Thanks for your help, bunglehead.

In this project, there is a large area. I have already split the whole system into 8 section. I think it's not possbile to split further, because it will make the control more difficult.

This is a redundant system, about 1/3 of the tags are for Connection 1 PLC, and 1/3 for Connection 2 PLC, the rest are internal tags. So there are 2 sets of same tags in the program, just for different connections, it's working, but I dont know it's the correct way or not.

Another problem is that some Alarms in the PLC program are not in continous range, sometimes i create a DBW in WinCC FLex but only use one bit for the alarm.

Thanks Wizard,

I have read the above links, but forour system,we use Ethernet for the connection, and for saving the cost,we use Simens software redundancy. Seem the method is not applicable for ethernet connection.

I will probably try to reduce the tags, and for the worst case, buy another MP377.