Four-state flip-flop

Flip-flops (US English) or bistables (British English) are well-known and widely used building blocks performing control, register, memory and toggle functions in logic circuits. The most popular ones are probably the CMOS 4013 (dual D type), the 4027 (dual J-K type), and the TTL 7474 (dual-D type). The latter also comes in LS-TIL, HC and HCT versions. Although D, J-K and S-R bistables have slightly different truth tables, they all spare a common characteristic: they have two stable states.