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The paper addresses two problems associated with the design of large-scale logic controllers. The first problem is concerned with the difficulty in comprehending logic-controller design specifications which are traditionally based on low-level design description languages. The second problem is concerned with the difficulty of designing high-speed programmable logic controllers which are usually based on the sequential scanning of the logic controller algorithm. An integrated solution to these two problems is developed. The Halstead metrics are described and adapted for evaluating the complexity of a design in terms of the properties of the design language. A decomposition methodology for structuring the design of the logic controller as a hierarchy of functional units is developed. The decomposition allows a relatively high level of functional description of a logic controller design through the use of abstraction. The effects of abstraction and decomposition on the complexity of a design specification is illustrated with an example. The hierarchy of functional units provide efficient building blocks for the low-complexity design of programmable logic controllers, which exploit hardware parallelism to achieve better performance.