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The original plant design used the Paradigm XL450 Man Machine Interface (MMI) linked to a Matsushita FP1 C56 PLC situated close to the Treatment Tanks.
Normally with a PLC / MMI combination the PLC contains the intelligence of the system and the MMI performs simple reporting and user interface functions. However, in this instance the capabilities of the MMI were used to the full.
So much so that the system program rested almost entirely within the MMI and the PLC was relegated to being an intelligent I/O unit.
The exceptions clearly being fast process handling requirements such as Emergency Stop and PID control.
A few benefits of the apporoach were:-
- Fault screens were in detail and in real English. 25 full pages were held in memory on the final solution.
- The MMI was programmed using a subset of C (almost scripting) providing a high level language control of the process. Code revisions were easily controlled and documented.
- The system became PLC non-specific as the main process code rested within the MMI. Most MMI's nowadays can support in excess of 20 different PLC protocols and Paradigm excelled in this area.
- The graphics screen chosen for the final MMI provided 600x200 pixels of resolution. This was sufficient to show the complete plant in great detail on a single page.
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