A turing machine for the 21st century

The Turing Machine, originally proposed in 1936, is a primitive model which nevertheless embodies concepts that form foundations for modern computer design.¹ Described in this paper is an implementation of a Turing Machine core that is useful as a vehicle for teaching finite state machines. It is adaptable to many levels of state machine design, from introductory digital circuit implementations with gates and flip-flops through industrial Programmable Logic Controllers (PLCs) to advanced designs using Programmable Logic Devices (PLDs) and other high-end components. The Turing Machine excels as a vehicle for learning finite state machine design due to its simple structure and easily-understood operation. The particular Turing Machine core described here is especially useful due to the visible display of the machine operation produced on a standard oscilloscope to portray how the user's design works. A basic Turing Machine consists of a theoretically infinite tape on which information is stored in cells, and a head that moves back and forth across the tape reading and modifying information found there. By controlling the head movement and what it writes to the cells in response to information present on the tape, various computing tasks can be demonstrated. In the Turing Machine core described here, there are two parallel tapes each holding sixteen cells. In each cell is stored one of four symbols (O, I, - , or ·) for use in the computation. The single head reads information at corresponding cells on both tapes, writes updated information to both cells, and then moves either left or right along the tapes, in response to instructions from the controlling state machine. The unique characteristic of this design is the display produced on a standard oscilloscope to record operation of the Turing Machine. The oscilloscope display shows the contents of the two tapes as two parallel strings of sixteen symbols each, chosen from O, I, -, or •, with one of those symbols in each string raised on the screen to indicate the head position. The display is similar to that produced by other instruments designed by the author and reported in earlier ASEE papers. As the Turing Machine operates, the head position moves back and forth, and the tape contents are updated as controlled by the state machine being used.