Failsafe CPU for deterministic safety lines
The Siemens 6ES7516-3FN00-0AB0 is a CPU 1516F-3 PN/DP — a failsafe controller that runs safety programs alongside standard logic on the same processor. PROFIsafe is integrated, so safety I/O communicates over the same PROFINET cable without a separate safety bus. This keeps the line moving: one controller handles both the weld-timer logic and the light-curtain monitoring, cutting the cabinet count and the cycle-time overhead of a second safety PLC. PROFINET IRT is supported, meaning the controller synchronises I/O updates to within a few microseconds — deterministic enough for coordinated multi-axis motion or high-speed cam switches on a body-shop transfer line. The MRP redundancy manager lets the CPU sit in a ring topology with up to 50 devices; a break in the ring heals in under 200 ms, so the line does not stop on a single cable fault.
Process image and memory sizing
The process image handles 32 kB of inputs and 32 kB of outputs — all I/O data lives in the image, no partial mapping. For a 300-I/O-point weld cell that leaves headroom for future sensors. The work memory is 16 kB max; that is the code and data space for the user program. On a typical safety-rated press application with 50 function blocks and 10 safety zones, expect to use about 60% of that. Backup time is 6 weeks at 40 °C ambient — the capacitor-backed real-time clock holds the timestamp and buffered data through a power loss. If the cabinet ambient runs hotter (50 °C+), the backup window shrinks; plan a battery-backed UPS for extended outages.
Protocol and expansion flexibility
The CPU supports LAD, FBD, SCL, STL, and GRAPH programming — the failsafe versions of LAD and FBD are included, so safety logic can be written in the same editor as standard logic. MODBUS TCP, UDP, TCP/IP, and RS 485 are onboard; no extra module needed for serial or Ethernet-based fieldbuses. A maximum of 8 communication modules or processors (CMs/CPs) can be added — PROFIBUS, additional PROFINET, or Ethernet interfaces. The 70 mm width and 147 mm height fit a standard S7-1500 slot; the 129 mm depth leaves room for the front connector and cabling in a 200 mm-deep enclosure.