What it is and what it fits
The Siemens 6ES7955-2AL00-0AA0 is a RAM expansion module for the SIMATIC S7-400 family, providing 4 Mbyte of total memory split as 2 MB for data and 2 MB for code. It is explicitly designated as a replacement part for the CPU417-4 V3 and the CPU417H V3 processors — meaning if you have one of those CPUs and need to expand memory, this is the official Siemens part.
What the 4 Mbyte rating means for fit
The 4 Mbyte capacity is not a generic add-on — it is specifically partitioned: 2 MB for data blocks and 2 MB for program code. That split matters for a CPU417-4 or CPU417H running large control logic or extensive data logging. If your application needs more than 2 MB of code space, this module provides exactly that ceiling. For a CPU417H in redundant mode, both CPUs need matching RAM modules, so this is the part to specify for each rack.
Physical integration note
At approximately 39 g, the module is lightweight and plugs directly into the S7-400 backplane slot. No additional wiring or configuration beyond slot assignment is required — the CPU recognizes the expanded memory on power-up. Verify the CPU firmware revision supports the 4 Mbyte configuration before ordering.