When it comes to connecting an S7 PLC to the cloud, there are 3 possible ways.
1) New PLCs: when you have a new PLC such as S7-1500, you can simply add a CP card and that can connect you to your desired could platform such as Amazon Web Service, Microsoft Azure, IBM Cloud or MindSphere which the cloud service offered by Siemens.
2) For the older PLCs, such as S7-300, you can make use of an IoT Gateway. If your PLC supports Profinet and comes with a Profinet port, you can easily use that port to connect the PLC to an IoT Gateway. SIMATIC CC712 is one of the popular models for IoT Gateway.
3) Some of the older S7 PLCs, don’t even have a Profinet port but almost every old S7 PLC in the market, have a Profibus port. So if this is the case, you can make use of SIMATIC CC716 which supports both Profinet and Profibus and connect your PLC to the cloud.
Of course, for each of these S7 PLCs, you need to configure the CP card or the IoT Gateway in the software (TIA Portal) as well.