logic families

Discussion in 'Electronics Resources' started by andy146, Oct 6, 2010.

  1. andy146

    Thread Starter New Member

    Oct 6, 2010
    1. What are the advantages of using one type of logic gates

    2. Can a TTL-NAND Gate be used with negative logic.
  2. Wendy


    Mar 24, 2008
    Logic has evolved, it didn't all spring up at once. Early logic families were RTL (resistor transistor logic) and DTL (diode transistor logic). They used low voltages (3.6V) and didn't have very good noise immunity, which created a whole bunch of problems. I stepped into electronics in time to see the end of RTL.

    TTL uses 5V ±.25V and has much better noise immunity, is fairly fast, but is very sensitive about its power supply voltage, and is a fairly power hungry. Because so many chips were made TTL is still fairly popular for schools and hobbyists.

    ECL came out shortly after for very high speed applications, and is one of the examples of negative logic. Conventional logic has caught up with ECL speeds, but ECL also keeps getting faster. I have seen examples of 40Ghz goggle rates, and much faster has been developed since. As a general logic family it also makes a decent space heater. If you look at ECL internal designs they resemble differential amps, and are a prime example on unsaturated logic. Their on/off voltages are -1.4V and -.6V, and are still in common use for extreme speeds.

    CMOS is probably the best for general usage, it has very loose voltage requirements (meaning you can power it up with a large variety of sources), extremely low current needs, and excellent noise immunity. It's speed has crept up nicely over time, which is where we are currently at.

    When I was getting into digital (before I went to school) I thought TTL was negative logic, but the gates kept being wrong! An AND gate is a OR gate if you look at it with the voltages reversed, but I was being stupid at the time.