When either the caller or the called partner is a non-ABAP program, it must be programmed to play the other partner in an RFC communication.
Sap has already provided some solution or technology to communicate with non sap system. These are all external applications like IDOCS , BAPI etc.
Using these techniques, you can simply send or receive data form a non sap system or vice versa. Which is nothing but a remote function call only.
Detailsed tutorials on IDOC and BAPI will be posted soon. Follow our blog to get notified.