Legacy Rockwell PLCs (PLC-5 and SLC-500) had a dedicated status file (S2) that was continuously updated by the controller with system values. The ControlLogix platform has removed this dedicated status file in order to reduce the controller processing load. In place of the dedicated status file, ControlLogix has provided two self-serve instructions that allow direct control over the system value access and are neatly organized into class objects.
The Get System Value (GSV) instruction retrieves a system value status from the controller and updates a specified destination tag with that value. The Set System Value (SSV) instruction updates a controller system value status from a specified source tag.
The following screenshot demonstrates the Get System Value and Set System Value elements within a Ladder Logic rung:
In the next section, we will dig deeper into the GSC and SSV instructions and how...
