To use apparatus intelligently it helps to understand what is going on inside; the discussion below focuses on giving some insight into what occurs when you do an analog conversion. As with most things, such discussion has many layers of increasing depth and detail. This discussion will go only one veneer down.

The analog to digital conversion is done on the John Bell Engineering Universal I/O board by an ADC 0817 IC which is connected to the address and data busses and to the RD (read) and WR (write) wires of the IBM-PC (Figure D.I). Port addresses 812–819 (assuming a base address of 800 decimal) are devoted to operation of the ADC.

There are basically three operations which can be done. Refer to Figure D. 1 in the following. If a byte is written to any of the port addresses 812–815 (e.g., Port[812]: = n;), the address is decoded by the card circuitry and the CS4 wire is activated. The WR wire is also active since this is a write operation. Thus the data wires (D0–3) at positions ABCD on the multiplexer are activated. These four wires select by means of a binary code, the position of the switch so that one of 16 analog input channels is connected to the input of the converter section. Thus the lower four binary digits of the byte (n) are used to set the multiplexer channel number. Just after the multiplexer is switched, the converter is started since the WR and CS4 wires are also connected to the Start input of the converter section.

Interrupts are an important capability of modern computers. They allow the processing of several independent tasks by the CPU. On large computers they allow multiuser and time sharing activities. On microprocessors they allow the running of a main program while periodically taking data or sending data to a slow device like a printer. Also computer start up, the keyboard, and TOD functions make use of the interrupt function.

In the discussion which follows, we will first trace the steps taken by the CPU when it receives an Interrupt Request (INTR) from other parts of the computer and then look into the ways we can cause interrupts to be generated and serviced.

Interrupts and the CPU

The interrupt sequence is similar to a jump to a subroutine except that it occurs when signalled by a wire leading to the CPU (INTR). When an interrupt signal is present on the INTR wire and the interrupt enable bit (IF) of the Flag register (see Figure 8.1) is 1, the CPU begins processing the interrupt. The interrupt enable bit is used to prevent the CPU from beginning to process the same interrupt again before it has completed the first one. Without it the computer would go into a continuous regression. The IF bit is set equal to 0 during an interrupt sequence and further interrupts are ignored until this bit is returned to 1. This can be done with the SEI instruction but is done automatically at the return from the interrupt service routine.

Thus far, there has been no need to understand the inner workings of the computer in order to do useful experiments. It has been a black box which responds in a reliable way when given instructions. Just as in using a car, many times this is sufficient; however, to utilize its capabilities as a tool in the laboratory fully, the internal operation of the computer should be understood. In this chapter, we will look under the hood to explore the internal organization of the IBM-PC and to learn to program the 8088 microprocessor directly.

Inside the IBM-PC

A first glance under the cover of the IBM-PC shows a circuit board with a row of connectors which contain other circuit boards standing vertically. The horizontal board (the mother board) contains the 8088 microprocessor chip and various other chips which control the keyboard and screen and contain the memory cells. The microprocessor is the CPU which controls the system and executes the program instructions. The boards in the connectors perform a variety of other functions. Figure 6.1 shows the general organization.

The different chips and circuit boards communicate with each other via the buss: a group of 62 wires which carry digital signals. There are 20 address lines, 8 data lines, and 34 auxiliary lines. The data lines contain the 8 bits of data which are to be transferred by the CPU. The bits on the address lines specify the binary number of the location from or to which the data will be transferred. The operation of the computer is, at the lowest level, a controlled transfer and manipulation of bits of data among various chips and devices.

A solid body moving through a fluid has a force pushing on it which depends on the type of fluid. You might imagine that it would be much harder to swim in honey than it is in water. The parameter which describes this difference is the viscosity (μ). The drag force also depends upon other parameters such as the surface area of the body and the fluid density, as you will discover in this chapter. The computer will be programmed to measure the speed of a sphere falling through glycerine and to calculate the viscosity. The measurements are made with photosensors and using machine language programming.

Force required to move a solid body through a fluid

In this section the physics of a sphere moving in a fluid will be discussed. There are two distinct regimes; if the sphere is moving slowly, the dominant force resisting its motion is due to viscosity. For rapid movement, the inertial resistance of the fluid due to its density is the dominant factor. The magnitude of the resistance and the functional dependence on sphere size, velocity, fluid density and viscosity can be estimated in a rough way for both cases. This gives insight into how the drag force behaves without getting lost in the mathematics. Indeed, with turbulent phenomena exact computations have not been possible.

