Glossary · PLC and control programming
Online change
Also known as: Download in RUN, Run-time edit
German: Online-Änderung
In PLC programming, an online change is a modification of the program or its data that is loaded into a controller while it continues to run, without stopping the machine or process.
- PLC programming
In one sentence
An online change loads a program modification into a running PLC without stopping the machine or process.
Example
In a continuous chemical process, an engineer adds an alarm condition to a running controller as an online change, because stopping would take hours.
How it applies
- Engineering: Online changes are essential in continuous processes where stops are expensive. Controllers handle them differently: some switch to the new code at a cycle boundary, some reinitialize changed data, some reject certain changes (such as interface or memory layout changes) in RUN.
- Risk: A change in a running machine takes effect immediately. A wrong change can cause unexpected motion or process upsets. Sites therefore require planning, a second person, a way back and a check of the machine state before the change.
- Change management: Online changes must go through Change control like any other change, and in process plants often through Management of change (MOC). The offline project must be updated to match the controller.
- Documentation: Record what was changed, by whom, when, and how it was tested. Keep a backup of the version before the change, to support Rollback capability.
Online change vs. offline programming
An online change works on the running controller. Offline programming works on a copy and requires a separate download, usually with the controller stopped or the machine in a safe state.