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Chapter 10 Controllers for automation 263

program are anticipated. A separate area of RAM is provided as a working memory; this
can be backed by a battery to aid fault finding after a system failure. Compared with the
requirements of present-day personal computers, the memory requirements of a PLC
are quite modest; a memory of 64K will hold up to 1000 instructions, which is adequate
for most applications. While a PLC is normally considered as a standalone item within a
system control architecture point that should be noted is that a PLC, depending on its
operating system, can be infected by malware. The malware may not attack the logic
ladders etc. but the underlying operating systems which can be a version of Windows or
Linux in modern PLCs as in the case of Stuxnet malware attack (Chen and Abu-Nimeh,
2011). The prevention of attacked on industrial control systems will be considered in
Chapter 11, Cyber Physical Systems.
Although similar to personal computers in terms of their hardware, a number of
specific features of PLCs make them suited to industrial control applications, including
the following.
The input and output channels can be wired directly from the PLC to external
systems without any additional interfacing.
The programming is undertaken in a ladder logic form, either from a local hand-
held controller or from a conventional terminal depending on the sophistication of
the controller.
Once the program is loaded it can be permanently stored in the PLC, so once the
PLC is commissioned it becomes an integral part of the machine and it will
become transparent to the operator.
The program structure permits easy reprogramming, allowing modifications and
revisions to be incorporated with minimal down time compared with the problems
associated with a wiring change when a hardwired logic system is used.

The provision of a number of suitable input/output channels is a significant element
of the overall PLC philosophy, and they form the direct interface to the equipment being
controlled. To provide this ability, a choice of input/output packages is commercially
available, including inputs rated at 5 V d.c. or 24 V d.c., and outputs rated at 24 V d.c. and
100 mA or 110 V a.c., 1 A, or 240 V a.c., 2 A. The designer needs to be aware of the system
requirements when a PLC is specified. It is standard practice for the input/output
channels to be optically isolated to provide a high degree of isolation, but relays or triacs
are normally used for the higher-power a.c. outputs. The isolation provided will protect a
PLC from external switching transients up to 1500 V. In a small self-contained PLC, the
inputs and outputs are physically located in the same unit and they normally have the
same rating. In a modular PLC, a range of input/output modules of different ratings can
be configured to suit the application.
The more advanced PLCs have additional input/output features; in particular, they
have the ability to handle analogue information and communications with remote
computers. The analogue-handling ability is useful in the direct monitoring of process
information (for example, of temperature or pressure), with the outputs being used as a