S7-400H CPU MPI Download Error: Step 7 Troubleshooting Guide

1. Problem Overview

A newly received Siemens S7-400H CPU will not accept a project download from STEP 7 V5.x over an MPI cable. The PG (Field PG with integrated CP card) reports a communication failure during the "Go Online" or "Download to Target System" operation, even though the PG/PC interface has been set to MPI and the cable is physically seated on the CPU's MPI port. The CPU's STOP, RUN, and SF / BF LEDs are visible, and the operator waits until the CPU reaches STOP before retrying. The fault reproduces on every attempt.

2. S7-400H Hardware Reference

The S7-400H is a fault-tolerant, redundantly configured PLC. Two CPUs of identical order number and identical firmware version run in parallel and synchronize process data and program execution over fibre-optic redundancy links. Before any STEP 7 download is attempted, the operator must understand the H-specific startup behaviour that does not exist on a standard S7-400 CPU.

3. Root Cause: H-CPU First-Power-On Self-Test

The single largest source of "I cannot go online with a brand-new S7-400H" reports is the H-CPU extended self-test. On every cold start (power OFF to ON), each H-CPU runs a RAM, register, and link-test routine before the operating system releases the MPI/PROFIBUS/PN interfaces to STEP 7 traffic. For an H-CPU the test is more extensive than a standard S7-400 CPU because the firmware must validate redundancy link integrity, link partners, and the standby CPU's RAM contents.

Self-test duration rules of thumb:

• CPU 412-3H, CPU 414-4H: typically 1 to 5 minutes for short tests, 5 to 15 minutes for a full RAM test on first commissioning.
• CPU 416-4H: 5 to 15 minutes typical, up to 25 minutes for full work-memory test.
• CPU 417-4H (30 MB work memory): up to 30 minutes for a complete RAM test on first power-up.

During the self-test the STOP LED may be lit with a blinking RUN LED, or the STOP and RUN LEDs blink alternately. No communication is possible on MPI, PROFIBUS, or PROFINET until the test is complete. A STEP 7 "Go Online" attempt during this window produces a dialog such as "Online: No connection to target CPU was established within the configured timeout" or a S7DOS / S7OLE error class.

4. Secondary Root Cause: Load Memory Re-Initialization

When the S7-400H CPU detects that load memory (the Flash-EPROM area or the integrated RAM load memory) has been wiped, it reinitializes the test RAM and runs an extended memory test. The larger the configured work memory, the longer this test runs. Siemens explicitly documents this behaviour in the SIMATIC S7-400H Fault-Tolerant Systems manual:

The longer it takes, the more memory size. The 417-4H test duration is longer than the 414-4H.

Mitigations supplied by Siemens:

1. Install the backup battery in PS 405 / PS 407 before first power-on. Type 1 backup battery (3.6 V Lithium) keeps RAM load memory alive and skips the re-initialization test.
2. If a battery is not available, supply external 24 V back-up to the CPU through the X1 backup connector on the front of the PS module.
3. Alternatively, pre-load the program to a Flash EPROM card (MMC for newer CPUs, FEPROM card for older CPUs) using the SIMATIC Field PG or a Prommer. The CPU will boot from Flash and skip RAM self-test.

5. Hardware Prerequisites for First Commissioning

Verify the following before going online:

6. STEP 1 — Configure the PG/PC Interface Correctly

Windows 10 64-bit and STEP 7 V5.5 require the legacy "Set PG/PC Interface" tool (32-bit STEP 7 dialog, not the Windows control panel).

1. Open SIMATIC Manager > Options > Set PG/PC Interface.
2. Select the access point: S7ONLINE (default for STEP 7 V5.x) or S7ONLINE (STEP 7 V16+) for TIA Portal.
3. Select the interface parameter assignment:
   • If the Field PG has an integrated CP 5611 or CP 5614 A2: choose CP5611.MPI or CP5614A2.MPI.
   • If using an external PC adapter USB (6ES7 972-0CB20-0XA0 or 6GK1 571-1AA00): choose PC-Adapter.MPI or PC-Adapter USB.MPI.1.
   • If using Ethernet to a CP 443-1: choose TCP/IP > Network Card (RFC1006). Industrial Ethernet is preferred for first download of large programs.
4. Click Properties. For MPI:
   • Address: 0 (PG default).
   • Highest MPI address: 31 (must match CPU "Highest MPI address" setting in HW Config).
   • Transmission rate: 187.5 kbit/s (default; 1.5 Mbit/s possible only with PROFIBUS cables and short distance).
5. Click OK, confirm "Diagnostics" reports the CP is READY.

7. STEP 2 — Verify CPU Self-Test Has Completed

Before any download attempt, confirm the H-CPU has finished its self-test.

1. Read the front-panel LEDs:
   • Solid STOP + solid RUN off = CPU is in STOP state with self-test complete.
   • Blinking STOP or alternating STOP/RUN = self-test in progress. Wait.
   • Solid SF lit = hardware fault, see CPU diagnostic buffer (Module Information > Diagnostic Buffer).
2. Open SIMATIC Manager, expand the project, right-click the H-CPU and choose Accessible Nodes. The H-CPU should appear in the list with its MPI address. If it does, the self-test is finished and the CPU is reachable.
3. Alternatively use PLC > Accessible Nodes with the access path set to MPI.

8. STEP 3 — Download Procedure for an H-CPU

The H-system download procedure differs from a standard S7-400. Both CPUs must receive the project, but the order matters.

