Flip-flop is a term referring to an electronic circuit (a bistable multivibrator) that has two stable states and thereby is capable of serving as one bit of memory. Today, the term flip-flop usually refers to clocked or edge-triggered devices (i.e., devices that are a conceptual combination of a transparent-high latch with a transparent-low latch).

A flip-flop is usually controlled by one or two control signals and/or a gate or clock signal. The output often includes the complement as well as the normal output. As flip-flops are implemented electronically, they require power and ground connections.


Set-Reset flip-flops (SR flip-flops)
The fundamental latch is the simple SR flip-flop , where S and R stand for set and reset respectively. It can be constructed from a pair of cross-coupled NOR logic gates. The stored bit is present on the output marked Q.
Normally, in storage mode, the S and R inputs are both low, and feedback maintains the Q and Q outputs in a constant state, with Q the complement of Q. If S (Set) is pulsed high while R is held low, then the Q output is forced high, and stays high even after S returns low; similarly, if R (Reset) is pulsed high while S is held low, then the Q output is forced low, and stays low even after R returns low

JK flip-flop


The JK flip-flop augments the behavior of the SR flip-flop (J=Set, K=Reset) by interpreting the S = R = 1 condition as a "flip" or toggle command. Specifically, the combination J = 1, K = 0 is a command to set the flip-flop; the combination J = 0, K = 1 is a command to reset the flip-flop; and the combination J = K = 1 is a command to toggle the flip-flop, i.e., change its output to the logical complement of its current value. Setting J = K = 0 does NOT result in a D flip-flop, but rather, will hold the current state. To synthesize a D flip-flop, simply set K equal to the complement of J. The JK flip-flop is therefore a universal flip-flop, because it can be configured to work as an SR flip-flop, a D flip-flop, or a T flip-flop.

The characteristic equation of the JK flip-flop is: Qnest = J¯Q¯+¯K¯Q
and the corresponding truth table is:

The origin of the name for the JK flip-flop is detailed by P. L. Lindley, a JPL engineer, in a letter to EDN, an electronics design magazine. The letter is dated June 13, 1968, and was published in the August edition of the newsletter. In the letter, Mr. Lindley explains that he heard the story of the JK flip-flop from Dr. Eldred Nelson, who is responsible for coining the term while working at Hughes Aircraft. Flip-flops in use at Hughes at the time were all of the type that came to be known as J-K. In designing a logical system, Dr. Nelson assigned letters to flip-flop inputs as follows: #1: A & B, #2: C & D, #3: E & F, #4: G & H, #5: J & K.
Another theory holds that the set and reset inputs were given the symbols "J" and "K" after one of the engineers that helped design the J-K flip-flop, Jack Kilby.



D flip-flop

The D flip-flop is sometimes refer to as data flip flop because it is in effect storege for one bit of data.The out put of D flip flop are always equal to the most recent value applied to the input.Hence it remembers and produces the last input.It is also referd to as delay flip flop, because it delays 0 or 1 applied to its input for a single clock pulse.

This flip flops is very useful, as it form the basis for shift registers, which are an essential part of many electronic devices. The advantage of the D flip-flop is that it "captures" the signal at the moment the clock goes high, and subsequent changes of data line do not influence Q until the next rising clock edge.

1ST, 2ND, 3RD, 4TH & 5TH GENERATION COMPUTERS

First Generation (1941-1956)

World War gave rise to numerous developments and started off the computer age. Electronic Numerical Integrator and Computer (ENIAC) was produced by a partnership between University of Pennsylvania and the US government. It consisted of 18,000 vacuum tubes and 7000 resistors. It was developed by John Presper Eckert and John W. Mauchly and was a general purpose computer. "Von Neumann designed the Electronic Discrete Variable Automatic Computer (EDVAC) in 1945 with a memory to hold both a stored program as well as data." Von Neumann's computer allowed for all the computer functions to be controlled by a single source.
Then in 1951 came the Universal Automatic Computer (UNIVAC I), designed by Remington rand and collectively owned by US census bureau and General Electric. UNIVAC amazingly predicted the winner of 1952, presidential elections, Dwight D. Eisenhower.
In first generation computers, the operating instructions or programs were specifically built for the task for which computer was manufactured. The Machine language was the only way to tell these machines to perform the operations. There was great difficulty to program these computers and more when there were some malfunctions. First Generation computers used Vacuum tubes and magnetic drums (for data storage).

