Editing Remote terminal unit (section)

===Communications===
An RTU may be interfaced to multiple master stations and IEDs ([[Intelligent electronic device|Intelligent Electronic Devices]]) with different communication protocols (usually serial ([[RS-232]], [[RS-485]], [[RS-422]]) or [[Ethernet]]). An RTU may support standard protocols ([[Modbus]], [[IEC 60870-5]]-101/103/104, [[DNP3]], [[IEC 60870-6]]-ICCP, [[IEC 61850]] etc.) to interface any third party software.

Data transfer may be initiated from either end using various techniques to ensure synchronization with minimal data traffic. The master may poll its subordinate unit (Master to RTU or RTU to IED) for changes of data on a periodic basis. Analog value changes will usually be reported only on changes outside a set limit from the last transmitted value. Digital (status) values observe a similar technique and only transmit groups (bytes) when one included point (bit) changes.  Another method used is where a subordinate unit initiates an update of data upon a predetermined change in analog or digital data. Complete data transmission must be performed periodically, with either method, to ensure full synchronization and eliminate stale data. Most communication protocols support both methods, programmable by the installer.

Multiple RTUs or IEDs may share a communications line, in a [[Multidrop bus|multi-drop scheme]], as units are addressed uniquely and only respond to their own polls and commands.

====IED  communications====
IED communications transfer data between the RTU and an IED (Intelligent Electronic Devices) . This can eliminate the need for many hardware status inputs, analog inputs, and relay outputs in the RTU. Communications are accomplished by copper or [[Fiber-optic communication|fibre optics lines]].

====Master communications====
Master communications usually occur between an RTU and a larger control system or a data collection system (incorporated into a larger system). Data may be moved using a copper, fibre optic or radio frequency communication system.