Abstract: The theory of differential current switches which applies to the designof multivalued ECL circuits is introduced. In this theory, the switching state of differential transisror pair and signal in ECL circuits are described by switching variable and quaternary signal variable, respectivcly. The connection opemtions bctween the two kinds of variables are introduced to describe the action process between switching element and signal in the circuits.Based on this theory, two kinds of interface circuits--2-4 encoder and 4-2 decoder are de- signed. The computer simulation for the designed circuits by using SPICE program confirm that both circuits have correct logic functions, desired DC transfer characteristics and transi- ent characteristics. These interface circuits are compatible with binary circuits in the integra- ted process, the power supply equipment, the logic stage and the transient characteristic. Therefore, they can be used as input-output interface of the existing binary ECL integrated circuits so as to decrease the number of pins of a chip and the connections between chips.