A11-A SIMPLE DESCRIPTION OF MINI--THE PDP SERIES OF SMALL COMPUTERS

 

By PragyanSharma Polavarapu

(A)INTRODUCTION

(1)The MINI computers of 1960s and 1970s were really very small compared to the old giant electronic calculating machines like ENIAC of 1940s . The old electronic calculator-computer machines like the respected ENIAC were of the size of big halls in houses. Wonder of wonders,the MINIs of 1960 and 1970s became reduced to the size of just table-top band radios! The transformation was made possible solely after the discovery of tiny sand particle-sized “TRANSISTOR” (silicon triode) and its use in the MINIs . Hundreds or even thousands of transistor triodes were used in creation of hundreds of logic circuits in registers and processors inside the computer.

(2)The transistor was invented in 1947 after a series of researches on tiny “crystal diodes” made from semiconductor materials like Germanium and Silicon (both non-metallic elements). The first transistors were junction diodes, created by joining “doped” p-type and n-type semiconductor wafers. Doping means entering atoms of other selected elements into crystal lattice structure of Germanium/Silicon by a carefully controlled accurate chemical/physical processes. Doped silicon wafers of n-type have excess electrons . Doped silicon wafers of p-type are dificient in electrons. Joining of the p-type and n-type materials creates triodes. They are of extremely minute size(of the size of sand particles or even smaller ) compared to the huge triodes created with “thermionic tubes”. The thermionic triodes used in old computers of 1940s were of the size of electric bulbs and made the old computers to be of a house size. The less than sand grain sized transistors used in MINIS performed more efficiently than the thermionic tube triodes.

(3)The MINI computers of 1960s started using the transistor triodes and created the “MINI computer revolution” . The MINIs were popular for about three decades in period 1960 to 1980s .But they too were costly at about 20,000 dollars and were not within reach of common people.

(4)In one respect the mini can be compared to a big mansion of say fifty rooms. Just as the mansion is constructed out of tiny bricks, a computer’s digital brain is also constructed using hundreds of thousands (now millions) of tiny sub microscopic “electronic bricks” called transistors. The first single transistors were rigidly fixed in tiny glass jackets and dozens of them rigidly fixed on Bakelite boards in electric circuit diagrams and connected by hair thin metal lines printed on the Bakelite boards . This technology of wiring the logic circuits with transistors on Bakelite boards later developed in to 50 to 60 billion worth multi-layered highly sophisticated”printed circuit board”(PCB) technology.

(5)As already said the MINI computer came in the size of an old multiband table radio . It has a small processing unit containing many inter-connected tiny PCBs containing assemblies of thousands of transistor triodes.such a processing unit (equivalent to ----of modern desktop computer )was roughly of size of a modern fritz. The MINI had also a key board like that of type writer to directly type “software instructions”(commands to computer’s digital brain given in a highly developed mathematics based software language) . One small room with a table and chair was sufficient.

(6)Inside the MINI computer, the software architecture and engineering plan was almost exactly the same as in the standard IAS architecture of the ENIAC of 1940s . The MINI computers which appeared in 1960s looked very sophisticated and appeared to be pointing to a new computer era. The MINIs were used only by senior engineers mainly for remotely controlling chemical and industrial processes.

(7) the first MINI s made of transistor circuits---,like the PDP-8 series poroduced by DE corporation---were a thousand times more powerful than the old computers of of 1940s. Compared to price of one million dollars for the giant size machines like ENIAC and EDVAC, the PDP-8 machines were priced at around 20,000 dollars! The later PDP models costed only around 5000-6000 dollars ! The first pdp-8 with its related electronic parts weighed only about 100 kg while some later models weighed much lower ,some weighing only about 25 kg! The MINI, in a compact beautiful size fitting into a researcher’s room, created a technical revolution and most surely also an intellectual revolution. Scientists and engineers were excited about the technical potentialities of the radio-size new wonder machine called computer .

(8) The primary software plan and architecture of electronic calculating machine (the IAS model) was invented while creating the ENIAC of 1940s. Since then this standard technical plan is implemented in all the hundreds of millions of computers even of today.

( B )OTHER FEATURES

(1)But the most wonderful development in computer technology , developed by the MINI computers was “interactive computing”. It is a great revolutionary idea , next only to the “IAS architecture” of 1940s mentioned above.

