The document discusses programmable logic controllers (PLCs). It defines PLCs as digital electronic devices that use programmable memory to store instructions to control machines and industrial processes (sentence 1). PLCs were invented to replace sequential relay circuits for machine control and are now widely used for automation in factories, amusement rides, and other industrial applications (sentences 2-3). The document then provides details on the history and development of PLCs, their basic components and functions, programming methods, applications, advantages and disadvantages (sentences 4-15). It concludes by discussing trends in PLC usage, such as their suitability for hazardous environments and the growing market for programmable automation controllers (sentences 16-18).