Page 719 - Mechatronics with Experiments

PROGRAMMABLE LOGIC CONTROLLERS 705
9.3 PROGRAMMING OF PLCs

Every PLC includes a program development software tool which allows the communication
between a PC and the PLC, development of the PLC application software, debugging,
downloads, and tests it (Figure 9.2). A notebook PC is used only as development tool
in this case, not as part of the real-time control. The development tools for different PLC
manufacturers currently are not interchangable. The application development engineer must
use the development tools supplied by the specific PLC vendor.
Assuming that we have the program development tools (i.e., a notebook PC and a
PLC specific ladder logic program development environment), the software that runs on
the PLCs to control a specific industrial application is the issue discussed here.
There are three main types of programming languages available for PLCs:
1. Ladder logic diagrams (LLD) which emulate the same structure of the hard-wired
relay logic diagrams. These have the widest use since most field technicians are
familiar with hardware relay logic diagrams and can understand LLD programs.
2. Boolean language is a statement list and is similar to BASIC programming language.
3. Flowchart language uses graphical blocks. It is more intuitive than the other two
languages. Although the use of flowchart languages has started to increase in recent
years, LLD is still the dominant language.
The flowchart type languages may eventually gain more widespread acceptance. Today,
every PLC has its own LLD language and they are not compatible across different PLCs. A
flowchart language designed based on standards can generate different run-time programs
for different PLCs simply by using a PLC specific compiler. The LLD programming
environment for all PLCs looks very similar from the application program development
point of view. In the rest of this chapter, only the LLD programs will be discussed, not the
specific development environment for a particular PLC.
There is a fundamental difference between the way a PLC executes its program and
the way a PC does. The program flow in a PC is controlled by the flow control statements
such as do-while, for loops, if-else blocks, function calls, jump or go-to statements. In high
level programming languages, it is possible that the program can be limited to a local loop.
Whereas a PLC program runs in the so-called scan mode (Figure 9.7 ). The whole program

Update inputs

Execute
ladder logic
program
. .
.
.

Update outputs

FIGURE 9.7: Ladder logic program execution model in a
Repeat every PLC. The PLC executes its program in scan mode. All of the
scan period code is checked for execution every scan period.