“Interactive computing” turned the MINI computer into a truly magical machine with a robot brain and greatly popularized computer among engineers.Two MINI computers, connected through a communication channel ,could interact like friends. Each could act independently and interact with the other computer.

(2)The most important condition for Interactive computing is that a live “FEEDBACK LOOP” for communication between the computers should exist . It can be a series of software command loops or some extra hardware . Sensors in each of the interacting computers awaken the other computer to particular tasks. This technology perhaps led to the development of “CLIENT-SERVER” type software. Among two computers ,one would first create an “output data” for the other computer. The receiving computer analyses the data received by it and instantly creates a new output and send to the first computer.Such interactive sessions would continue as long as necessary.

(3) In 1960s the most useful “CLIENT-SERVER TECHNOLOGY” was developed perhaps based on the principle of interactivecomputing. Many users could simultaneously utilize the same mainframe computer by a procedure called “TIME SHARING”.

We know that a computer does logical and mathematical work in millionth of second units .For a computer,a “second” could be divided in to several hundred pieces each containing a tiny “time slice”. For humans such “time slice” may be unmanageably small ,but it is big for the computer’s CPU which works in millionth of second units . It is a big time slice. A few such time slices are alloted to each human user . Each user’s quota of time gets repeated thousands of times in each second and he even feels that he is the only person utilizing the entire time of the “SERVER” (the chief central computer). The client/server environment is also type of interactive computing .

( 4 ) Perhaps, only after the idea of “interactive computing” became popular in MINI s, other great technologies like MSDOS, WINDOWS ,WORD PROCESSING , ICON based computing (using MOUSE and DIALOGUE BOXES ) etc were developed. The basic idea under all such technologies is existence of a “feedback loop” to convey data signals to both the interacting computers. This means that one computer’s output influences “next action” in the other computer and creates the feeling that both computers,like humans, are able to converse.

( 5 ) In modern days the special highly standardized codes called “SOFTWARE LANGUAGES” are used to send commands to the digital brain of computer. Various software languages like FORTRAN, BASIC ,COBOL were developed. These languages with their RESERVED WORDS are in advance loaded in the computer and managed the entire working of the computer most accurately.

The old software languages like BASIC and COBOL construct software programs resembling single room houses .The modern soft ware languages like C and JAVA can construct programs resembling strong multistoried houses(STRUCTURED PROGRAMMING). Languages like C ++,JAVA ++ are far more stronger like engineers using new machinery and new techniques to construct still bigger houses.

(6)In radio or television,after we hear/see a program the program simply “evaporates into thin air”and disappears without trace like water flowing out of an open pipe . But in computer a program (text file, picture file,sound file,video file ) after they are read/seen/heard by us are stored not just for hours and days but even years. They are stored in computer’s files and do not disappear until we erase them .

MISCELLANEOUS

(1)Several models named PDP-8/S, PDP-11,PDP-12,PDP-14 etc were released . Each new model had some new facility perhaps suited for a particular groups of users. It is said that about 300,000 PDP machines were sold . This indicated that the PDP machines were useful for various types of commercial/scientific/technical/ data processing purposes.

(2)The most important point is that with the availability of a small tabletop computer of the size of a old valve radio, further research about many potentialities small computer got accelerated. Small networked computer systems appeared for interactive use in many fields of science and inaugurated the dawn of the modern era of IT as we see today.

(3) In the early MINI computers the magnetic and other data storage devices could store only small quantities of data . But they were really pathways to great research in computer theory. “Magnetic core memories” made of thousands of extremely small magnetized ferrite rings 1.5 mm to 2.5 mm in diameter were interconnected through hair thin wire paths/printed metal lines etc . The magnetic rings were arranged in vertical/horizontal rows with switch on/switch off facilities.

(4)Later large storage devices like magnetic tapes and optical discs were created. The technology of converting text data/picture data and sound data into digital strings of billions of 0s and 1s was invented. In modern computers such huge data is nowadays stored in button size computer chips containing hundreds of millions of transistor storage units (flash memories).Whole books (thousands of pages),whole photo albums can be stored. Even entire videos and commercial films are stored in such digital storage devices.