Selecting a man-machine interface for a programmable logic controller (PLC)-based process control system that will meet the needs of a cement industry plant over the projected lifetime of the control system requires consideration of many issues. Some of the issues are soft and include such things as opinions, personal prejudices, and judgments of the people involved. Other issues are hard facts such as the technical performance characteristics and specifications. The authors suggest evaluating both categories of issues while keeping them separate. Examining issues such as cost, modular design, data handling techniques, communication network concepts and open architecture and obtaining an evaluation copy of the software that will allow taking the system on a 'test drive' are also suggested. The authors suggest a format that can be used to arrange the critical issues into a table that will help the process of comparing the many issues in summary form so that the optimum solution can be selected for the specific requirements of the facility involved.
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