CPU 317TF-2 DP — FM 8 and PROFIsafe backbone
The Siemens 6ES7317-6TF14-0AB0 is a CPU 317TF-2 DP for the S7-300 rack, carrying FM 8 (8 function modules) and native PROFIsafe support — this is the safety-rated variant of the 317-2 DP line, designed for fail-safe applications up to SIL 3. The FM 8 count means it can handle up to eight technology function modules (e.g., high-speed cam switches, positioning) without an external FM carrier. It supports LAD, FBD, STL, SCL, GRAPH, HiGraph, and CFC programming languages.
Memory, I/O, and retentivity constraints
The CPU provides 244 bytes each for inputs and outputs, with 32 modules overall — enough for medium-size distributed I/O on the backplane. The I²t rating is 1 A²·s, which governs the short-circuit energy the backplane bus can handle; keep the total module draw below that threshold to avoid bus faults. Retentive data is supported but must be restarted at each restart — no preset retentivity, so the application must explicitly configure retentive areas in the hardware config. Forcing is enabled, useful for commissioning without modifying the program.
PROFIBUS DP and routing
The CPU has an isolated RS 485 interface for PROFIBUS DP, supporting both DP master and slave operation via CP and loadable FB. It can act as DP client or server, with routing enabled — so the CPU can route S7 communication through the DP network. Up to 2 DP connections via CP, plus 8 CPs for LAN (PROFINET) and 8 CPs for PtP — the CPU itself does not have an onboard MPI or PROFINET port, so all Ethernet/PROFINET connectivity requires an external CP module.