Configuring S7-400H CPU 414-4HJ as a Solo Station in STEP 7

Overview: What is S7-400H Solo Mode?

The Siemens SIMATIC S7-400H is a high-availability, fault-tolerant PLC platform built on two redundant CPUs executing the user program in parallel and continuously synchronizing their internal state over fiber-optic redundancy links. Solo mode (also referred to as single-station mode, solo operation, or single-mode) is a documented operating condition in which one slot of an H-station contains a powered H-CPU while the partner slot is either physically empty, populated by a non-powered CPU, or temporarily taken out of service.

When started in solo mode, the H-CPU detects the absence of its redundant partner through the rack's slot addressing and the redundancy link's "partner-not-found" state. The H kernel then boots the user program with redundancy-related blocks disabled. Self-test routines still execute, system OBs (OB70–OB87) are still active, and H-status diagnostics remain available, but no master/standby switchover, no link monitoring, and no partner comparison will occur.

Solo mode is officially supported in the Siemens "S7-400H Fault-Tolerant Systems" manual and is the recommended path for:

• Pre-commissioning and program development when only one H-CPU is on hand
• Lab or bench testing of fault-tolerant user programs on a single controller
• Education and training environments where two full H-CPUs are not justified
• Pre-staging a station that will later be upgraded to a full redundant pair (chassis, slots, and STEP 7 project can be reused)

Reference documentation is the Siemens "SIMATIC S7-400H Fault-Tolerant Systems" manual, which can be retrieved from the Siemens Industry Online Support portal. The final section of that manual, titled "Commissioning and Operation as a Solo Station," is the authoritative procedure that this article extends with field-tested wiring and STEP 7 details.

Prerequisites and Bill of Materials

To configure an S7-400H in solo mode, the hardware and software must satisfy the same electrical and mechanical constraints as a full H-station, except that only one CPU is populated and only one redundancy link interface is required.

Hardware Setup: UR2-H Rack and Slot Plan

The UR2-H (Universal Rack H) consists of two mechanically separated backplanes, each with 9 slots. The two segments are tied together internally to share the PS-405 modules and the synchronization signal lines. In a full H station, the CPU in slot 3 of segment A partners with the CPU in slot 3 of segment B; all other slots follow the standard S7-400 numbering.

Insert the PS-405 in slot 1 of segment A, the H-CPU in slot 3 of segment A, and leave segment B entirely empty (or fitted with a blank cover to preserve airflow and EMC behavior). Slot 2 of segment A must remain empty; this is the standard PS-405-to-CPU gap.

Mount the rack on a 35 mm DIN rail or directly to the cabinet backplate. UR2-H racks dissipate heat through both segments; ensure at least 40 mm clearance above and below for convection, especially when only one segment is populated.

Power Architecture: PS-405, SITOP, and MCB Sizing

The PS-405 10 A module accepts 24 V DC on its input terminals (X1, X2) and produces the 5 V DC and 24 V DC rails needed by the S7-400 backplane and I/O. In an H station each PS-405 normally feeds one segment. In solo mode only one PS-405 is installed, and it must supply the entire load on the populated segment plus the I/O modules on both segments if those segments contain powered I/O.

If the projected load exceeds the PS-405 10 A rating, or if redundant 24 V feed is desired for safety, a Siemens SITOP module should be wired in parallel with the PS-405 24 V input. The recommended practice is:

1. Feed the PS-405 from a dedicated MCB (e.g. 16 A, C-curve) on the DC distribution.
2. Feed the SITOP from a separate MCB (e.g. 10 A) on the same or a second DC bus.
3. Tie the two 24 V outputs together at the PS-405 input terminals using appropriately sized conductors (6 mm² Cu or larger, depending on length).
4. Use a decoupling diode pair or SITOP's own redundancy module (e.g. 6EP1961-3BA21) to prevent one source back-feeding the other.

Verify the MCB trip curves against the PS-405 inrush current. The 405-0KA02-0AA0 draws up to 30 A peak for <5 ms during power-up. A C-curve 16 A MCB is generally coordinated; B-curve devices may nuisance-trip on cold-start with high-capacitance DC bus designs.

STEP 7 Project Configuration

Solo mode configuration follows the standard S7-400 station build flow in STEP 7 V5.x. TIA Portal does not support S7-400H; use STEP 7 V5.5 SP4 or later (or V5.4 SP5+ with the appropriate HSP) for projects containing the 414-4HJ04-0AB0.

Step 1 — Create a New Project

1. Open SIMATIC Manager.
2. File → New → Project. Name it (for example S7-400H_Solo_Test) and choose a project path with read/write access.
3. Insert a new SIMATIC 400 Station. The station object class is S7-400, not S7-300 or S7-400H; STEP 7 will infer H-mode from the CPU inserted in HW Config.

