Siemens SIMATIC S7-300 Series Reference Manual page 106

CPU 31x-2 as DP Master/DP Slave and Direct Communication
S5 DP Master
If you are using an IM 308 C as a DP master and the CPU 31x-2 as a DP slave,
the exchange of consistent data requires the following:
In the IM 308 C, you must program FB 192 to enable the exchange of consistent
data between DP master and DP slave. FB192 outputs/fetches all CPU 31x-2 data
in a single block!
S5-95 as a DP master
If you are using an AG S5-95 as a DP master, you must also set its bus
parameters for the CPU 31x-2 as a DP slave.
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PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01