TABLE 3-1 RELATIONSHIP BETWEEN SOME DECIMAL AND BINARY NUMBERS

Decimal

• For monographs on compuler interfacing, see: Com-purtrs for SpectroJcoputJ, ed. R. A. G. Carrington. New York: Wiley, 1974; J.• Finkel,Computer-Aided Exper-imentation. New York: Wiley-Interscience, 1974; J. S.

MatlJoD, H. B. Mark, J r, and H. C. MacDonald, J r, Computer Fundamentabfor Chemu/$. New York: Marcel Dekker, 1973; and A. J. Diefenderfer,PrincipleJ of Elec-tronic Instrumentation, 2d ed. Philadelphia: Saunders, 1979,Chapters 10-12.

70-64=6 6- 4=2 2- 2=0

often employ 16-bit words and microcom-puters 8. Computer memories are described in terms of the number of words they can retain. Thus, an 8K memory has space for approximately 8000 words.

A single word contains a pattern of bits that cause the computer to take a specific action. Generally, a word contains a datum or a command and an address, both written in binary notation. The latter gives the loca-tion of an instrucloca-tion or a datum word in the memory. For example, for a computer em-ploying 12-bit words, a word might take the form

Here, the first three bits could be the code for a command such as add, subtract, multiply, or store. The next two, in this example, are not used. The last seven bits give the location of the number upon which the command is to be exercised. For example, the word just shown might command the computer to add the number stored in the indicated location (storage number 74) to another number.

Major Components of a Computer Figure 3-40 is a block diagram showing the arrangement of the major components of a digital computer.

The Central Proceaiag Unit. The heart of a computer is the central processing unit, which contains a control unit and an arith-metic unit. The former controls the overall sequence of operations by means of a program, which is stored in the memory unit The control unit ~receives instructions from the input device, fetches instructions and data from the memory: unit, transmits instructions to the arithmetic. unit for computation, and transmits the results of these computations to the output and often to memory as well

The program, the instructions, the math-ematical operations, and the memory of a dig-ital computer are all based upon the binary system made up of just two states, on and 00;

or 0 and 1. The \:cntral processing unit of a computer contaiJls a vast array of transis-torized switches, ~ach of which has only two settings, 0 or I, corresponding to on or 01T.

These switches are employed using binary

logic to perform the various functions of the computer; generally, the switches control volt-ages that range from 0

±

0.5 V for the off or .

o

position and 5

±

2 V in the on or I posi-tion. The band between 0.5 and 3.0 V is sufficiently large to prevent interference by random noise.

Memory. Storage of data and instructions for a computer take two forms. One makes use of magnetic tapes or disks. Access to these is relatively slow, and they are used primarily for storage of programs and data for future use.

The main memory of computers is either a magnetic-core type or an integrated-circuit type. In both, instructions or data are stored in binary form with a separate core or circuit being required for each bit. Generally, the individual cores or circuits are arranged in square arrays of 32 x 32 or 1024 storage posi-tions. For a computer employing words made up of 16 bits; one of these arrays would corre-spond to 64 words. The minimum memory capacity required for most computational applications is 4096 words (a 4K memory).

Thus, the number of individual binary cores or circuits is enormous.

The individual storage unit or core in a magnetic-core memory is made of a ferrite ceramic, shaped in the form of a doughnut.

The inside and outside diameters of the ring are 0.013 and 0.020 in~ respectively. When an electrical signal passes through a wire that is threaded through the ring, magnetization of the ferrite material occurs; the direction of magnetization (clockwise or counter clock-wise) depends upon the direction of the current and remains fixed after the current ceases. Thus, the core has two stable states and can store binary information. A sensing wire running through the ring is then employed to sense the direction of magnetiza-tion and thus retrieve the informamagnetiza-tion stored in the element.

An integrated-circuit memory element gen-erally consists of a circuit containing four transistors, two capacitors, and two switches.

•• •

Tiny single chips containing 1024 to 4096 of these elements are employed. Binary storage and retrieval depends upon which half of the circuit is conducting at any time. An integrated-circuit memory has the disadvan-tage of being volatile; that is, loss of electric power results in loss of the stored information.

