The IEC 60870-5 standards address the basic goals of telecontrol systems and their particular environmental conditions. IEC 60870-5 does not define one particular protocol profile; but rather like EPRI/UCA, it specifies a number of frame formats and services that may be provided at different layers.
IEC 60870-5 is based on a three-layer Enhanced Performance Architecture (EPA) reference model for efficient implementation within RTUs, meters, relays, and other IEDs. Additionally, IEC 60870-5 defines basic application functionality for a user layer, which is situated between the OSI Application Layer and the application program.
This user layer adds interoperability for such functions as clock synchronization and file transfers. The following descriptions provide the basic scope of each of the five documents in the base IEC 60870-5 telecontrol transmission protocol specification set.
Standard profiles are necessary for uniform application of the IEC 60870-5 standards. Such profiles have been and are being created. The T101 profile is described in detail following the description of the applicable standards.
IEC 60870-5-1 (1990) specifies the basic requirements for services to be provided by the data link and physical layers for telecontrol applications.
In particular, it specifies standards on coding, formatting, and synchronizing data frames of variable and fixed lengths that meet specified data integrity requirements. ¾ IEC-60870-5-2 (1992) offers a selection of link transmission procedures using a control field and optional address field; the address field is optional because some point-to-point topologies do not require either source or destination addressing.
IEC 60870-5-3 (1992) specifies rules for structuring application data units in transmission frames of telecontrol systems. These rules are presented as generic standards that may be used to support a great variety of present and future telecontrol applications. This section of IEC 60870-5 describes the general structure of application data, and describes basic rules to specify application data units without specifying details about information fields and their contents.
IEC 60870-5-4 (1993) provides rules for defining information data elements and a common set of information elements, particularly digital and analog process variables that are frequently used in telecontrol applications.
IEC 60870-5-5 (1995) defines basic application functions that perform standard procedures for telecontrol systems, which are procedures that reside beyond Layer 7 (application layer) of the ISO reference model.
These utilize standard services of the application layer. The specifications in IEC 60870-5-5 (1995) serve as basic standards for application profiles that are then created in detail for specific telecontrol tasks.
Each application profile will use a specific selection of the defined functions. Any basic application functions not found in a standards document but necessary for defining certain telecontrol applications should be specified within the profile. Examples of such telecontrol functions include station initialization, cycle data transmission, data acquisition by polling, clock synchronization, and station configuration.