Periodic Tasks Effects on Communication in Allen Bradley Posted 19 February Yes those scan times can limit or knock you off of online programming. Early on, I inadvertently changed my scan time at one of Rockwell's road shows to 1ms and completely locked myself out of the PLC. Had to take the PLC to program mode to be able to get online and change it back. The 3ms scan time is probably the one that is killing you as there is very little time in between for comms. You may want to see if one of your routines might be better served by an event rather than fast scan times. Rockwell does recommend using periodic tasks, but they do need to be setup with realistic scan times.