CPU 318-2 DP — FM 16 and 64 KB Work Memory
The Siemens 6ES7318-2AJ00-0AB0 is a CPU 318-2 DP from the SIMATIC S7-300 family, built around the SFB type with FM 16 (function module 16) — the FM rating tells you this CPU is designed for high-speed closed-loop control and motion tasks, not just basic logic. It carries 64 kbyte of work memory, enough for mid-complexity applications like packaging machine sequences or multi-axis coordination. The CPU supports LAD, FBD, STL, SCL, and CFC programming languages. CFC support lets you wire up function blocks graphically. No MPI port on this unit, but the RS 485 interface serves as the PROFIBUS DP master connection.
Fieldbus and Integration Constraints
The RS 485 port is the primary fieldbus interface — this CPU speaks PROFIBUS DP natively as a master. For additional PROFIBUS or PROFINET connections, it supports up to four CP 342-5 communications processors, which gives you up to 16 PROFIBUS connections total. Point-to-point links via CP 342-5 PtP modules are limited to eight. The I/O address space is 244 bytes each for inputs and outputs — that's the maximum process-image size, not the total fieldbus data. If your application needs more than 244 bytes per direction, you'll need to use direct peripheral access (PIO) or distribute the load across multiple CPs. The overall rack count is 4, so the expansion chassis limit is tight for large distributed systems.
Programming and Engineering Toolchain
This CPU requires STEP 7 V5.0 or later. The GSD file for PROFIBUS DP integration is siem807f.gsg. Forcing of I/O and memory variables is supported, and routing across subnets is enabled. The retentive memory area is adjustable and covers inputs, outputs, memory bits, data blocks, timers, and counters. The time-of-day clock has a granularity of 1 hour with a lower limit of 1 ms — adequate for time-stamped alarms but not for high-resolution event logging.