Logic 1 and Logic 0 are not simply 5V and 0V or even Vcc and Ground. Within any family of ICs the voltages and currents indicating 1 and 0 cover defined ranges unique to that logic family. The range of voltages allowed for a particular logic level depends on the amount of current flowing into or out of the logic gate inputs or output, the larger the current the output is supplying, the lower the output voltage will be.
Each output will supply a certain amount of current before the output voltage falls too far to be called logic 1, and each gate input will need to be supplied with a certain amount of current to raise the input voltage sufficiently to be recognised as logic 1.
Examples of typical logic levels at inputs and outputs in a range of logic families are illustrated in Fig. 3.3.1. These levels are fairly standard throughout a particular family, although there can be minor differences in these and other parameters, between products from different manufacturers. In addition there are sub families within these families that may have different defined levels. When designing digital circuits, or replacing ICs in critical equipment, it is therefore essential to consult the appropriate manufacturer’s data sheets.
Logic 1 levels for inputs and outputs are shown in red and logic 0 in green. To highlight the fact that true ECL gates, have negative logic levels, these colours have been changed to yellow and blue respectively.
Notice that the logic levels for outputs (left column) and inputs (right column) in all of the families are different. This ensures that provided that the output voltage of a gate is within its defined logic limits for 1 or 0, any compatible gate input connected to that output will recognise the correct 1 or 0 levels. The difference between levels at the output and input in any particular family is called the ‘Noise Margin’.