A typical RTU has a network interface (usually either Ethernet, proprietary, or both), a simple processor, some environmental sensors, some override switches, and a bus which it uses to communicate with devices and/or interface boards. This bus is sometimes called a device bus or a field bus. Standards include the ISO Controller Area Network (ISO 11898), MODBUS, and others. Many vendors provide proprietary buses for their equipment; examples include Allen-Bradley's Data Highway and HSQ Technology's MISERnet. Sometimes a device or field bus standard can be used to interconnect RTUs and host systems as well as field devices and RTUs.
Interface boards come in analog and digital flavors, and are typically designed for input only, output only, or both. These main types of interface boards are often abbreviated as "DI" (digital input), "AO" (analog output), and so forth. These categories are further subdivided based on the range of inputs, the amount of protection against voltage surges, and the amount of intelligence on the interface board. Sometimes an RTU or PLC has integral interfaces used to allow the deployment of only a RTU (without a bus and interface modules) to monitor or control just a few devices.
The interface boards are in turn connected to physical objects using wires. A typical application found in most SCADA implementations is to connect high current capacity relays to a digital output (or "DO") board to switch power on and off to devices in the field. The DO board switches voltage to the coil in the relay, which closes the high current contacts, which completes the power circuit to the device. Analog inputs are usually 24V with a current range between 4-20 milliamps; the RTU or host system then translates this into the appropriate units such as gallons of water left or temperature before presenting the data to the user via the HMI or MMI.
Modern RTUs are usually capable of executing simple programs autonomously without involving the host computers of the DCS or SCADA system to simplify deployment, and to provide redundancy for safety reasons. A RTU in a modern water management system will typically have code to modify its behavior when physical override switches on the RTU are toggled during maintenance by maintenance personnel. This is done for safety reasons; a miscommunication between the system operators and the maintenance personnel could cause system operators to mistakenly enable power to a water pump when it is being replaced, for example.
RTUs and PLCs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.