CODESYS basically provides two options to transfer a modified application to the controller: download and online change.

A download results in a recompilation of the application. In that time, a syntax check is performed and application code is also created and downloaded to the controller. This leads to the running program being stopped. A download is the recommended method of data transfer because a defined starting state is always created due to the program stop and the reinitialization.

In the case of an online change, only the modified parts are downloaded again to the controller. A running program is not stopped for this. You should perform an online change only in the case of minor changes to the application. For extensive changes, the behavior of a program cannot be safely predicted. For more information, read the notes in the description of the “Online Change” command.