Viewing a Control Schema

A schema is a collection of interconnected functional operator blocks that form a control strategy. It is a graphical block-diagram that represents the algorithm that is to be run on the programmable controller. To display a schema file, select the Open item in the File menu and choose a file with a .scm extension. Schema Viewer will display each file in a separate window. Another way to open a schema is to open the hardware interface file (.hwi extension) which contains the module to which the schema is linked. Then double click on the module to display the linked schema. The hardware interface file may be accessed on its own by opening a document, or by opening a workspace and then choosing the HWI file containing the desired schema.

A sample schema is shown in the figure below. It contains a number of operator blocks, including a PID Control operator, an Analog Input operator, two Bargraph operators, several Numeric Display operators, and more. These operator blocks are interconnected---the output signal of one operator feeds the inputs of others. For instance, in the figure below, the output of the PID Control operator feeds the Right Numeric Display operator.

Schemas contain several types of signals that connect operator blocks including analog signals (floating point values) drawn in red, digital signals (boolean value, i.e., true/false or high/low) drawn in blue, and structures (grouped data) drawn in green. The signal connections are made at an operator's input or output pin. An I/O pin, linked or unlinked, is drawn as a short line attached to a small circle: . The unfilled circle indicates that the pin is not connected. When the pin is connected, Schema Viewer fills in the circle: .

 Example Schema.

Each operator block has a list of properties associated with it. These properties determine the characteristics and behavior of each operator. To view an operator's property sheet, position the mouse cursor on the operator and double-click the left mouse button. For example, the property sheet for the Water Level Constant operator of the figure above is shown in the figure below.

Properties of Analog Input 1 Object.

For a complete reference of schema operators, their associated properties, and other aspects of schema design, refer to the Design Pad G4 Operator Reference Instruction Bulletin (Fairmount Automation Technical Bulletin PDG-TR-2011/13).