Input-Output Systems. Input-output devices provide the means by which the user (or his instrument) communicates with the com-puter. Familiar input devices include the electric typewriter, punch cards or tapes, magnetic tapes or disks, and the electrical or mechanical signals from analytical instru-ments. Output devices include recorders, elec-tric typewriters, cathode-ray screens, and meters. It is important to appreciate that most of these devices provide or use an alUllog signal while, as we have pointed out, the computer can respond only to digital sig-nals. Thus, an important part of the input-output system is an analog-to-digital converter (ADC) for inputting data, which the com-puter can use, and a digital-ta-analog counter (DAC) for converting the output from the computer to a usable signal.

By definition, an analog signal is one that varies continuously, whereas a digital signal is one that varies in discrete increments. An example of an analog signal is the voltage output of a glass electrode, which varies con-tinuously from about 0 to about 1.0 V depending upon the pH of the solution in which it is immersed. Most, but not al~ analyt-ical instruments are analog; examples of ex-ceptions include the output from the decay of a radioactive species or from a Geiger tube when it is employed to measure the intensity of an X-ray beam.

Most output devices such as recorders, plotters, and meters require analog rather than digital signals.

Where the analog signal is the rotary motion of a shaft, a digital signal is readily produced by means of a mechanical turn counter. One method of achieving a digital

signal is to mount on the shaft a circular disk that is notched to give opaque and transparent slots. An array of light beams and pho-tocells then produces a series of digital electri-cal pulses as the disk rotates. Such a device might be employed to monitor the wavelength setting of a monochromator or the null point of a potentiometer.

More commonly, analog-to-digital convert-ers are electronic and are based upon count-ing square-wave pulses from a generator.

Here, the number of pulses or their frequency is determined by the analytical signal. Alter-natively, the time required for a ramp voltage to change from zero to a chosen level may be measured.

Digital-to-analog converters are electronic devices that convert a digital number into a voltage proportional to the number. The volt-age can then be employed to drive a record-er, metrecord-er, or other output device.

Computer Programming

Communication with a computer entails set-ting an enormous aggregation of switches to appropriate off or on positions. A program consists of a set of instructions as to how these switches are to be set for each step in an instruction or a computation. Each of these instructions must be written in a form to which the computer can respond-that is, a binary mJJChine code. Machine coding is te-dious and time consuming and is often prone

o110001 100 100 001 0100 001100 100010 0100 001 100 100 011

o111 001 100 100 100

to errors. For this reason, assembly languages have been developed in which the switch-setting steps are. assembled into groups which can be designated by code word. For exam-ple, the abbreviation for subtract might be SUB and might correspond to 101 in machine language. Clearly, SUB is a good deal easier for the programmer to remember than 101.

Assembly programming, while simpler than machine programming, is still difficult and tedious. As a consequence. a number of high-level languages, such as FORTRAN and Basic, have been developed. These languages, which are easily learned, have been designed to make communication with the computer relatively straightforward. Here, instructions in Basic or FORTRAN are translated by a computer program (called a compiler) into machine language which can then control the computer for computations. Unfortunately, loss of efficiency accompanies the use of higher-level languages.

Figure 3-41 illustrates the application of the three languages for obtaining a sum.

Minicomput ••.•

A minicomputer is sufficiently small and inex-pensive that its use can be limited to one lab-oratory or even to a single instrument. A minicomputer will have at least 4K or memory and often considerably more.

Usually, it can be used on a time-shtJred basis.

That is, its capabilities are such that it can be

LOA Z Assembler ADA B AOAC STA 0

FIGURE 3-41 Relationships among machine, as-sembly, and a high-level language. (From S. Perone, J. Chromatog, Sci., 7, 715 (1969). With permission.)

used in more than one way and by more than one user at the same time without interfer-ences. A single minicomputer is frequently used to control several instruments in one laboratory.

MiniComputers can usually be pro-grammed in one or more high-level lan-guages. Often, however, interfacing them with instrument signals may require the user to construct subroutines in assembly language.

The central processing unit in a mini-computer generally ranges in cost from 51000 to 510,000. The addition of storage, input and output devices, and other peripherals usually doubles these costs at a minimum and may multiply them by a factor as great as five to ten.

Microcomput ••••

The heart of a microcomputer is the micro-processor, a device that first appeared on the market in about 1973; the applications of microprocessors have grown exponentially since that time.⁹ A microprocessor is a com-plete central processing unit, which has been miniaturized and formed on a single integratcd-circuit chip having an area of a few square millimeters. Combination of a micro-processor with a memory and an input-output device. leads to a remarkably inexpensive microcomputer, which can be conveniently employed in conjunction with a single instrument; such a computer is said to be dedicated. Most new analytical instruments are being sold with one or more programmed microprocessors as an integral part of the equipment. With these, no programming by

• For a summary of the construction and applications of microprocessors and minicomputers. see: R. E. Dessy. P.

