1. A programmable controller comprising a controller having a data bus, a microprocessor coupled to said data bus, a first ROM coupled to said data bus for storing a system program, a second ROM coupled to said data bus for storing a user program, a first RAM coupled to said data bus for storing data, said second RAM being removably connected with a socket to said data bus, a display unit having front panels for displaying measured and set value operating outputs, means for specifying the operating mode and means for changing the set value and manual operation of the output, a programmer removably connected to said controller having a second RAM for storing said user program, a third ROM for storing a control program, a keyboard for use in programming, a programmer display for use in programming, and a changeover switch for changing said programmer from a program mode to a test run mode, whereby said programmer, when connected to said controller, may be switched from a program mode for writing the user program in the second RAM using said keyboard to the test run mode for said controller thereby executing the user program written in said second RAM and checking such execution using said programmer display whereas said controller executes the user program stored in said second ROM when said programmer is disconnected from, said controller, said system program of said controller being provided with a basic function program equivalent to the function performed by a cascade controller having two process controllers connected in series, a selective control function program equivalent to the function performed by a selective type controller having a plurality of process controllers connected in parallel, whereby a control operational function of the controller is determined by selecting a control function program by a command from a user program.