CPU 315-2 PN/DP with FM 8 — what the key ratings mean for fit
The Siemens 6ES7315-6FF01-0AB0 is a SIMATIC S7-300 CPU 315-2 PN/DP with an integrated FM 8 function module — the FM 8 designation means it carries eight counter/frequency channels on-board, saving a separate module slot and the associated backplane current draw. The CPU handles 244 byte inputs and 244 byte outputs, which sets the process-image limit for cyclic data exchange with the distributed I/O; if your application needs more than that per cycle, you are looking at a larger CPU frame or a 317/319 class unit. Programming is supported in FBD, LAD, STL, and SCL — the full IEC 61131-3 set except S7-GRAPH (which is a separate license). The STEP 7 version floor is V5.1 SP6 or higher; if your engineering station is still on V5.0 or an early V5.1 service pack, the hardware catalog will not recognize this CPU. No MPI port on this variant — communication is via the integrated PROFINET interface and the RS 485 port (PROFIBUS DP). The backup time for the retentive memory is 6 weeks at 40 °C ambient — enough to survive a plant shutdown without battery replacement, but if the cabinet sits at 50 °C+ the backup duration shortens. The I2t rating of 0.5 A²·s is the thermal let-through energy for the internal power supply; it matters when sizing the external 24 VDC breaker feeding the CPU, but for most installations a standard 2 A MCB (C-curve) is fine.
Physical integration and mounting
The module dimensions are 40 mm wide, 125 mm high, 130 mm deep — standard S7-300 single-width format. It snaps onto the DIN rail and engages the backplane bus connector on the right side. The 40 mm width means it occupies one slot in the rack; if you are replacing a 6ES7314-1AF11-0AB0 (also 40 mm wide), the physical swap is direct — same footprint, same bus connector position. The PROFINET interface is on the front, RJ-45; the RS 485 (PROFIBUS) is a 9-pin D-sub. No rewiring needed for the backplane, but the PROFIBUS address and PROFINET device name must be set in the hardware config before download.