Controller Software Structures

Just as the original analog controllers influenced initial hardware designs, so also did they influence software. Process control vendors found most of these software languages difficult for users who needed easy tools for configuring rather than programming. This led each vendor to develop function blocks to “arrange” control strategies by connecting them in various ways with “soft wiring.” Smaller function blocks have more flexibility, but making any changes will probably require a more significant rearranging of the control strategy, a painful recalculation of scan times, and the consideration of any other interactions. Sometimes it requires checkout and verification as is needed with computer code. Typically, vendors who took the “variable time slot” approach tended to have smaller function blocks. The object-oriented configuration allows the readers to build layers of larger blocks out of smaller blocks. Vendors have created the equivalent of many instrument hardware types by building software function blocks that perform the same computations.