2 IEEE TRANSACTIONS ON MANUFACTURING TECHNOLOGY, VOL. MFT-6, NO. 1, MARCH I977

Calculators and Minicomputers-What’s the Difference?

HOWARD M. RATHBUN

T HE PURPOSE of this article is to compare calculators and minicomputers and describe some of the trade-offs an

engineer faces when he has to choose between these devices for an interfacing or control application. However, before we go too far I think it is necessary to comment- on the terms “calculator” and “minicomputer.”

Several years ago it was very clear in everyone’s mind what a calculator was and what a minicomputer was. However, with recent advances in technology, the distinction between calculators and minicomputers has become less distinct. Both are actually members of the larger class of devices known as computers since they both have stored programs and can make decisions. In some cases their capabilities actually overlap each other. Therefore, for the purposes of this article, it is probably best to give examples of both categories so you will have a better understanding of the types of devices we are talking about.

Examples of calculators include the Wang 2200 and WCS series, the Tektronics 4051, the IBM 5100, the Olivetti P6060, the Monroe 1800 series, and the Hewlett-Packard 9800 series. There is no consistency between these companies as to what they call these devices. They are referred to as portable com- puters, desktop computers, programmable calculators, or simply calculators. In any case, they all share several common characteristics. With one exception, they all contain a built-in keyboard and display, storage media, memory, processor, input-output (I/O) channels and a built-in language in a compact, self-contained box.

The number of different minicomputers is much larger than the variety of calculators, but for examples we can mention the DEC PDP 8 and PDP 11, and HP 2100 and 21MX, and the Data General NOVA 2 and NOVA 3. The main characteristic of this class of computers is that they consist of a box with a memory, processor, I/O channels and a front panel containing toggle switches and lights. They do not contain the built-in keyboard, display, language, or storage media that is found in the calculators.

A TYPICAL CALCULATOR CONFIGURATION

Let us look at typical configurations from the hardware, or external standpoint. A calculator is contained in one box, with the single exception of ‘Wang. This evolved from the traditional mechanical or electromechanical calculator which was in one box. The modern day calculator includes a key

Manuscript received March 1, 1977. Jack Hickey, Editor-inChief of Chilton’s Instruments & Control Systems Magazine has granted his permission to reprint this article. It appeared originally in the February 1977 issue of Instrument & Control SystemsMagazine.

The author is with the Hewlett-Packard Co., Loveland, CO 80537.

board, which may be a fully alphanumeric keyboard similar to a Teletype or typewriter, or it may contain a ten-key number pad with a number of predefined special function keys. The alphanumeric keyboard provides more flexibility in the language features and capabilities of the calculator. But is also costs more.

Next, the calculator includes a built-in display. This can range in complexity from a 15digit numerics-only display to a 32character alphanumeric display or a multiple line cathode- ray tube (CRT). In any case, the display provides messages to the user in terms that he can understand.

Some calculators contain a built-m printer with line widths ranging from 16 to 80 characters. These printers are either mechanical or thermal and provide a fast output method for listing programs, messages to the operator, or test results. In addition to the internal memory, which all calculators and minicomputers must contain, they have some form of re- movable storage media which can be used for saving pro- grams and data.

Magnetic cards were one of the earliest methods of recording information and data. Now the trend is towards small mag- netic tape cartridges and flexible disks. They provide a fast and efficient way of saving programs and data on a removable media, which can be reloaded into the calculator whenever required.

Finally, all the calculators mentioned contain a built-in language. The languages range from assembly language, in the case of the Monroe calculators or the Reverse Polish Notation (RPN) in the HP 9815, to a higher level language found in the more flexible and powerful calculators. The most common language is Basic which is found in the HP 9830, the Wang series, the Tek 4051, and the IBM 5100. The IBM 5100 is unique in that it can be obtained with two languages, which are switch selectable, the first being Basic and the second being APL. The HP 9825 has another language, HPL, which is similar in many ways to Basic but contains many of the more powerful features found in Fortran.

In all cases, the language is contained in firmware so that it is always present in the calculator. In general then, all calcu- lators provide everything the user needs to write and run a program in one package.