The IBM 650 Magnetic Drum Calculator

Second Generation Computers (1956-1963)

The invention of Transistors marked the start of the second generation. These transistors took place of the vacuum tubes used in the first generation computers. First large scale machines were made using these technologies to meet the requirements of atomic energy laboratories. One of the other benefits to the programming group was that the second generation replaced Machine language with the assembly language. Even though complex in itself Assembly language was much easier than the binary code.
Second generation computers also started showing the characteristics of modern day computers with utilities such as printers, disk storage and operating systems. Many financial information was processed using these computers.
In Second Generation computers, the instructions (program) could be stored inside the computer's memory. High-level languages such as COBOL (Common Business-Oriented Language) and FORTRAN (Formula Translator) were used, and they are still used for some applications nowadays.

The IBM 7090

The IBM 7090, announced in 1958, was a transistorized version of the vacuum-tube-logic 709 and the first commercial computer with transistor logic (the first such computing device, according to [53], was the IBM 608, but that was not a general-purpose stored-program computer).

Grayson Kirk, President of Columbia University (right) and Kenneth M. King, director of the University's new Computer Center, 1963, in the machine room at the IBM 7090 console, "one of the largest computers in existence" at the time. Photo: found in an unattributed newspaper clipping in the Columbiana archive.




The IBM 7090 Console in the Columbia Computer Center machine room, 1966. Pictured: A group of particle physicists who discovered the violation of charge-conjugation invariance in interactions of intermediate strength: Charles Baltay and Lawrence Kirsch of Nevis Lab (back row); Juliet Lee-Franzini of SUNY Stony Brook and team leader Paulo Franzini of Nevis Lab [V1#7].
Photo: Columbia Computer Center Newsletter, V1#7, Aug 1966, Columbiana Archive.

Third Generation Computers (1964-1971)

Although transistors were great deal of improvement over the vacuum tubes, they generated heat and damaged the sensitive areas of the computer. The Integrated Circuit(IC) was invented in 1958 by Jack Kilby. It combined electronic components onto a small silicon disc, made from quartz. More advancement made possible the fittings of even more components on a small chip or a semi conductor. Also in third generation computers, the operating systems allowed the machines to run many different applications. These applications were monitored and coordinated by the computer's memory.


The IBM 360/91

Installation of the IBM 360/91 in the Columbia Computer Center machine room in February or March 1969. Photo: AIS archive.


Fourth Generation (1971-Present)

Fourth Generation computers are the modern day computers. The Size started to go down with the improvement in the integrated circuits. Very Large Scale (VLSI) and Ultra Large scale (ULSI) ensured that millions of components could be fit into a small chip. It reduced the size and price of the computers at the same time increasing power, efficiency and reliability. "The Intel 4004 chip, developed in 1971, took the integrated circuit one step further by locating all the components of a computer (central processing unit, memory, and input and output controls) on a minuscule chip."
Due to the reduction of cost and the availability of the computers power at a small place allowed everyday user to benefit. First, the minicomputers which offered users different applications, most famous of these are the word processors and spreadsheets, which could be used by non-technical users. Video game systems like Atari 2600 generated the interest of general populace in the computers.
In 1981, IBM introduced personal computers for home and office use. "The number of personal computers in use more than doubled from 2 million in 1981 to 5.5 million in 1982. Ten years later, 65 million PCs were being used." Computer size kept getting reduced during the years. It went down from Desktop to laptops to Palmtops. Mackintosh introduced Graphic User Interface in which the users don’t have to type instructions but could use Mouse for the purpose.
The continued improvement allowed the networking of computers for the sharing of data. Local Area Networks (LAN) and Wide Area Network (WAN) were potential benefits, in that they could be implemented in corporations and everybody could share data over it. Soon the internet and World Wide Web appeared on the computer scene and fomented the Hi-Tech revolution of 90's.

Desktop Computer.

Fifth generation computers

Fifth generation computers are mainly future computers. Of course some modern computers also belong to this generation. The aim of these computers is to develop devices that respond to natural language input and are capable of learning and self-organization. In these computers massive numbers of CPUs are used for more efficient performance. Voice recognition is a special feature in these computers. By using superconductors and parallel processing computer geeks are trying to make artificial intelligence a reality. Quantum computing, molecular and nanotechnology will change the face of computers in the coming years.

Fifth generation computer.