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To know about the advantages of TTL logic family, one should have a basic idea about RTL, DTL etc. Diode logic (DL) uses diodes to implement logical functions like AND and OR. But the disadvantage is that it can not perform NOT operation. As AND and OR are not complete functions by themselves, they can not perform several logic functions without NOT. Hence, there was a need for some device which can perform a NOT function as diodes can not. That device is a transistor. Then came the DTL which uses a transistor along with diodes. As a transistor can act as an inverter, NAND (NOT-AND) & NOR (NOT-OR) operations can be performed. But this logic uses several diodes which will slow down its operation. Due to the delay offered by them, the logic levels may sometimes change i. e. 0 t0 1 or 1 to 0. Then came TTL. This logic uses a multi emitter transistor, a transistor with many emitter terminals. As every emitter is nothing but a diode, this logic eliminates the use of all diodes. This is the major advantage. As transistor becomes ON and OFF much rapidly than a diode, switching time will be faster.
TTL, or Transistor-transistor logic replaced resistor-transistor logic, and used much less power. The TTL family is very fast and reliable, and newer faster, less power-consuming, etc. types are always being developed.
ECL is Emitter Coupled Logic. Compare with TTL which turned the transistors completely on (saturated) or off for the two states. Changing from the off state or the saturation state requires significant time. In ECL the transistors are biased so they operate near the middle of their range and coduct a little more or a little less for their states. ECL is faster than TTL, very power hungry in comparison, and were used in many early mini and main frame computers for speed. I believe that CMOS has eclipsed ECL in speed, and certainly consusmes much less power.
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