Janse·Van. Vuuren, and J. A. Titus. AMI.Chem.46 (11~

917A (1974); 46 (12~ 10SSA (1974); B. Soucek. Micro-procnson and Microcompuler<.New York: Wiley. 1976;

and An Introduclion 10Microcompulers. Adam Osborne and Associates. P.O. Box 2036. Berkeley. CA 94710. 1977.

vol O. I.and 2.

the user is required to obtain automated in-strument control.

Microcomputer memories tend to be small-er than those employed in minicomputsmall-ers.

Memory increments of 256 words are avail-able; typically, eight of these are employed to provide a capacity of 2048 words. Only one or two of the memory increments is required for the control of many instruments. Word lengths for microcomputers are commonly eight bits.

'Microcomputers, quite adequate for the control of a single instrument and with data-acituisition speeds of 1000 to 3000 words per second, are available for 51000 or less.

tIn more complex instruments, one or more microcomputers may be controlled by a minicomputer. The slave microcomputer then performs one or more control functions at the command of the minicomputer.

.A~Plicetiona of Compute •••

Computer interactions with analytical instru-ments are of two types, passive and active.tO

In passive applications, the computer does not participate in the control or the experi-ment but is used only for data handling, processing, storing, file searching, or display.

In an active interaction, the computer is significantly involved in the control of the experiment. Instruments with computer con-trol are said to be automated.

Passive Applications. Data processing by a computer may involve relatively simple math-ematical operations such as calculation of

~ncentrations, data averaging, least-square analysis, statistical analysis, and integration to obtain peak areas; as we have seen. several

10This c1assification has been IUgated by Perone in his summary or computer applicatioDl. See: S. P. Perone and D. O. Jones. Digital Compuler III ScinIlific llUll'lUMntation.New York: McGraw·Hill, 1973. Chapter 12.

of these operations can also be performed with operational amplifiers. More complex calculations may involve solution of several simultaneous equations, curve fitting, and Fourier transformations.

Data storage is another important passive function of computers. For example, a power-ful tool for the analysis of complex mixtures is obtained by linking gas-liquid chromatog-raphy (GLe) with mass spectrometry (MS).

The former is capable of separating mixtures on the basis of the time required for the indi-vidual components to appear at the end of suitably packed columns. Mass spectrometry permits identification of each co~ponent according to the mass of the fragments formed when the compound is bombarded with a beam of electrons. GLCjMS equip-ment may produce as many as 100 spectra in less than an hour, with each spectrum being made up of tens to hundreds of peaks. Con-version of these data to an interpretable form is time consuming; thus, the data must be stored in digital form for subsequent printing.

Identification of a species from its mass spectrum involves a search of files of spectra for pure compounds until a match is found;

this process is also time consuming, but can be accomplished quickly by using a computer. Here, a magnetic disc or magnetic tape file storage is required. For example, programs have been described which permit the search of several thousands of spectra in a minute or less. Such a search will frequently produce several possible compounds. Further comparison of spectra by the scientist usually makes identification possible.

Another important passive application uti-lizes the high-speed data fetching and cor-relating capabilities of the computer. Thus, for example, the computer can be called upon to display on a cathode-ray screen the spec-trum of anyone of the components after it has exited from a gas chromatographic column.

Active Applicatiol& In active applications,

the computer exercises at least some and sometimes all of the control of an instrument during an analysis. Most modem instruments contain one or more microcomputers that perform such functions. Examples would in-clude control of the slit width and wavelength setting ora monochromator, the temperature of a chromatographic column, the potential applied to an electrode, the rate of addition of a reagent, and the time at which the integra-tion of a peak is to begin. Referring again to the GLCjMS instrument considered in the last section, a computer is often used to ini-tiate collection of mass spectral data each time a compound is sensed at the end of the chromatographic column.