A TYPICAL MINICOMPUTER

A typical minicomputer departs from the calculators described in many ways. The typical minicomputer is essen- tially a box with a memory, power supply, processor, a front panel which contains some lights and control switches, and a number of interface channels for peripherals. Thus a mini- computer by itself can do very little. True, it is usually

RATHBUN:CALCULATORSAND MINICOMPUTERS

Fig. 1. Like most calculators, the HP Model 9825A desktop program- mable calculator includes display and built-in language.

possible for one to enter a small program via the control panel and make it run, but this is not the typical nor intended method of operation.

To make a minicomputer useful, it must be attached to one or more peripherals. The most important peripheral is a key- board and some storage media for entering and removing pro- grams and data. The most common device used for this pur- pose has been a Teletype@. Now, however, CRT terminals with a built-in keyboard are gaining in popularity.

Punched paper tape has been the storage media for pro- grams and data for a long time, but this is also changing to small, magnetic tape cartridges and flexible disks. In any case, these media are external peripherals connected to the minicomputer. Printers are also always external to the mini- computer. The vendor usually has a wide range of choices from relatively slow character impact printers to very high speed line printers.

Finally, the computer does not contain a built-in program- ming language: Instead, the programming language must be loaded into the computer before a program can be prepared. Thus a minicomputer is only a portion of the system needed to solve a problem. It is not ready to work when it arrives. It must be first configured with the rest of the peripherals that are needed to effectively use it. This is also one of the advan- tages of the minicomputer because the user is able to choose from a very large array of peripherals.

The calculator provides a minimum built-in configuration which, for many users, is enough. Calculator manufacturers do provide an extensive set of peripherals such as plotters, printers, and storage devices which greatly expand the capa- bilities of the calculator systems.

FEATURES AND CHARACTERISTICS

If you were to study the data sheets of typical calculators and minicomputers you would discover a wealth of features and characteristics. For example, every calculator can be used as a calculator, without using any special purpose stored pro- gram. In other words, you can simply walk up to a calculator,

Fig. 2. IBM 5100 programmable calculator.

turn it on and do simple, or not so simple arithmetic calcu- lations.

Another common feature is the number range of the calcu- lator. Most present day calculators have a 12-digit accuracy with a range from lO-99 to 10.99. Next, some of the data sheets list the various execution times. Typical add times for calculators are on the order of 1 ms. This is the time it takes to add two 12-digit numbers in the floating point nota- tion which is carried internally in the calculator.

With the minicomputer, it is not possible to merely turn it on and perform a calculation from its control panel. A pro- gram must first be loaded into it. The number range in the newer minicomputers is typically 16-bits; however, other sizes such as 8, 12, 24, and 32 bits are avaiiable. Because a 16-bit word can only hold a five-digit number, then some provision must be made to do arithmetic with more accuracy than five-digits. This is done either through software (the running program) or through an optional hardware feature.

Finally, add times quoted for minicomputers are in the order of a few ps, but these are for two 16-bit numbers. However, the time a program would take to add two 12-digit floating point numbers in a minicomputer is also in the ms range.

Minicomputers contain other standard or optional features not found on calculators such as memory protect, and power- fail auto-restart. Memory protect allows the user to protect part of the memory so that programs and data in that portion of the memory cannot be destroyed by other programs. The power-fail auto-restart allows the minicomputer to safely store temporary results in the event of a power failure and then reload them and restart the program when the power returns.

HANDLING PROGRAMS

The manner in which calculators and minicomputers handle programs is probably one of the most distinguishing charac- teristics between these devices. Calculators contain a pro- gramming language built into their firmware in read-only memory (ROM). This is supplied with the calculator by the manufacturer and cannot be changed. Because not every user needs the same capabilities, some manufacturers have

4 IEEE TRANSACTIONSONMANUFACTURING TECHNOLOGY,MARCH 1977

Fig. 3. Wang WCS/20 programmable calculator with desk and separate printer.

provided optional plug-m ROM’s, or language extensions to their calculators. This allows the user to customize his lan- guage by inserting additional ROM%.

With minicomputers, all languages are supplied in the form of paper tape or some magnetic media such as a tape cartridge or disc. This provides flexibility because most minicomputers offer several languages (e.g., Basic, Fortran, Algol, or Cobol). The manufacturer also provides a library of routines and pro- grams. In addition, the minicomputer permits users to ,write their own programs in assembly language.

