A Single unit Programmable Logic Controller (PLC) operating simultaneously till either the random inspection period or the random extended available period is over whichever occurs first after the failure of the online unit is considered in this paper. Postulating the operative phase, partial system failure phase, repair phase, and replacement phase of that online unit as states of a Markov Renewal process (MRP) in a time cycle reliability indices and availability characteristics were obtained and it has been shown that profit will be constant in the context of increase in cost per unit time of inspection, repair and replacement and numerically illustrated and presented in a table.