Computer control can be relatively simple, as in the examples just cited, or more complex. For example, the determination of the concentration of elements by atomic emis-sion involves the measurement of the heights of emission peaks that occur at wavelengths characteristic for each element. Here, the computer can cause a monochromator to sweep a range of wavelengths until a peak is located, and can then seek the exact wavelength at which the maximum output signal is obtained. Measurements are made at this point until an average is obtained that will give a suitable signal-to-noise ratio. The computer then causes the instrument to repeat this operation for each peak of interest in the spectrum. Finally, the computer processes the data and prints out the concen-tration of the elements present.

Because of its great speed, a computer can often control variables more efficiently than can a human operator. Furthermore, with some experiments, a computer can be pro-grammed to alter the way in which the mea-surement is being made, according to the nature of the initial data. Here, a feedback loop is employed in which the signal output is fed back through the computer and serves to control and optimize the way in which addi-tional measurements are made.

1. Design a circuit whose output V. is given by - V. = 2V,

+

4V2

+

1V3

+

3V4

2. Design a circuit for calculating an average value for three voltage inputs multiplied by 100.

3. Design a circuit for performing the following calculation:

y=!(2x,

+

3X2)

R" Rf2

R, V,

R.

V.

.".

R3 V3

(a) write an expression giving the output voltage in terms of the three

I

input voltages and the various resistances. .

(b) indicate the mathematical operation performed by the circuit when R1

=

Rfl

=

100 kn; R4

=

Rf2

=

SOkn; R2

=

2S kn;

R3 =10 kn.

S. Show the algebraic relationship between the voltage input and output for the following circuit:

v, 3kU SkU

v,

4kU V3

6kU V.

6. What would be the output voltage of the circuit shown in Figure 3-26a after 30 s, if the input voltage is 2S mY, the resistor is 200 kQ, and the capacitor is 0.26 JlF?

12. The linear slide wire AB has a length of 100 an. Where along its length should contact C be placed in order to provide a potential of exactly 2.50 V?

8. Show that when the four resistances are equal. the following circuit becomes a subtracting circuit.

13. Design a circuit that will produce the following output:

V.= 2

.f

^{VJ dt}

⁺

³

f

^V2dt

14. Design a circuit that will produce the following output:

V.= 3

f

^{VI dt - 4(V2}

⁺

^V3)

15. The following data were obtained for repetitive weighings ofa 12S4-g object with a top-loading balance. Calculate the signal-to-noise ratio for the measurements, assuming the noise to be random.

9. Derive a relationship between

v..

I't. and ~ for the circuit shown in Problem 8 when Ru = Ru and R^J = R2•

10. Derive a relationship between V. and l'/ for the following circuit:

1253 g 1256 1.257 1.254

12S6g 1.252 1.254 1.255

16. How many measurements would have to be averaged to increase SIN in Problem 15 to l000?

17. The following data were obtained for a current measurement on a noisy system. What is the signal-to-noise ratio. assuming that the noise is random?

~v.

1.34flA 1.76 121 1.35

1.10flA 1.63 1.77 1.19

18. How many measurements would have to be averaged to obtain a signal-to-noise ratio of 10 for the mean in Problem 16; to obtain a ratio of l00?

19. The resistance of a dc circuit is 15000.What is the maximum resist-ance that a current-measuring device can havc if thc current in this circuit is to be measured with a relativc error of less than 2

%?

Thc cxpected current range is 0 to 20 pA.

20. Devise an operational amplifier circuit employing a 0 to 10 m V mctcr that will meet the requirements specified in Problem 20.

21. Thc resistance of a dc circuit is 2000.What is thc maximum resist-ance that a current-measuring device can havc if thc current in this.

circuit is to be measured with a relative error of less than 5

%?

The expected current range is 0 to 50 pA.

22. Devise an operational amplifier circuit employing a 0 to 10 m V meter:

that will meet the requiremcnts specified in Problem 21. .

This chapter reviews several topics concerned with electromagnetic radiation, including the fundamental properties of this radiation and the mechanism of its interactions with matter.

The material will serve as a general introduc-tion to the various instrumental methods dis-cussed in Chapters 5 through 14}

PROPERTIES OF

ELECTROMAGNETIC RADIATION

Electromagnetic radiation is a type of energy that is transmitted through space at enor-mous velocities. It takes numerous forms, the most easily recognizable being light and

ra-~iant beat. Less obvious manifestations include X-ray, ultraviolet, microwave, and radio

radiations. .

Many of the properties of electromagnetIC

Many of the properties of electromagnetIC

In document Principles of Instrumentation - Skoog (Page 50-67)