The communication between EPAM and one or several process controllers (PLCs or other data sources) is implemented by means of read/write operations for individual variables. Variables are associated with the EPAM objects so that their values can be visualized or controlled.
In other words, the communication driver requests actual values from the PLC by means of variables. Any modified setpoints are sent immediately to the PLC by means of individual variables, and then read back. A setpoint value can therefore be reset by the PLC, which will then be displayed immediately in the visualization system.
Variables are interrogated cyclically and only the modified values are refreshed on screen. Only those variables that are required at the time are interrogated, i.e. the variables of all the screen pages (windows) that are opened at the same time.
Variable read and write operations are executed immediately. When the values are written or read in the PLC (cyclically or not) depends on the relevant communication driver.
| • | Variable names are symbolic |
| • | A variable name is always unique and defines a distinct data item |
| • | Variable names are structured according to a specific schema |
| • | Variables in EPAM4 have a specific data type |
See also