Advanced considerations of IEC 60870-5

The chapter presents the features of the International Electrotechnical Commission (IEC) 60870-5 protocol. The chapter describes the application level functions, interoperability, provisions, and network operations of IEC 60870-5. Command transmission is described by the standard, and there are two standard command procedures—(1) direct command and (2) select and execute command. Direct commands cause immediate changes, provided that there are no logical forces preventing the operation of the particular outputs. Select and execute command sequences may be used for greater security against inadvertent error, both by human operators and by telecontrol system error. Application functions are implemented by the user process layer of both controlling and controlled stations. These functions are specified in detail by IEC 60870-5-101, which largely refers to and makes selections from IEC 60870-5-5. The main functions include station initialization, data acquisition, general interrogation, and cyclic data transmission. Initialization is required when a station is first powered up or after a reset. Its purpose is to ensure the orderly commencement of monitoring and control operations. The procedure followed is basically to reset the link layer, reestablish link communications, and then commence application level services. The link layer services are responsible for establishing the link, and they proceed to do this immediately after being powered up or reset. General interrogation occurs when the controlling station issues an interrogation command C_IC to the controlled station. On receiving the interrogation command, the controlled station transmits its process information to the controlling station. Because of the early use of the IEC 60870-5-101 (or T101) protocol, there has been an increasing level of interest to use standard network profiles to carry protocol messages. This has occurred no doubt with the ever increasing penetration and usage of local and wide area networks, which may offer a ready-made communications infrastructure that is attractive for supervisory control and data acquisition (SCADA) because of the savings that can be achieved over installing separate communications infrastructure.