MPI is RS 485 + PROFIBUS FDL (lower layer telegrams) with its own SAP definition.
PROFIBUS DP has standardized SAPs (command codes) embedded in the telegrams that tell the slave device what to do. When a device is undergoing a certification process in a testlab all these SAPs are tested to make sure all slave and master devices speak the same language (PROFIBUS DP).
MPI is using the complete bottom layer of PROFIBUS DP, but the command codes are different. Only MPI devices understand these commands. MPI devices are mostly masters and utilized between a PC and Siemens PLCs to upload/download software or to communicate with panels. The connector layout is also exactly the same as PROFIBUS DP, but the default baudrate is 187.5 kbps and the basic cable provided in the delivery package are of less quality compared to PROFIBUS cable. With the early PLCs the MPI interface wasn’t even electrically isolated.
With a tool like ProfiTrace, MPI telegrams are clearly visible and all tool functions apply, because they are actual PROFIBUS telegrams. The SAPs are not decoded, because ProfiTrace does not recognize them