Patent US 4,247,909

A programmable controller of the type used to solve relay ladder diagrams is taught. The programmable controller includes a line solver which has as a part thereof relatively simple low capacity shift register memories for storing interim intercontact node power status information without necessitating the use of a separate large capacity remotely located memory within the programmable controller. Because of the presence of left and right node shift registers as storage devices in the line solver, the line solver may be used to provide an ultimate solution for even a complex rung of a ladder diagram by utilizing only one line solver operation per each entered contact or device. No extra line solver operations are required for intercontact connections nor for transferring information between the line solver and the main memory on an interim solution basis other than to take input information from the main memory and to provide final output information to the main memory.