Remote terminal units (RTU) collect data automatically and connect directly to sensors, meters, loggers or process equipment. They serve as slave units to supervisory controllers or supervisory control and data acquisition (SCADA) masters. Remote terminal units are located near the monitored process and transfer data to the controller unit on command. They often include integral software, data logging capabilities, a real-time clock (RTC) or totalizer, and a battery backup. Intrinsically safe remote terminal units are designed to operate safely in hazardous environments. Devices with weather tight enclosures are designed to prevent the ingress of moisture, dust or other environmental contaminants. Closed loop systems use proportional, integral and derivative (PID) control; proportional and integral (PI) control; proportional and derivative (PD) control; or proportional (P) control. Redundant RTUs are complete remote terminal units that contain all of the transceivers, encoders, and processors needed for proper functioning in the event that a primary RTU stops working.