converters, programmable gain amplifiers, digital switches, etc.).

A variety of sensors produce voltages precisely related to such physical quantities as temperature, pressure, light intensity, current, etc. For a computer to get information from sensors, voltages must be converted to digital codes, something most efficiently done by an ADC. While this chapter only shows how to interface and control an ADC through the parallel ports, several references in Appendix A provide general discussions of sensors, operational amplifiers and the internal operation of converters.

ADC's are characterized by an input voltage range, conversion time and resolution (the number of binary bits in the converted value). The Analog Devices AD573JN is a $20, 10-bit, 20μsec converter which accepts input voltages in one of two ranges, bipolar -5 to +5 or unipolar 0 to +10. The AD573 was chosen because its speed and 10-bit resolution are needed for applications in later chapters.

Introduction

The last several chapters of this book describe a detailed approach to data acquisition using the I/O ports presented in Chapter 3. The parallel data collector, as it's called, consists of the ports, the control/data interface (Chapter 7), the ADC module (Chapter 8) and an applications module. Any computer with suitable ports can operate the hardware and support the software.

Chapter 9 presented an example applications module which determines counts, times and voltages. A characteristic of the measurements is that software waits for data ready and then inputs and stores values. Because of the time required for these operations, the minimum counting interval, shortest measurable time, and maximum conversion rate depend on the computer's speed and the software's efficiency. Cases may arise which require smaller intervals, shorter times and/or higher rates. This chapter presents an applications module which automatically converts voltages and stores the values in its own memory. The system increases the maximum rate and works identically on original PC's, Apples II's and the latest computers. The approach also applies to counting and timing measurements.

The Fast Voltage Measurer (FVM) determines the voltage of one source at 8-bit accuracy with the rate and number of conversions programmable. The maximum 40K measurements/second arises from the AD573's 20μsec conversion time, and the maximum number of measurements equals 32K, the storage capacity of four 6264 8K IC's. Data acquisition starts when both Software Start/Stop (S-S/S*) and External Start (EX-S) become high. Software monitors for the end of data acquisition after which the stored values are read and processed as desired.

What does someone do who wants to use computers for data acquisition? No matter which way the goal is pursued, it's necessary to know how computers communicate with the external circuits which make measurements. This chapter summarizes choices and introduces the approach used in this book.

Ports and Expansion Slots

Measurement Circuits (MC's) can be connected to computers through game ports, serial ports and expansion slots. In each case, software must send values to and get values from the MC's. The following sections review and compare the approaches.

Game Port Communication

On an Apple II, communication between its game port and an MC occurs as follows. Up to three digital signals generated by an MC are connected to the three game port push buttons. The computer ascertains a signal's high or low state by peeking a specified address and then seeing if the value is greater than 127 which means the signal is high. Similarly, one or more of the game port's four enunciator outputs are connected to an MC. The computer makes an output high by peeking a specified address and low by peeking a different address.

Game port data acquisition is practical when the number of input and output bits can be limited which is the case with the few commercially available systems. They are typically self-contained units starting with, for instance, a temperature probe and ending with a data plot. Their low cost, reliability and operational simplicity make them attractive for instructional laboratories.

Serial Communication

The most widely adopted standard in computers is RS-232 serial communication. For virtually any machine, serial ports either are or can be installed (at low cost).

Chapters 4-6 are tutorials on three specific devices: the AD573 Analog-to-Digital Converter, the 6264 static RAM and the Intel 8253 Programmable Interval Timer. The devices by themselves are insufficient for reasonable data acquisition systems. For instance, it's desirable to use several 8253's for counting and timing measurements (as discussed in Section 6.8.3), and it's advantageous to use a PIT to control RAM storage of data (as discussed in Section 5.6.7). This chapter describes a framework which supports multiple devices.

The Parallel Data Collector (hereafter called the PDC) is a modular hardware and software system which facilitates the design, development, testing and use of a variety of data acquisition systems. The parallel ports described in Chapter 3 comprise the first module and is the only part of the system including software which depends on the particular host computer. A second module, the Control/Data Interface (CDI), produces from the ports a control and data structure sufficiently elaborate to support multiple PIT's, memories and/or ADC's (as well as other category devices). A third module contains two ADC IC's. The plan is to use the parallel ports, CDI and ADC as service modules over and over with a variety of applications. Figure 7.1 is a block diagram of the system.

This chapter describes the CDI including specifications, design, software, construction and testing. Chapter 8 presents the ADC. Both modules are built on individual wire wrap cards, and complete drawings are provided. Specific card rack, power supply and interconnecting cables are recommended.

After the framework is established, a general purpose measurement system is presented in Chapter 9. And a “fast” voltage recorder is given in Chapter 10.

