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2020/04/07 WCC Lite user manual
18 Programmable logic controller
A programmable logic controller (PLC) is a digital device adapted for control of processes which
require high reliability, ease of programming and real­time responses. Such functionality has long
since replaced hard­wired relays, timers and sequencers which would be required to complete
various tasks.
Programmable logic controllers usually had to conform to IEC 61131­3 standard which defines four
programming languages: function block diagram (FBD), ladder diagram (LD), structured text (ST)
and sequential function chart (SFC). This standard does not support distributed control systems
therefore IEC 61499 standard was published in 2005. The standard is considered an extension of
IEC 61131­3 standard.
WCC Lite supports PLC functionality while conforming to specifications of IEC 61499 standard.
18.1 IEC 61499
IEC 61499­1 defines the architecture for distributed systems. In IEC 61499 the cyclic execution
model of IEC 61131 is replaced by an event driven execution model. The event driven execution
model allows for an explicit specification of the execution order of function blocks. If necessary,
periodically executed applications can be implemented by using the E_CYCLE function block for
the generation of periodic events.
IEC 61499 enables an application­centric design, in which one or more applications, defined by
networks of interconnected function blocks, are created for the whole system and subsequently
distributed to the available devices. All devices within a system are described within a device model.
The topology of the system is reflected by the system model. The distribution of an application
is described within the mapping model. Therefore, applications of a system are distributable but
maintained together.
Like IEC 61131­3 function blocks, IEC 61499 function block types specify both an interface and
an implementation. In contrast to IEC 61131­3, an IEC 61499 interface contains event inputs and
outputs in addition to data inputs and outputs. Events can be associated with data inputs and outputs
by WITH constraints. IEC 61499 defines several function block types, all of which can contain a
behavior description in terms of service sequences:
•
Service interface function block
– SIFB: The source code is hidden and its functionality is only
described by service sequences;
•
Basic function block
­ BFB: Its functionality is described in terms of an Execution Control
Chart (ECC), which is similar to a state diagram (UML). Every state can have several actions.
Each action references one or zero algorithms and one or zero events. Algorithms can be
implemented as defined in compliant standards.
•
Composite function block
­ CFB: Its functionality is defined by a function block network.
•
Adapter interfaces
: An adapter interface is not a real function block. It combines several events
and data connections within one connection and provides an interface concept to separate
specification and implementation.
•
Subapplication
: Its functionality is also defined as a function block network. In contrast to CFBs,
subapplications can be distributed.
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1.4.0 | ELSETA | Programmable logic controller