CPU 313C — compact controller with 8 function modules
The Siemens 6ES7313-6CG04-0AB0 is a SIMATIC S7-300 CPU 313C compact PLC. It carries 8 integrated function modules (FM 8) — a count that tells the sourcing buyer this CPU packs dedicated hardware for counting, positioning, or closed-loop control without needing a separate FM module slot. The CPU supports LAD, FBD, STL, SCL, GRAPH, and CFC programming languages — STEP 7 V5.5 + SP1 or higher (or V5.3 + SP2 with HSP 203) is required. This means the existing codebase from earlier S7-300 projects can migrate directly, but the engineering station must be at that revision level. PROFIBUS DP connectivity is handled via a CP module and loadable FC/FB — the CPU itself does not have a DP interface onboard. It acts as both DP client and server, but routing requires an active interface.
I/O capacity and retentive data handling
The CPU addresses 244 bytes of inputs and 244 bytes of outputs — a fixed I/O image that cannot be expanded. Forcing is supported, which helps during commissioning when you need to override a sensor state temporarily. Retentive data (inputs, outputs, memory bits, DBs, timers, counters) must be restarted at each restart. The retentive range is adjustable from 10 to 499 — set this in the CPU properties before download. The I²t rating of 0.7 A²·s limits the inrush current the CPU can handle from the backplane bus — relevant when calculating the total load on the S7-300 power supply module.
Communication interfaces and configuration
The CPU has an isolated RS 485 interface onboard — this is the MPI/PROFIBUS port. MPI is not supported, so the interface is strictly for PROFIBUS DP or point-to-point communication via CP modules. Up to 4 CP modules (communication processors) can be added for LAN, and up to 8 for PtP connections. The latest GSD file is available from Siemens for PROFIBUS integration. PROFIsafe is not supported — this CPU cannot be used in safety-rated applications without a separate fail-safe module.
Physical dimensions and sourcing
The depth clears most enclosure backplates without interference.