$A_DBB[n]就是NC这边的变量，本来与PLC的DB块没有关联，但通过FC21可以相互交换数据
请参考标准功能手册
A separate, internal data area is provided to allow the highspeed exchange of data between
the PLC and NCK. The size of the internal data field is preset to 1024 bytes. The accesses
(read/write) from PLC take place via the FC 21. The occupation of this range (structure) must
be defined identically in the NC part program and the PLC user program.
These data can be accessed from the NC parts program by commands $A_DBB[x],
$A_DBW[x], $A_DBD[x], $A_DBR[x] (see /PGA1/ Parameter Manual System variables)).