Control and supervisory layers
A PLC evaluates inputs and controls outputs according to its application program. SCADA displays process state, passes operator commands and, depending on configuration, supports alarms, trends and records. This is a division of responsibilities, not a brand choice: a controller can work with different interfaces. Define command validation, data ownership and update intervals at the start of a project.
Example: a pump start command
An operator requests a pump start on screen; the PLC evaluates local/remote mode, relevant permissives and feedback. The display should show actual running status, not merely the requested command. If the command is rejected, its reason should be visible. This example describes ordinary process control; it does not imply that emergency stops or other safety functions are provided by ordinary SCADA logic.
Correct display behaviour on connection loss
When connectivity fails, the last temperature reading must not appear current. Track the update timestamp, connection status and data quality together. Test how the controller operates without the display and whether reconnecting can resend commands. For the recording system, include time synchronisation, user permissions and restoration from backup in acceptance checks. Availability depends on the complete architecture, not the software name.