Page 711 - Mechatronics with Experiments

PROGRAMMABLE LOGIC CONTROLLERS 697
space. In other words, once the location of each module in the PLC rack is known, the I/O
of all modules are memory mapped to the PLC’s memory space using the configuration
functions of the PLC program development environment. Then, those memory locations
(input/output data) are used in the PLC’s application program logic.
Although there have been claims for the past 30 years that PLCs would be a thing
of the past and that personal computer (PC) based control would take over the industrial
control world, PLCs continue to be strong in the market. Let us compare PLC based control
with the PC based control.
1. PLCs have modular design. If a different type of I/O signal needs to be processed,
all we need to do is to add a different I/O interface module and modify the software.
Furthermore, the I/O modules include the terminals necessary for field wiring. In PC
based systems, for different I/Os, we need to insert a different PC card and provide
a separate terminal block for wiring which connects to the PC card through a ribbon
cable. This tends to be a messy and non-standard wiring process.
2. PLCs have a rugged design suitable for harsh industrial environments against high
temperature variations, dust, and vibrations.
3. Programming of PLCs is mostly done using ladder logic diagrams which are under-
stood by millions of technicians in the field. This proves to be one of the biggest
advantages of PLCs. Even though ladder logic diagram (LLD) programming does
not have the programming environment capabilities provided for PCs, it is well
established, proven to work well, and a large base of technical personnel can work
with it.

The real trend observed in industry is not competition between PLCs and PCs in
industrial control, but rather the complementary use of them. PCs are used in conjunction
with PLCs at two different levels,
1. PCs are used as networking and user interface devices (Figure 9.2).
2. PCs implement the control logic, replacing the role of the CPU on the PLC while the
PLC provides the I/O interface (Figure 9.2). In this configuration, PLC has the I/O
modules and a scanner module which updates the I/O between the PLC rack and the
PC. The PC implements the control logic which may be developed using any of the
programming tools under the PC platform, that is using C, Basic, or PLC specific
graphical program development tools. The key issue is to guarantee hard real-time
performance in the PC using a real-time operating system. As real-time operating
systems become more robust and low cost over time, this model of PC and PLC
combination may be widely accepted in industry.

9.2 HARDWARE COMPONENTS OF PLCs

9.2.1 PLC CPU and I/O Capabilities
Perhaps the biggest reason for the success of PLCs in the industrial control market is
the fact that the hardware of almost all PLCs have very similar designs. The hardware
design is based on a backplane which carries the power and communication bus. A snap-on
input/output (I/O) card into any one of the slots makes the necessary electrical contacts for
power as well as interface (Figures 9.1, 9.2). Each PLC needs:

1. rack(s) with slots: a backplane for the communication bus,
2. a power supply module,