1. A controller for process control comprising: a programmer including memory means for encoding and storing a plurality of computing elements which may be combined to form programs necessary for determination of functions of the controller, key board means having plural keys for reading those computing elements stored in said memory means required for a selected program in response to depression of selected keys labeled with instrumentation terms identifying the program function, and means for producing a computing program for a desired function from said read out computing elements in response to depression of said keys of the key board; program memory means removably connected to said programmer for storing the computing programs produced by said programmer, said program memory means comprising a physically unpluggable programmable memory; and an arithmetic processing portion having register means for storing input signals, arithmetic means selectively connectable to said programmable memory for interpreting the codes of the computing programs stored in said program memory means and for executing an interpreted program on the basis of the data stored in said register means, and output means for outputting data obtained as a result of executing the computing program by said arithmetic means, said arithmetic processing portion being at no time electrically connected to said programmer.