System Documentation

DOE-HDBK-1132-99

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

considered.

4.3

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

savings.

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.

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