Sensors allow a PLC to detect the state of a process. Logical sensors can only detect a state that is either true or false. Examples of physical phenomena that are typically detected are listed below.
Recently, the cost of sensors has dropped and they have become commodity items, typically between $50 and $100. They are available in many forms from multiple vendors such as Allen-Bradley, Omron, Hyde Park and Turck. In applications sensors are interchangeable between PLC vendors, but each sensor will have specific interface requirements.