Step 2 — Open HW Config

1. Open the station and double-click Hardware to launch HW Config.
2. Open the hardware catalog (View → Catalog).
3. Insert the UR2-H rack: SIMATIC 400 → RACK-400 → UR2-H (6ES7 400-2JA00-0AA0).
4. Insert the PS-405 in slot 1: PS-400 → PS 405 10A (405-0KA02-0AA0).
5. Insert the H-CPU in slot 3: CPU-400 → CPU 414-4H → 6ES7 414-4HJ04-0AB0.
6. Leave segment B slots empty for solo operation.

Step 3 — Configure CPU Properties

Double-click the CPU in slot 3 to open its properties dialog. Configure the following sub-tabs:

Step 4 — Save and Compile

1. Station → Save and Compile (Ctrl+S).
2. Confirm zero errors in the compile log. H-CPU-specific checks will appear if the rack type is wrong (UR1 instead of UR2-H) or if redundant I/O is configured without a partner slot available.
3. Close HW Config.

Downloading the Project to the Solo Station

The first download sets the H-CPU's station configuration. Use one of the following paths:

For a bench setup the MPI route is the fastest. After downloading, the CPU executes a cold restart. During restart the CPU checks slot 3 of segment B for a partner; finding none and seeing the Solo Operation flag, it transitions to RUN without entering the "partner missing" blocking condition.

The diagnostic buffer after a successful solo startup should contain (in order):

1. Power ON event
2. CPU self-test completed without error
3. Partner CPU not found (informational event, not a fault)
4. Solo mode active
5. Cold restart executed (OB102 loaded, if present)
6. OB1 cycle started

Programming Considerations for Solo H-CPUs

The 414-4HJ04-0AB0 accepts the standard STEP 7 instruction set (LAD/FBD/STL). Several H-specific blocks differ from their non-H counterparts and behave identically in solo:

Code written in solo mode is portable to a full H station without modification, provided no H-specific functions rely on partner handshake data. This is the principal advantage of solo development: programs validated on a single CPU run unchanged when the partner CPU is added and Solo Operation is later disabled.

Verification and Commissioning Checks

Run the following checks before declaring the solo station ready for program testing:

1. Power rail check. Measure +5 V DC and +24 V DC at the backplane test points with a calibrated multimeter. Tolerances: 5 V ± 2 %, 24 V ± 5 %.
2. CPU status LEDs. FRCE off, RUN solid green, STOP off, SF (red) off, BF (red, if PROFIBUS present) off.
3. Diagnostic buffer sweep. Open the online diagnostic buffer and confirm no "STOP due to" or "module removed" entries. Informational "partner missing" entry is acceptable.
4. OB1 scan time. Open the online CPU information panel; record OB1 minimum, maximum, and current cycle times. Maximum must be < the configured watchdog (default 300 ms).
5. Force table sanity. Open the Force table and confirm no residual force values from prior tests.
6. MPI/PROFIBUS handshake. From STEP 7, perform a "PLC → Accessible Nodes" scan. The CPU should appear with its configured MPI or PROFIBUS address.
7. Watchdog test. Temporarily insert a 400 ms delay in OB1 and verify the CPU does not enter STOP. Restore the original OB1 after the test.

If all seven checks pass, the solo station is ready for program loading and functional testing.

Troubleshooting Matrix

Migration Path: Solo to Full Redundant H Station

When a second 414-4HJ04-0AB0 CPU becomes available, the solo station can be upgraded without rewriting code:

1. Power down segment A (PS-405 off, MCB open).
2. Install a second PS-405 in slot 1 of segment B and wire its 24 V feed through a dedicated MCB (or share the redundancy module output).
3. Insert the second H-CPU in slot 3 of segment B.
4. Install fiber-optic redundancy link modules in both CPUs if they are not already populated, and connect them with the supplied LC patch cords.
5. Re-open HW Config and set Solo Operation = Disabled; recompile.
6. Download the updated configuration. The H-system will detect the partner on the next cold restart and enter redundant RUN mode.

The STEP 7 project, user program, and I/O configuration are reused unchanged. The diagnostic buffer entries from solo operation remain as historical records but do not affect the redundant runtime.

Field-Engineered Caveats

• Cable length on the UR2-H backplane. Segment B is internally wired with an inter-segment bus. Do not modify the pre-installed bus connectors.
• EMC. Even in solo mode, the UR2-H chassis must be bonded to cabinet ground at both end brackets. Floating chassis can produce intermittent SF entries from internal diagnostics.
• Memory. The 414-4HJ04-0AB0 has a fixed work memory. If programs exceed the memory in solo, they will also exceed it in redundant mode; the additional overhead of redundancy blocks is negligible on this CPU.
• Firmware. Verify firmware version ≥ V4.5 on the CPU. Earlier firmware versions (V3.x) have known issues with solo startup behavior; consult the Siemens firmware update portal for the latest FW.
• Replacement parts. When sourcing a replacement PS-405, the "0KA02" suffix is the 10 A variant. The "0KA01" suffix is the 4 A variant; do not substitute without recalculating the load.

FAQ