ON 20030119@11:10:07 PM at page:
http://techref.massmind.org/techref/logic/fam.htm
JMN-EFP-786 James Newton Says
http://techref.massmind.org/techref/logic/TTLs.htm
TTL (Transistor-Transistor Logic)

ON 20030119@11:11:15 PM at page:
http://techref.massmind.org/techref/logic/TTLs.htm
JMN-EFP-786 James Newton Says

(TTL) A common semiconductor technology for building discrete digital logic integrated circuits. It originated from Texas Instruments in 1965. <BR>
<BR>


ON 20030119@11:23:16 PM at page:
http://techref.massmind.org/techref/logic/TTLs.htm
JMN-EFP-786 James Newton Says

As integrated circuits (ICs) grew "real estate" became more important. Since it takes as much room on the IC chip to construct a diode as it does to construct a transistor, the designers looked for a way to avoid large numbers of input diodes.
<PRE>
     +Vcc
      /       +Vcc
      \        /
      /        \
      \        /
A -|<-+        \
      |        +--- Output = not A * B * C * D
B -|<-+      |/
      +-->|--|
C -|<-+      |\
      |        Y
D -|<-+       GND
</PRE>

They realized that the opposed diodes are the same as two junctions of a transistor. In fact, an inverter would have a single input diode, and we could replace the two opposed diodes with an NPN transistor to do the same job.
<P>
In fact, this works quite nicely. The figure to the left shows the resulting inverter.
<P>
In addition, we can add multiple emitters to the input transistor without greatly increasing the amount of space needed on the chip. This allows us to construct a multiple-input gate in almost the same space as an inverter. The resulting savings in real estate translates to a significant savings in manufacturing costs, which in turn reduces the cost to the end user of the device.
<P>
One problem shared by all logic gates with a single output transistor and a pull-up collector resistor is switching speed. The transistor actively pulls the output down to logic 0, but the resistor is not active in pulling the output up to logic 1. Due to inevitable factors such as circuit capacitances and a characteristic of bipolar transistors called "charge storage," it will take a certain amount of time for the transistor to turn completely off and the output to rise to a logic 1 level. This limits the frequency at which the gate can operate.
<P>
The designers of commercial TTL IC gates reduced that problem by modifying the output circuit. The result was the "totem pole" output circuit used in most of the 7400/5400 series TTL ICs. The final circuit used in most standard commercial TTL ICs is shown to the right. The number of inputs may vary — a commercial IC package might have six inverters, four 2-input gates, three 3-input gates, or two 4-input gates. An 8-input gate in one package is also available. But in each case, the circuit structure remains the same.



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