ECL logic, or emitter coupled logic is a family of very high speed logic. with ECL logic, a transistor is never completely on or off, meaning the transistor is never saturated, making it very fast. It also makes it weird and quirky, interesting enough to have a look at.
First of all, ECL logic generally works on -5V instead of 5V, though later it was also available using a more normal 5V power. Second of all, the logic level is not 0V and -5V, but roughly -0.8V and -1.6V. Third, most ECL logic has 2 outputs, a normal and an inverting one. This can be used to make logic design easier or to have an differential pair output for noise immunity.
ECL was used in multiple well known designs, for example the Cray-1 supercomputer used ECL.
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