Page 700 - Mechanical Engineers' Handbook (Volume 2)

2 Programmable Controllers (PLCs) 691

ferent courses of evolution of programmable controllers in North America, Europe, and Japan
and the wide variety of applications of programmable controllers in modern industry. These
languages include:
• The SFC elements described in Section 1.4 for sequential control
• Ladder Diagrams (LD) for relay replacement functions
• Function Block Diagrams (FBD) for logic, mathematical, and signal-processing func-
tions
• Structured Text (ST) for data manipulation
• Instruction List (IL) for assembly-language-level programming

The IL language will not be described in this book; for further details the IEC standard 3
should be consulted.
Figure 10 shows the application of the LD, FBD, and ST languages to implement a
simple command execution and monitoring function. In general, a desired functionality can
be programmed in any one of the IEC languages. Hence, languages can be chosen depending
on their suitability for each particular application.
An exception to this portability is the use of iteration and selection constructs
(IF. . .THEN. . .ELSIF, CASE, FOR, WHILE, and REPEAT) in the ST language.
The functionality shown in Fig. 10 can be encapsulated into a reusable function block
by following the declaration process defined in the IEC language standard. An example of
the graphical and textual declaration of this functionality is shown in Fig. 11.
In addition to providing mechanisms for the programming of mathematical functions
and function blocks, the standard provides a large set of predefined standardized functions
and function blocks, as listed in Tables 8 and 9, respectively. The intent is for these to be
used as ‘‘building blocks’’ for user programs.
In addition to being used directly for building functions, function blocks, and programs,
the LD, FBD, ST, and IL languages can be used to program the ‘‘actions’’ to be performed
under the control of SFCs as described in Section 1.4. These SFCs can then be used to build
programs and reusable function blocks using the mechanisms defined in the IEC language
standard. 3
It will be noted in Figure 11 that data types are defined for all variables. The IEC
standard provides facilities for strong data typing, with a large set of predefined data types
as listed in Table 10. In addition, facilities are provided for user-defined data types as listed
in Table 11. The standard allows manufacturers to specify the language features that they
support. Users should consult the standard to determine which language features are required
by their application and check their language requirements carefully against the manufac-
turers’ specifications when making their choice of a programmable controller system.

2.4 Programmable Controller Standard, IEC 61131
To propagate consistent characteristics and capabilities for PLCs in the marketplace, an
international standard describing them has been developed under the auspices of the IEC. 1
It is impossible, in a chapter of this length, to cover all the hardware, software, and pro-
gramming language characterisitics, features, and scope defined in the IEC standard for
programmable controllers. The standard comprises seven parts 3,7–12 under the general title
‘‘Programmable Controllers,’’ covering various aspects of PLCs.