IEC 60870-5 is one among the six parts of IEC 60870 standard which defines the systems used for telecontrol especially SCADA systems in power system automation and electrical engineering. Part 5 describes the communication profile for sending telecontrol messages between two systems having permanent directly connected data circuits. Telecontrol means transmitting supervisory data and data acquisition requests for controlling power transmission grids. This part contains seven documents defining the standard for telecontrol, teleprotection and associated telecommunications for electrical power systems. IEC 60870-5-101 and 60870-5-104 are the protocols satisfying these standards and is known as IEC 101 and IEC 104 respectively.
The requirement of standard in the field of power system automation is interoperability between different vendor devices. Since big control networks are associated with the power systems, it is not possible to accomplish the entire system with a single vendor’s products. Standardization simplifies the process of commissioning, maintenance and extension works.
IEC 101 &IEC 104
IEC 101 provides a communication between a central telecontrol station (master/controlling station)
and telecontrol outstations (slave/controlling station) which uses standard asynchronous serial channel. And it supports physical layer in point-to-point, multidropped, star .
IEC 104 is the extension of IEC 101 which preserves the application layer and made changes in lower four OSI layers. IEC 104 uses the open TCP/IP interface which provides network connectivity.
IEC 101 uses asynchronous serial communication interface and allows two alternative transmission procedures between telecontrol station (master/controlling station) and telecontrol outstations (slave/controlling station)
Master station controls the data traffic in the network. Master requests data and slave responds to this request
In this mode of transmission both the master station and slave station act simultaneously but the configuration is restricted to point-to –point and multidropped.
IEC 104 combines the application layer of IEC 101 and standard transfer functions of TCP/IP which provides connectivity to WAN. The main advantage of 104 over 101 is its network connectivity.
IEC 60870-5 contains distinct monitoring and control directions with overall system having a hierarchical structure involving centralized control station. In this protocol every station is either a controlled station or a controlling station.
IEC 101/104 communication is between controlled and controlling stations. Controlled station is monitored or commanded by a station controller (RTU, PC with Software based controllers) and also called Remote station, RTU, 101-Slave, 104-Server etc. Controlling stations will be mostly a PC with SCADA system or may be RTUs which control all controlled stations. Then Monitor direction is from controlling station to controlled station and Control direction is vice versa.
Application Data Objects
IEC 60870-5 has information on a set of information objects that are suited to both general SCADA applications and electrical system applications in particular. Each different type of data has a unique type identification number. Only one type of data is included in any one Application Service Data Unit (ASDU). The type is the first field in the ASDU. The information object types are grouped by direction (monitoring or control direction) and by the type of information (process info, system info, parameter, file transfer).
Addressing in both link level and application level is there, link address is the identification number of device. And in application level ASDU address for the identification of end station.
Link address can be one or two bit length having range 1-255 and 1-65535 respectively. Typical values are 1 for IEC 101 and 2 for IEC104. FF and FFFF are the broadcast address for mapping all the stations at the link level.
Each device on the communication network has a Common Address of ASDU (COA or ASDU address). The common address of the ASDU combined with the information object address contained within the data itself combine to make the unique address for each data element.
Present systems are connected through IEC 104 only, to convert IEC 101 to IEC 104 converter RTUs are used. The main advantage of IEC 104 over IEC 101 is the network connectivity. Following is the architecture of such system.