to confirm system requirements. These tests can reveal premature hardware
failures and system configuration inadequacies. They also provide a starting
point for plant operators'training.
4.2.11 System Documentation . Proper vendor hardware, software, and system
configuration documentation should be obtained to keep the system in operation
after it has been installed. On-line documentation retrieval systems should be
Programmable Logic Controller . Functional differences between PLC and DCS have
been narrowing over a period of time. PLCs have evolved into units offering many of
the functions traditionally reserved for DCSs. DCS has also taken advantage of new
technology and incorporated functions like relay ladder logic, function blocks, and
structured text programming that have traditionally been performed by PLCs. The
following systems issues should be considered in deciding whether to use DCS or PLCs:
In large processes that require producing multiple products and/or batches,
including recipe management, the DCS outperforms the PLC. However, in small,
dedicated batch production with limited recipes, where batch management is not
critical, PLCs may be a cost-effective and practical solution.
Initial hardware and software costs are invariably less for PLCs than for DCSs.
However, for a large system that requires heavy integration and custom
programming, PLC software cost may be higher and may cancel initial cost
DCS networks are designed to offer high availability and full redundancy in all
systems components, with no single point of failure. Tight coupling between
operator interface, controllers, and system software provides greater security and
assurance that all components work well together.
The PLC is predominantly used as embedded automation by the original
equipment manufacturer because of its flexible application and low hardware
cost. Most small machine operations cannot support an expensive DCS solution.