Transistor-Transistor Logic (TTL)

Transistor-Transistor Logic (TTL)

Transistor-Transistor Logic, or TTL, refers to the technology for designing and fabricating digital integrated circuits that employ logic gates consisting primarily of bipolar transistors. It overcomes the main problem associated with DTL, i.e., lack of speed.

The input to a TTL circuit is always through the emitter(s) of the input transistor, which exhibits a low input resistance. The base of the input transistor, on the other hand, is connected to the Vcc line, which causes the input transistor to pass a current of about 1.6 mA when the input voltage to the emitter(s) is logic '0', i.e., near ground. Letting a TTL input 'float' (left unconnected) will usually make it go to logic '1', but such a state is vulnerable to stray signals, which is why it is good practice to connect TTL inputs to Vcc using 1 kohm pull-up resistors.

The most basic TTL circuit has a single output transistor configured as an inverter with its emitter grounded and its collector tied to Vcc with a pull-up resistor, and with the output taken from its collector. Most TTL circuits, however, use a totem pole output circuit, which replaces the pull-up resistor with a Vcc-side transistor sitting on top of the GND-side output transistor. The emitter of the Vcc-side transistor (whose collector is tied to Vcc) is connected to the collector of the GND-side transistor (whose emitter is grounded) by a diode. The output is taken from the collector of the GND-side transistor. Figure 1 shows a basic 2-input TTL NAND gate with a totem-pole output.

Figure 1. A 2-input TTL NAND Gate with a Totem Pole Output Stage

In the TTL NAND gate of Figure 1, applying a logic '1' input voltage to both emitter inputs of T1 reverse-biases both base-emitter junctions, causing current to flow through R1 into the base of T2, which is driven into saturation. When T2 starts conducting, the stored base charge of T3 dissipates through the T2 collector, driving T3 into cut-off. On the other hand, current flows into the base of T4, causing it to saturate and pull down the output voltage Vo to logic '0', or near ground. Also, since T3 is in cut-off, no current will flow from Vcc to the output, keeping it at logic '0'. Note that T2 always provides complementary inputs to the bases of T3 and T4, such that T3 and T4 always operate in opposite regions, except during momentary transition between regions.

On the other hand, applying a logic '0' input voltage to at least one emitter input of T1 will forward-bias the corresponding base-emitter junction, causing current to flow out of that emitter. This causes the stored base charge of T2 to discharge through T1, driving T2 into-cut-off. Now that T2 is in cut-off, current from Vcc will be diverted to the base of T3 through R3, causing T3 to saturate. On the other hand, the base of T4 will be deprived of current, causing T to go into cut-off. With T4 in cut-off and T3 in saturation, the output Vo is pulled up to logic '1', or closer to Vcc.

Outputs of different TTL gates that employ the totem-pole configuration must not be connected together since differences in their output logic will cause large currents to flow from the logic '1' output to the logic '0' output, destroying both output stages. The output of a typical TTL gate under normal operation can sink currents of up to 16 mA.

The noise margin of a logic gate for logic level '0', Δ0, is defined as the difference between the maximum input voltage that it will recognize as a '0' (Vil) and the maximum voltage that may be applied to it as a '0' (Vol of the gate driving it). For logic level '1', the noise margin Δ1 is the difference between the minimum input voltage that may be applied to it as a '1' (Voh of the gate driving it) and the minimum input voltage that it will recognize as a '1' (Vih). Mathematically, Δ0 = Vil-Vol and Δ1 = Voh-Vih. Any noise that causes a noise margin to be overcome will result in a '0' being erroneously read as a '1' or vice versa. In other words, noise margin is a measure of the immunity of a gate from reading an input logic level incorrectly. For TTL, Vil = 0.8V and Vol = 0.4V, so Δ0 = 0.4V, and Voh = 2.4V and Vih = 2.0 V, so Δ1 = 0.4V. These noise margins are not as good as the noise margins exhibited by DTL.

As mentioned earlier, TTL has a much higher speed than DTL. This is due to the fact that when the output transistor (T4 in Figure 1) is turned off, there is a path for the stored charge in its base to dissipate through, allowing it to reach cut-off faster than a DTL output transistor. At the same time, the equivalent capacitance of the output is charged from Vcc through T3 and the output diode, allowing the output voltage to rise more quickly to logic '1' than in a DTL output wherein the output capacitance is charged through a resistor.

The commercial names of digital IC's that employ TTL start with '74', e.g., 7400, 74244, etc. Most TTL devices nowadays, however, are named '74LSXXX', with the 'LS' standing for 'low power Schottky'. Low power schottky TTL devices employ a Schottly diode, which is used to limit the voltage between the collector and the base of a transistor, making it possible to design TTL gates that use significantly less power to operate while allowing higher switching speeds.

