CPU CENTRAL UNIT 6ES7412-3HJ14-0AB0 6ES7 412-3HJ14-0AB0 6ES7412-3HJ14-OABO

Siemens 6ES7412-3HJ14-0AB0 | SIMATIC S7-400H CPU 412-3H — High-Availability Redundant CPU, 768KB Memory, 1 MPI/DP + 2 Sync Interfaces, S7-400H and S7-400F/FH

Overview

The Siemens 6ES7412-3HJ14-0AB0 is the CPU 412-3H — the central processing unit of the SIMATIC S7-400H high-availability controller system. A single 6ES7412-3HJ14-0AB0 unit cannot operate alone in its intended role.

It requires a second identical CPU, and the two operate in a redundant pair: both CPUs run the same program simultaneously, both process the same inputs, and their outputs are compared synchronously.

If one CPU fails, the other takes over seamlessly, continuing the process without the slightest disturbance visible to the controlled process.

This hardware redundancy is the defining feature of the S7-400H and the reason it exists as a separate platform from the standard S7-400. In most factory automation, a PLC fault causes a machine to stop — inconvenient and costly, but recoverable.

In continuous process plants — refineries, chemical reactors, power generation, water treatment — a controller failure can mean process loss, product contamination, equipment damage, or safety hazards.

The cost of a single unplanned stoppage can reach hundreds of thousands of dollars in lost production, and restart after an unplanned shutdown may take hours. Systems controlling these processes cannot tolerate even brief controller outages, which is why the S7-400H is designed from the ground up for continuous operation through any single hardware fault.

The CPU 412-3H is the entry-level H-CPU in the S7-400H range — providing 768KB total memory (512KB program, 256KB data) with MPI/DP communication.

For larger programs or applications requiring more data storage, the larger CPU 414-3H (6ES7414-3HM14-0AB0) and CPU 417-4H (6ES7417-4HL14-0AB0) offer progressively larger memory and additional interfaces.

Key Specifications

The Synchronisation Architecture — How Two CPUs Become One Fault-Tolerant System

The three-interface design of the CPU 412-3H reflects the technical demands of hardware redundancy.

The single MPI/DP interface handles normal automation communication — connecting the CPU to PROFIBUS DP field devices, to programming terminals, to HMI panels, and to other PLCs in the plant network. The two synchronisation interfaces are dedicated to inter-CPU communication.

The two sync module interfaces on each CPU 412-3H connect to synchronisation modules (IM 460-4, 6ES7460-4AA01-0AB0) that form the physical data links between the two redundant CPU racks. Two independent sync links are used — not one — because the synchronisation channel itself must be redundant.

A single sync cable failure would otherwise disable the entire redundancy function. 

With two independent sync paths, the pair of CPUs can synchronise through either path independently, and a fault in one sync path generates a maintenance alarm without forcing a switchover.

The sync communication carries all the state information needed to keep both CPUs identical: every output state, every timer and counter value, every memory bit — updated at every program scan.

This bumpless synchronisation is what enables a true switchover (not a restart) when one CPU fails: the surviving CPU already has all the current data and continues execution from exactly where the failed CPU left off.

S7-400H and S7-400F/FH — Redundancy and Safety Combined

The 6ES7412-3HJ14-0AB0 is compatible with two system configurations:

S7-400H (High-availability): The standard redundant configuration — two CPUs operating in parallel, fail-over on hardware fault, target availability in the range of 99.99% or better.

The H system addresses availability (keeping the process running) but not functional safety (preventing hazardous process states).

S7-400F/FH (Fail-safe High-availability): Combines hardware redundancy with IEC 61508-certified functional safety.

The F/FH system adds safety-certified F-modules and safety program blocks that implement the safety instrumented functions (SIF) required for hazardous area processes. 

