A major impediment to the success of modern Direct Digital Control (DDC) systems, as applied to HVAC and lighting, is their lack of system function. In 1988 Hartman published a proposed standard for an Operator's Control Language (OCL) that standardizes the way certain types of software is written. This improves DDC system performance by (1) providing more powerful algorithms, and (2) simplifying the language used to create complex control sequences. It also makes all levels of operation transparent to the operator.

OCL is a part of the specification for all Hartman projects and provides for continuity between projects. This language has become the cornerstone for DDC system function to some controls manufacturers. It provides a platform for effectively implementing control sequences in all Hartman designs.