IEC60870-5-104

'High level' interface:

This interface is a so-called ‘single-block solution’. All functions are encapsulated in one PLC block. The block implements the most important services and functions. This implementation is sufficient for over 90% of applications.

Pro: Very little PLC programming work is required in order to create an application; all services, such as general query, counter query, time synchronisation, command execution, spontaneous data transmission etc. are already implemented in the block and are executed automatically; the mapping of the IEC<->PLC process data and that of the data points is configured via function calls; the PLC programmer does not need to be very well acquainted with the protocol standard;

Contra: The PLC application has only a small influence on the execution of the protocol; no influence on the execution of the services – these are automatically implemented internally; time stamps are automatically generated by the block and cannot be changed (handed over by externals); only the direct command execution, for example, is possible; poorer performance if there are many data points.

This interface is recommended if you:

• are not familiar with the protocol standard;
• are implementing a simple application with few data points (<1000);
• are not placing any great performance demands on the application;
• are not sending any special command execution such as Select/Execute or data + time stamp from external devices;
• do not require any functions that are not supported according to the compatibility list;

'Low level' interface

This interface starts lower down on the protocol stack and enables the sending and/or receipt of individual frames (ASDUs). This interface is placed inside the protocol structure above the transport layer (4) and implements already the APCI functions (protocol control information of the application layer, see lower table ). Application functions like for example the general request and count request are not implemented in the interface, but the user can implement them by himself.

Pro: Very flexible; all properties in the ASDU frame can be changed (e.g. an own time stamp, Select/Execute or a special command execution etc.); high performance can be achieved because only the necessary services are implemented; high performance if there are many data points;

Contra: Larger amount of programming work; all services, such as general query, counter query, time synchronisation, command execution, spontaneous data transmission etc. must be implemented by the PLC programmer himself (programmed out); the PLC programmer must be familiar with the protocol standard.

This interface is recommended if you:

• are familiar with the protocol standard;
• are implementing a protocol converter application;
• are implementing virtually all available standard functions in the application;
• are using special functions, such as the relaying of the time stamp from a Modbus device or the gaining of control over the command execution;
• require functions that are not supported according to the compatibility list;
• have many data points (>1000) and need high performance;

Protocol structure of endsystem:

Protocol structure of the end system:

Layers 5 and 6 are not used.