This chapter includes all research on the current state of the art. Section 2.2 details the history of refrigeration, the refrigeration cycle and the different types of refrigeration equipment available and most commonly used. Section 2.3 covers refrigeration and freezing of various products. Section 2.4 provides information on programmable logic controllers (PLCs). This section includes a brief history of PLCs, how they work, the different programming languages ​​used and some commonly used instructions. Section 2.5 describes different types of signals used in control, including digital, analog, serial, fieldbus, and SCADA.2.2 Refrigeration Processes2.2.1 BackgroundFor most of history, ice and snow, packaged in cellars insulated with wood or straw, were used for refrigeration. It was not until 1755 that the first artificial refrigeration system was developed by William Cullen (Koelet, 1992). Throughout the 1800s, many mechanical refrigeration systems were designed and developed, and the vapor compression cycle most commonly used today dates back to this time. There are different refrigeration cycles including gas cycle, vapor absorption cycle and vapor compression cycle. The gas cycle uses a refrigerant, often air, which does not change state, but remains in gas form throughout the cycle. The vapor absorption cycle, which preceded the vapor compression cycle, is a heat-operated cycle. It requires little electricity, but a lot of thermal energy, and is generally only used where waste heat is abundant (Ameen, 2006). The vapor compression cycle is favored over all others in domestic and industrial installations because it is the most efficient system available. It works by pumping a refrigerant, which changes...... middle of paper ...... two devices, which were connected together using an RS-232 cable, to communicate. A fieldbus, which transfers data serially, reduces the number of cables needed over long distances. The transferred data is available on all nodes at the same time. Fieldbus is a flexible system for easily connecting new units to the bus.2.5.5 SCADASCADA (supervision control and data acquisition) is a graphical application used to monitor and control industrial systems from a centralized location. It receives real-time data from the PLC and can be used to enter values ​​and enable/disable outputs. It can provide alarms to let the user know when a process has completed or if a fault has occurred. SCADA systems often use an HMI (human machine interface) to allow the user to control valves, pumps, etc. to which the HMI is connected (Inductive Automation, 2014).