<Tr> <Td> TTL </Td> <Td> 0 V to 0.8 V </Td> <Td> 2 V to V </Td> <Td> V = 5 V ± 10% </Td> </Tr> <Tr> <Td> ECL </Td> <Td> V to − 1.4 V </Td> <Td> − 1.2 V to 0 V </Td> <Td> V is about − 5.2 V </Td> </Tr> <P> Nearly all digital circuits use a consistent logic level for all internal signals . That level, however, varies from one system to another . Interconnecting any two logic families often required special techniques such as additional pull - up resistors or purpose - built interface circuits known as level shifters . A level shifter connects one digital circuit that uses one logic level to another digital circuit that uses another logic level . Often two level shifters are used, one at each system: A line driver converts from internal logic levels to standard interface line levels; a line receiver converts from interface levels to internal voltage levels . </P> <P> For example, TTL levels are different from those of CMOS . Generally a TTL output does not rise high enough to be reliably recognized as a logic 1 by a CMOS input, especially if it is only connected to a high - input - impedance CMOS input that does not source significant current . This problem was solved by the invention of the 74HCT family of devices that uses CMOS technology but TTL input logic levels . These devices only work with a 5V power supply . </P>

What is the difference between active low and active high logic explain the difference