Optically switchable directional couplers can function as logically complete building blocks for constructing all-optical computational engines. Logically, such devices operate as all-optical five-terminal gates, where the two output signals are logical functions of two throughgoing input signals and a control input signal. Such an optically controlled exchange element is a promising functional unit for constructing general-purpose digital optical logic circuits. The implementation of such all-optical five-terminal gates using Ti:LiNbO3 electro-optic directional couplers is described. These optical logic gates utilize mature technology developed for the telecommunications industry, facilitating construction of robust optical logic hardware. Single-mode optical fiber is used for all logical interconnections. Effectively dc-coupled control circuitry converts optical pulses into electrical pulses capable of switching the transfer state of low-voltage lumped electrode directional couplers. The use of such Ti:LiNbO3 five-terminal optical logical gates in the construction of various simple circuits such as oscillators and divide-by-N circuits is shown. Such circuits demonstrate many of the issues arising in the construction of all- optical digital computing systems, and are fundamental subsections of an all-optical bit serial computer design currently under construction.