Good morning Eh Doh,

When you upload a program (from the PLC to the PC) if the A-B PLC is in the run mode it will continue to run, and you will recieve a message letting you know that you are performing an upload while the PLC is running and data table values may change during the time it takes to upload the entire data table.

That said, when you download a program (from the PC to the PLC) to an A-B PLC, you are erasing and replacing the PLC's memory, and if your PLC is in the run mode you will get a message stating the PLC is in Run Mode and asking you if you really want to change the PLC to program mode.

Hope this helps,

Shawn Tierney,
Instructor at The Automation School