What this part is and where it fits
The Siemens 6ES7955-2AM00-0AA0 is a RAM expansion module for the SIMATIC S7-400 family, providing 8 Mbyte of memory split as 4 Mbyte data and 4 Mbyte code. It is specifically designated as a replacement part for the CPU417-4 V3 and CPU417H V3 processors. This is not a general-purpose memory card for all S7-400 CPUs — it targets the high-end 417-series controllers used in redundant (H) and high-performance applications. The 8 Mbyte capacity supports larger code bases and data blocks than the base CPU memory.