are used:

● PUT: Instruction for writing data to a remote CPU or a remote PC

● GET: Instruction for reading data from a remote CPU or a remote PC. Partners can be S7-CPUs, PCs, PGs, TDs/OPs or SCADA systems, such as WinCC.

In other words, if you have ony a CPU300 with integrated interface, the S7-1200 must be client (takes initiative with PUT and GET) where the CPU300 is server (waits for S7-1200 commands).

For communication purposes, the Datablocks in the S7-1200 should be configured as not optimized (S7-300 compatible).

PS: I did not an explicite test of S7-1200 with S7-300.

• Using Profibus most of the time a communication between a PLC and some distributed I/O's are established. So a master (PLC) and a slave (I/O's) communication is the best way. The slaves give a reply when the master is asking about new data or something else. So your master will be set as an active station on the bus and the slave will be passive.

• But, meanwhile the slaves get more and more intelegent. And now there are two intelligent stations which are trying to communicate. Sharing some I/O's cyclically Master-Slave still is the best way. But from time to time you want to send some information from PLC to the inteligent slave (or read some special information). This is another kind of communication. Now there must be data exchange between two active stations at the bus.