The iFIX process database is a representation of the process created by linking together blocks of process control logic. The process database consists of blocks and chains. A block (also called a tag) is a coded set of process control instructions provided by GE that perform a specific task. Most blocks can hold only one data point. Each block requires you to provide several parameters. In general, there are two types of blocks:
Primary blocks – start a logical chain and provide the scheduling for the chain. Typically have a scan time and handle all input and output from the database.
Secondary blocks – take data from the upstream block and perform some type of action or calculation.
A chain is a series of connected blocks that create a control or monitoring loop. For example, in a particular control loop, you need to read a data point, manipulate it with a standard formula, and write it out. The chain that executes this control strategy might consist of an Analog Input block connected to a Calculation block connected to an Analog Output block.
In iFIX, calculations needed for scaling or combining multiple I/O data points can also be handled directly in the graphics dynamic links. The following figure displays both control strategies.
For complete information on creating and maintaining a database, refer to the Building a SCADA System manual.