Hardware for data acquisition and control

There are two styles of hardware for using a microcomputer to acquire data and control equipment. One is exemplified by the IBM-PC system you have used in the laboratory. The ADC, the DAC and the digital I/O cards are inside the computer and are under direct control of the microprocessor. They have control and data registers which are directly addressable via the buss. External devices (sensors, switches, etc) are connected to the cards. Creative programming can turn the computer into, for example, an oscilloscope (ADC and display) or a signal generator (DAC) as the laboratory exercises have shown.

Other buss systems are in use which, like the slots in the IBM-PC, allow a microprocessor to be connected to various data acquisition and control devices by simple board replacements. Some of the more widely used ones are S100, STDBUS, MULTIBUS, and QBUS.

The second style is to have a separate box next to the computer which has the ADCs, DACs, digital I/O lines and a programmed microprocessor controller. It communicates with the computer via a serial or parallel communication system (Section 9.2 has a description of a parallel system). The box takes care of the data acquisition and control while the computer is used to send control bytes to tell the box what to do and to receive the data for further processing. The limitation of this style is in the speed of communication to the computer and in the number of things the box has been preprogrammed to know how to do.

The microprocessor has become commonplace in our technological society. Everything from dish washers to astronomical telescopes have chips controlling their operation. While the development of applications for computers has been in constant flux since their introduction, the principles of computer operation and of their use in sensing and control have remained stable. Those are the primary subjects of this book. Once a basic understanding of the principles has been built, further detailed knowledge can be acquired later as the need arises.

This book is designed to be accompanied by extensive laboratory work. Over the years the engineering curriculum has focused more and more on the lecture/recitation format. This has led to an ever increasing emphasis on theoretical developments and a loss of contact with the physical basis of engineering and science. The laboratory provides a vital experience in linking theory with physical reality. It also provides the satisfaction of building something and making it work.

Not all computers are suitable for laboratory use. Large mainframe computers are fast and can handle large amounts of data but are awkward to connect to laboratory equipment. At the other end of the scale, microprocessors are included in many laboratory devices but are programmed to perform only a restricted set of duties. Mini and microcomputers have enough speed and memory for all but the most demanding applications but yet are small enough to be dedicated to individual projects and therefore are widely used in the laboratory.

With the technological strides of recent years, microcomputers (or personal computers) have prodigious capabilities.

This device is commonly called a UART (Universal Asynchronous Receiver/Transmitter). There is one for each COM port of the computer. They allow communication between computers directly or via telephone modems.

Asynchronous Communications Adapter

The Asynchronous Communications Adapter is a 4“H × 5”W card that plugs into a System Expansion Slot. All system control signals and voltage requirements are provided through a 2 × 31 position card edge tab. A jumper module is provided to select either RS-232-C or current loop operation.

The adapter is fully programmable and supports asynchronous communications only. It will add and remove start bits, stop bits, and parity bits. A programmable baud rate generator allows operation from 50 baud to 9600 baud. Five, six, seven or eight bit characters with 1, 1–1/2, or 2 stop bits are supported. A fully prioritized interrupt system controls transmit, receive, error, line status and data set interrupts. Diagnostic capabilities provide loopback functions of transmit/receive and input/output signals.

Figure (22) is a block diagram of the Asynchronous Communications Adapter.

The heart of the adapter is a INS8250 LSI chip or functional equivalent. The following is a summary of the 8250's key features:

• Adds or Delete Standard Asynchronous Communication Bits (Start, Stop, and Parity) to or from Serial Data Stream.

• Full Double Buffering Eliminates Need for Precise Synchronization.

• Independently Controlled Transmit, Receive, Line Status, and Data Set Interrupts.

• Programmable Baud Rate Generator Allows Division of Any Input Clock by 1 to (216–1) and Generates the Internal 16× Clock.

• Independent Receiver Clock Input.

The purpose of an instrument is to make measurements of a particular parameter in a physical process. This requires at least a sensor which responds to the parameter and a display which lets the user record readings which are in some way proportional to the parameter being measured. A thermometer is an instrument which indicates the temperature by quantitatively showing the expansion of a liquid with a temperature increase. A more complete description of the measurement process is shown in Figure 2.1. The arrows show possible but not necessary routes for the flow of information. The computer is able to do many of the tasks which were formerly done by separate units of an instrument. This lets the designer reduce the number of components required to a bare minimum as the experiments in this book show. Many times all that is needed is a sensor to translate the process into an electrical signal.

Another way to think of the computer is as an interface between the experimenter and the experiment (or the user and the measurement). It is able to translate the unintelligible signals from the sensor into a form which is understandable using human senses. One of the best ways of communicating information is by picture. ‘A picture is worth a thousand words.’ (In fact, it takes roughly a thousand words of computer memory to display a video graphics screen.)