On large systems, the designer is often responsible for interfacing several controllers to create an overall system control. Although this can certainly provide a workable solution, it does increase both cost and complexity. When combining multiple controllers -especially if obtained from different manufacturers - the designer must be careful not to create inadvertent conflicts in operating logic.
Although there are situations where the "multiple box" approach is needed, there are others where control complexity can be reduced while still retaining desirable operating characteristics.