Viscous resistance of a fluid arises from shear in the velocity profile of flow. If two flat plates have fluid between them, as shown in Figure 7.1, a force is required to move the top one at a constant speed in relation to the bottom one.

In addition to Turbo Pascal, a graphing program is needed which is capable of drawing engineering graphs on the screen and the printer. There are several ways to do this. One is to use the primitive graphics functions of Turbo Pascal itself to build a library of graphing procedures. This has the advantage of making the graphics functions available in the program generating the data but is a much larger task than it initially appears to be. The Turbo Graphics Toolbox available from Borland International provides some plotting features, but generally it concentrates on graphics rather than graphing which limits its usefulness.

Using a spread sheet program is another way of graphing data. It has the advantage of also allowing the data to be transformed before graphing. However, the graph formats are frequently limited (eg, a single set of abscissa values and no logarithmic scales) and a file of the data must be written before it is used.

There are a number of stand alone graphing programs on the market now which can make beautiful graphs. Some of them allow data manipulation as well. Please see the list of programs which follows.

A wish: Someone please make an engineering graphing program (memory resident?) which can be used from within a program to make quick graphs on the screen or printer of data and models.

General IBM-PC and 8088 programming

IBM-PC DOS Technical Manual, IBM, Boca Raton, FL.

IBM-PC Hardware Technical Manual, IBM, Boca Raton, FL.

iAPX86,88 User's Manual, Intel Corp., Santa Clara, CA, 1981.

Bradley, David, Assembly Language Programming for the IBM Personal Computer, Prentice-Hall, Englewood Cliffs, NJ, 1984.

Norton, Peter, Programmers' Guide to the IBM-PC, Microsoft Press, Bellevue, WA, 1985.

General computing

BYTE Magazine.

Good general overview of microcomputing with frequent references to laboratory applications.

General numerical analysis

Press, W., Flannery, B., Teukolsky, S. & Vetterling, W. Numerical Recipes, The Art of Scientific Computing, Cambridge Univ. Press, New York, 1986.

General electronics

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

The best reference for designing laboratory electronics.

Lawson, C. L. & R. J. Hanson, Solving Least Squares Problems, Prentice-Hall, Englewood Cliffs, NJ, 1974.

Physical data

Handbook of Chemistry and Physics, ed. R. Weast, 52nd edn, Chemical Rubber Co, Cleveland, OH, 1971.

American Institute of Physics Handbook, ed. D. E. Gray, McGraw-Hill, New York, 1957.

Mark's Standard handbook for Mechanical Engineers, eds. T. Beaumeister, E. A. Abalone & T. Baird, 8th edn, McGraw-Hill, New York, 1978

Physics

Any general introductory physics text will provide a good background.

Sensors and transducers

Doebelin, E. O., Measurement Systems, McGraw-Hill, New York, 1983.

A thorough overview of general design and specific devices.

Specific hardware

Witten, I. H., Welcome to the Standards Jungle, BYTE, pp. 146–78, February, 1983.

This a close look at serial data communication.

Although cryptic, data sheets contain all of the detailed information about a particular device. But, be warned!, they are sometimes inaccurate due to typos and poor editing. The data sheets on the following pages of the 8255 PPI seem to be accurate.

1. ▪ I MCS-85TM Compatible 8255A-5

2. ▪ 24 Programmable I/O Pins

3. ▪ Completely TTL Compatible

4. ▪ Fully Compatible with Intel Microprocessor Families

5. ▪ Improved Timing Characteristics

6. ▪ Direct Bit Set/Reset Capability Easing Control Application Interface

7. ▪ Reduces System Package Count

8. ▪ Improved DC Driving Capability

9. ▪ Available in EXPRESS

10. — Standard Temperature Range

11. — Extended Temperature Range

12. ▪ 40 Pin DIP Package or 44 Lead PLCC (See Intel Packaging: Order Number: 231369)

The Intel 8255A is a general purpose programmable I/O device designed for use with Intel microprocessors. It has 24 I/O pins which may be individually programmed in 2 groups of 12 and used in 3 major modes of operation. In the first mode (MODE 0), each group of 12 I/O pins may be programmed in sets of 4 to be input or output. In MODE 1, the second mode, each group may be programmed to have 8 lines of input or output. Of the remaining 4 pins, 3 are used for handshaking and interrupt control signals. The third mode of operation (MODE 2) is a bidirectional bus mode which uses 8 lines for a bidirectional bus, and 5 lines, borrowing one from the other group, for handshaking.

General

The 8255A is a programmable peripheral interface (PPI) device designed for use in Intel microcomputer systems. Its function is that of a general purpose I/O component to interface peripheral equipment to the microcomputer system bus.