The input-output section of a minicomputer is extremely flexible and fast. Priority interrupt systems are the rule, rather than the ‘exception. Speeds of up to a million char- acters per second are not uncommon. Calculators are slower than minicomputers in this respect and have much less flexi- bility in the input-output section. However, there are many applications that do not require extremely high speeds and here, calculators can be used.

PREPARING A PROGRAM

The value of either device is obtained only through the use of a program. Manufacturers of both machines supply pro- gram packages to solve general problems, but usually the user will have to write his own.

The method of writing a program on the two devices is very different. In a calculator, the user merely turns it on and starts to key in the program through the keyboard. As each line of the program is keyed in, it appears on the display. When the user is satisfied that the program line looks correct, he presses a button marked “store” or perhaps “end of line.” One of the features of most calculators is that before the line is stored, it is checked by the firmware for proper syntax. If the syntax is correct, then the line is stored in the correct location in memory. If the syntax is not correct, the calcu- lator will give some sort of error message.

After the complete program is entered, the user normally would make a listing of it. This is done by pressing a button or typing in a command called LIST. Finally, the user can save the program by pressing a record button or keying in a

Fig. 4. Olivetti P6060 programmable calculator.

Fig. 5. Monroe 1880 programmable calculator.

command called RECORD. To run the program, he presses the RUN button. If there is an error in the program, the appropriate line is recalled and corrected.

With the minicomputer, the procedure will vary depending upon the machine. For example, disc operating systems (DOS) or real-time executive systems (RTE) offer much more flexibility when preparing programs than a small system with- out them.

For this discussion, let us assume that we are talking about a typical, small minicomputer system. The program must first be prepared off-line; i.e., it could be prepared on a Teletype with a paper tape punch or keyed into a magnetic tape car- tridge on.. a terminal. In either case, the error correcting is limited and there is no syntax checking while the program is being prepared.

Then the program must be assembled or compiled. This process converts the original source program to machine lan- guage. To assemble or compile the program, the assembler or compiler must first be loaded into the computer. This is another program. The second program then reads in the source program and assembles or compiles it. This process will find any syntax errors. If there are no errors, then the assembler or compiler produces another punched paper tape or magnetic tape cartridge in machine language. Now the program is ready to run.

It is possible that the program prepared by the assembler or compiler can be simply loaded into the computer and run.

IEEE TRANSACTIONS ON MANUFACTURING TECHNOLOGY, VOL. MFT-6, NO. 1, MARCH 1977 5

Often, however, the assembled program will call upon some library routines which are contained on another tape. In this case, still another program called the loader, must first be brought into the computer. The loader then in turn will load the machine language program followed by any of the library routines from the library tape which might be required.

Finally, the user is ready to run the program in the com- puter. When the program is run, it will undoubtedly contain errors which require program changes. To make the changes, a program called the editor must be loaded. The editor allows the user to change the original source tape. Then you must go back to the beginning and start the process over by reassem- bling and reloading the tape.

The advantage of compiling a program is that the original program can be written in a high-level language like Fortran, but the final version is in machine language which runs faster. The disadvantage, of course, is that the user must understand these somewhat complicated procedures. And, programs cannot be created and run as quickly.

Most minicomputers offer Basic as a programming language. It is not necessary to create the program off-line, assemble it, and load it-as with the other languages. Instead, it is loaded in by the Basic program itself, much like the calculator. The Basic is a program which must be loaded into the minicom- puter in the first place.

,-.- A QUICK REVIEW

The typical calculator can be described as a friendly easy- to-use quick-to-learn system. It is a stand-alone system with all of the necessary peripherals built in.

The minicomputer requires peripherals to make it functional. It offers a very high degree of flexibility, not only with respect to the peripherals available, but with respect to the languages used and their features. One of the prices the user must pay is the additional programming time and user involvement. This in turn, often requires a computer specialist to do the pro- gramming. Calculators are programmed by the user.

Because the calculator is easy to program and debug, it lends itself to applications with a large amount of program development, where programs are run only a few times or where program execution time is not important.

Programming on minicomputers is more involved and complex. They lend themselves to production type programs which are generated, refined only once and then used many times. They are also useful when program execution speed is important.

Thus after several years of evolution, we find that calcu- I’ators are still distinct in many respects from minicomputers even though some of their capabilities such as speed, and input-output characteristics are similar or overlapping.