David Blaine's Secrets Revealed:::::Voodoo Ash

However i personally salute this great artist in true spirit.

David Blaine's Secrets Revealed

1. Voodoo Ash

Effect: A name is written on a piece of paper on a notepad by the spectator, the paper is torn off unseen by the magician, crumpled up and placed in an ashtray. It is then set alight.

The ashes are then rubbed on the magicians arm and the name of the person written on the paper mysteriously appears.

Preparation: You will need a small notepad, pen, ashtray, lighter and a small piece of soap or wax.
Method: The spectator is asked to write the name of a person or favourite anything of theirs on a notepad.

Then to tear of the sheet of notepaper, crumple it up and place it in the ashtray, where the magician then sets the paper alight.

The magician takes the notepad of the spectator and should be able to see the imprint of the name written on the paper above in the next sheet of the notepad.

Looking at the pad should not be made obvious and some misdirection carried out while he carefully takes the small piece of wax or soap and writes the name on the imprint on his arm or hand (this action should not be visible to anyone ).

The magician can then take some ash from the ashtray and rub it over the wax where it will stick and create an impression of the spectators word to their amazement.
This is a great trick but depends on the magician creating an atmosphere with an entertaining routine and patter about black magic to enhance the effect and create misdirection to allow him time to pull off the trick.

thanks to rahul

SPEED UP UR ACROBAT READER (ALMOST LIKE NOTEPAD)

Do u get irritated when acrobat reader takes 5/10 seconds to load when you want to open a pdf document. There is a way to speed up the loading.

1. Go to the installation folder of acrobat reader
(C:\program files\adobe\acrobat\reader\.. whatever)

2. Move all the files and folders from the "plugins" directory to the "Optional" directory. (I repeat.. cut and paste the files NOT copy & paste).

Also make sure that acrobat reader is not open else it will lock the files and not allow you to move the files).

Now your acrobat reader will load very fast
and almost as good as notepad..


Thanks to RAHUL

one4all: End of an era: Left and Indian political colossus Jyoti Basu passes away

one4all: End of an era: Left and Indian political colossus Jyoti Basu passes away

End of an era: Left and Indian political colossus Jyoti Basu passes away

Veteran Marxist leader and former West Bengal Chief Minister Jyoti Basu died at the AMRI Hospital of multiple-organ failure at 11.47 a.m. on Sunday. He was 95.

"Jyoti Basu is no more," said party state secretary Biman Bose.

Basu was admitted to the hospital on January 1 with acute respiratory failure bordering on pneumonia. He was also undergoing treatment at the hospital for age-related ailments.

Born on July 8, 1914, Jyoti Kiran Basu's name was shortened by his father to Jyoti.

In 1996, Basu seemed all set to become the first Communist Prime Minister of India. However, the CPI (M) Politburo decided not to participate in the government, a decision that Basu later termed as a historic blunder.

Basu was elected to the Bengal Legislative Assembly in 1946, contesting the Railway constituency. He served as Leader of Opposition for a long time when Dr. Bidhan Chandra Roy was the Chief Minister of West Bengal.

Basu's admirable eloquence both as an M.L.A and the Leader of Opposition drew the attention of Dr. B. C. Roy, who had a strong affection for this young leader though his stand was completely against the policies of the then State Government.

He led several agitations against the State Government and earned an enviable reputation as a politician, particularly among students and youth.

Beside organising the movements of railway labourers, he led a historical movement of teachers who were protesting against their meagre salary.

Between 1967 and 1969, Basu was the Deputy Chief Minister of West Bengal in the United Front governments. He worked under Chief Minister Ajay Mukhopadhay.

When the Congress returned to power in West Bengal in 1972 under Siddhartha Shankar Ray, Basu faced defeat in the Baranagar Assembly Constituency and complained about unprecedented rigging. His Party CPI (M), decided to boycott the Assembly till a fresh election was conducted in 1977.

From June 21, 1977 to November 6, 2000, Basu served as the Chief Minister of West Bengal of various Left Front governments. In 1996, Basu almost became of Prime Minister of India.

Basu resigned as Chief Minister of West Bengal in 2000 for health reasons, and was succeeded by fellow CPI (M) politician Buddhadeb Bhattacharya.

The 18th Congress of CPI (M), held in Delhi in 2005, re-elected Basu to its Politburo, although he had asked to be allowed to retire from it. On September 13, 2006, Basu entreated the CPI (M) to allow his retirement due to his age, but the resign was turned down.

At the 19th Congress in early April 2008, Basu was not included in the Politburo, although he remained a member of the Central Committee and was designated as Special Invitee to the Politburo. (ANI)