The IEC 60870-5-101 protocol or T101, introduced in November 1995 in its first version, was developed exclusively for electrical systems and contains a hierarchical structure, in order to support, at link level, a balanced communication and only point-to-point, to avoid collisions, and it can be used to configure unsolicited messages. It also supports non-balanced communication or multidrop, where only the master can start communicating. As for the transport layer, data is transmitted in bit-serial mode on networks with low bandwidth.
In this protocol, the process of acquiring data is performed by master scans on slave devices, and those scans occur in typical one-second periods. First there is an integrity check, and then only information that changed are sent, called exceptions, such as when measurements exceeded the dead band or changes of a device's status.
The structure of frames or data packets, as shown on the next figure, is more complex than the one used in Modbus protocol. These telegrams contain characters for message's start and end, the total size of the packet, control, and information address, in addition to markers indicating whether data from Classes 1 and 2, digital and analog information respectively, is available.
Structure of a message
Each frame can carry a single ASDU (Application Service Data Unit), located in the application layer, where digital and analog information are formatted according to the needs of an electrical process, as shown on the next table.
Structure of an ASDU
Object |
Information |
Description |
Data Identification Unit |
Types of ID |
Defines a data type that contains the specific format of data objects, that is, simple point, double point, or measurement |
Variable for Structure Qualification |
Indicates whether this type of information contains multiple objects or not |
|
Cause of Transmission |
Displays the cause for spontaneous or cyclical transmissions |
|
ASDU's Common Address |
Indicates separated segments and the address in a device |
|
Data Object 1 |
Data Object's Address |
Provides the address of an element of a data object |
Data Elements |
Contains details about an information element, according to its type |
|
Timestamp |
Used for real-time monitoring of sequences of events |
|
Data Object N |
Data Object's Address |
-- |
Data Elements |
-- |
|
Timestamp |
-- |
Because these are real-time applications, the size of messages is usually 250 bytes, which is considered small. Another important aspect of the Standard is interoperability, which is the convenience of communication among devices from different manufacturers, allowing interconnected IEDs to understand the syntax and semantics received from another device with the same technology, eliminating the use of converters. Therefore, if two devices have the same protocol but different interoperability, they may generate several communication errors or they cannot even complete a connection.
The IEC 60870-5-104 protocol is an extension to IEC 60870-5-101 protocol, with changes to transport, network, and physical layers to adapt to TCP/IP networks. It users the default TCP/IP interface for LAN (Local Area Network) networks and routers for WAN (Wide Area Network) networks.