Siemens SIMATIC S7-300 Series Reference Manual page 25

Loss of GD packets Following Change in the MPI Subnet During Operation
Warning
!
Loss of data packets in the MPI subnet:
Connecting an additional CPU to the MPI subnet during operation can lead to loss
of GD packets and to an increase in cycle time.
Remedy:
1. Disconnect the node to be connected from the supply.
2. Connect the node to the MPI subnet.
3. Switch the node on.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
CPUs
1-9