When both availability (no unplanned shutdowns) and functional safety (SIL 1/2/3 certified protection) are required in the same controller, the S7-400F/FH addresses both simultaneously. This configuration is common in the oil and gas, chemical, and nuclear industries where process hazards require certified safety protection alongside continuous operation.

Memory Configuration — 512KB Program, 256KB Data

The memory split — 512KB for program and 256KB for data — reflects the typical workload balance in the continuous process control applications that the S7-400H targets. Process automation programs tend to be larger than equivalent discrete automation programs because they include:

Extensive analog signal processing (scaling, filtering, alarm limit evaluation for dozens or hundreds of process variables), complex control algorithms (cascade PID loops, ratio controllers, advanced process control), large data structures for process historian integration (buffering measurement histories before transmission to SCADA), and comprehensive diagnostic and alarm management routines that report plant condition in detail to operators.

The 512KB program memory accommodates these complex programs. For very large plants, the addition of loadable function blocks (FBs) stored on memory cards extends the effective program size beyond the built-in memory limit.

FAQ

Q1: Does the S7-400H automatically switch over to the standby CPU when the master fails, and how fast is the switchover?

Yes, switchover is automatic and does not require operator intervention. When the active (master) CPU detects a fault in itself, it signals the standby CPU to take over. The standby CPU — which has been running the same program with the same data in synchrony — continues execution without interruption.

The switchover time in a correctly configured S7-400H system is within the PLC scan cycle time, typically in the range of single-digit milliseconds, which is imperceptible to field devices and process instrumentation.

No process values are lost, no outputs change state due to the switchover itself, and no manual restart is required.

Q2: Can the S7-400H be maintained — modules replaced or firmware updated — while the process is running?

Yes. The S7-400H's redundant architecture supports online maintenance — replacing failed modules in one CPU rack while the other remains running and controlling the process.

This means hardware maintenance can be performed during production, including: replacing a failed CPU (the 6ES7412-3HJ14-0AB0 itself), swapping power supplies, replacing I/O modules, and updating firmware — all without stopping the process. 

After maintenance, the repaired rack is re-synchronised automatically with the running rack.

This online maintainability is the core operational advantage of the H system over simply providing a hot standby.

Q3: Is the CPU 412-3H directly compatible with standard S7-400 I/O modules and communication modules?

Yes. The S7-400H CPU racks accept standard S7-400 signal modules (SMs), communication processors (CPs), and function modules (FMs), subject to redundancy-mode restrictions.

In H-mode operation, modules installed in both racks should be identical, and some module types (particularly FMs with communication bus functions) require specific H-compatible versions. 

Standard S7-400 I/O modules (SM421, SM422, SM431, SM432 series) are compatible and commonly used. The complete compatibility list is documented in the S7-400H system manual.

Q4: How is the S7-400H programmed — is special software required beyond STEP 7?

The S7-400H uses STEP 7 as its programming environment, with the addition of the S7-REDCONNECT library.

The REDCONNECT software provides the function blocks and configuration tools for setting up the redundant communication links (connections between the H system and PROFIBUS/PROFINET slaves, SCADA systems, etc.) in a way that both CPUs can manage the communication path simultaneously, with automatic failover of communication connections in addition to CPU failover.

Without REDCONNECT, the H system will still run standard STEP 7 user programs, but the redundant communication configuration features will not be available.

Q5: The CPU 412-3H is discontinued. What is Siemens's recommended migration path?

Siemens recommends the SIMATIC S7-1500H system (particularly the CPU 1513H-1 PN and CPU 1517H-3 PN) as the current-generation replacement for the S7-400H.

The S7-1500H provides equivalent hardware redundancy with significant improvements in performance (faster scan cycles, higher memory), native PROFINET IO-redundancy, integration with TIA Portal engineering, and a significantly more compact physical footprint.

Migration from S7-400H to S7-1500H is a hardware and software project — the STEP 7 program must be migrated to TIA Portal, and the hardware configuration rebuilt for the new platform — but Siemens provides migration tools and services to support the transition.