Communication Between R/3 and R/2 (MVS/VSE) 
Communication Between R/3 and R/2 (MVS/VSE)
Purpose
The following topics provide an overview about the prerequisites for the partner systems.
As of R/2 Release 5.0D, you can also use an RFC call instead of a CPI-C call.
The following limitation applies to an MVS/VSE host:
These prerequisites depend on whether the calling system is R/3 or R/2:
From R/2 to R/3 From R/3 to R/2