楼主的plc是否已进行过多次修改的操作，如是则内存资源被占用量大。
一.压缩一下内存，试一试。
1.Select the S7 program in the "Accessible Nodes" window or the online view of the project.
2.Select the menu command PLC > Diagnostics/Setting > Module Information.
3.In the dialog box which then appears, select the "Memory" tab. In this tabbed page there is a button for compressing the memory if the CPU supports this function.

二.将PLC总清，重新下载。