1. In SIMATIC Manager, expand S7 Program > right-click Blocks > Download.
2. STEP 7 prompts: Select target module. Choose CPU 1 only (the master). Do not select "Both CPUs" in this dialog.
3. Confirm the dialog: Stop target system and download. The CPU enters STOP, the program is written to load memory, then to work memory. The CPU remains in STOP.
4. Repeat for CPU 2: select the redundant CPU in the "Accessible Nodes" tree and download the same blocks.
5. Once both CPUs have the identical project, both CPUs will perform an update (initial link-up). Wait until the IFM1F / IFM2F link LEDs on the front panel show steady green.

9. STEP 4 — Verify the H-System is Linked

1. In SIMATIC Manager, right-click the H-CPU → Module Information.
2. Tab Diagnostics Buffer: the last entries should read Update end (initial) and System redundancy OK. Event IDs 0x73F8 and 0x73F9 indicate successful redundancy initialization.
3. Tab Communication: should report both redundancy links as OK with current peer CPU as the active master.
4. Tab Performance: redundancy status Standby/Standby or Master/Standby.

10. Memory Reset (MRES) — When the CPU Still Won't Go Online

If the H-CPU has been powered without a battery for an extended period and the self-test still fails to complete within 30 minutes, or if the CPU remains in STOP with SF lit, perform a memory reset.

1. Turn the mode selector to STOP.
2. Hold the mode selector in the MRES position for at least 3 seconds until the STOP LED flashes slowly.
3. Release, then turn the selector to MRES again within 3 seconds. The STOP LED will flash quickly while the reset runs.
4. Wait until the STOP LED is solid. The CPU is now in clean state with empty work memory.

11. Flash-EPROM Procedure (When RAM Will Not Stabilize)

Some technicians report that the RAM-based load memory refuses to accept the download, but writing to a Flash EPROM (or MMC for newer CPUs) succeeds. The procedure is:

1. Insert the FEPROM / MMC card into the slot on the front of the CPU (powered off).
2. Power on. The CPU will detect the FEPROM and copy its contents into RAM load memory at startup.
3. Use the SIMATIC Manager PLC > Save to Memory Card menu (or for MMCs, SIMATIC Card Reader) to write the project to the FEPROM.
4. Power cycle the CPU. It will boot from FEPROM without requiring a battery for load memory.

This is a viable first-commissioning workaround for new CPUs whose RAM test is incomplete or flaky.

12. Ethernet Instead of MPI — Faster, More Robust First Download

For a 30 MB work-memory project on a CPU 417-4H, an MPI download at 187.5 kbit/s can exceed 45 minutes. Industrial Ethernet at 100 Mbit/s is roughly 500x faster and avoids many timing-sensitive issues.

1. Install a CP 443-1 EX30 (or later) in a free slot of the H-rack.
2. In HW Config, assign an IP address from the project, e.g. 192.168.0.10 for CPU 1's CP and 192.168.0.11 for CPU 2's CP.
3. Configure the PG/PC interface as TCP/IP > Network Card (RFC1006) with the S7ONLINE access point.
4. Compile and download the project. Ethernet does not bypass the H-CPU self-test but eliminates the MPI-specific timing limitations.

13. STEP 7 / TIA Portal Version Compatibility Matrix

The official compatibility matrix lives at the SIMATIC S7-400H firmware / software compatibility list.

14. Troubleshooting Matrix

15. Field-Proven Caveats

• Battery polarity. The Type 1 backup battery (Siemens order code 6ES7 971-1AA00-0AA0) has a defined orientation. Reverse insertion will not damage the system but the CPU will behave as if no battery is installed.
• CP model. A CP 5611 in a Field PG M2/M3 supports MPI 187.5 kbit/s only. To download over MPI at 1.5 Mbit/s the Field PG must be a later model with CP 5614 A2.
• STEP 7 V5.5 on Windows 10: install STEP 7 V5.5 SP4 + HF11 or later for Windows 10 1909+ compatibility. Earlier SPs report 0xE0 01 01 even on healthy hardware.
• Mixed-version H-pair. An H-system refuses redundant operation if the two CPUs have different firmware. The diagnostic buffer of CPU 1 will read 0x73F2 "Standby CPU has different firmware". Both CPUs must be re-flashed to the same firmware version.
• Redundant IO configuration. The first download must include the system data (SDB) for the H-specific IO configuration. Without SDB the redundant IO channel-error masking is incorrect, and the link-up reports 0x10C5.
• STOP-to-RUN after link-up. It is normal for the master to remain in STOP after both CPUs have been downloaded. Manually switch master to RUN after verifying link-up is complete.

16. Recommended First-Commissioning Workflow

1. Unbox the H-CPU and the PS 407.
2. Insert the Type 1 backup battery into the PS module before powering up.
3. Mount CPU 1 and CPU 2 in the rack; connect the redundancy fibre-optic cable.
4. Power on. Wait for the self-test to complete (LED pattern stops blinking).
5. Connect PG via MPI to CPU 1. Use Accessible Nodes to confirm CPU 1 is reachable.
6. In SIMATIC Manager open the project that matches the H-system configuration (HW Config CPU type and firmware match physical).
7. PLC > Compile and Download Objects. Download to CPU 1 first.
8. Connect to CPU 2 via Accessible Nodes. PLC > Download to Target System. Select Blocks only.
9. Switch both CPUs to RUN. Verify the diagnostics buffer shows "System redundancy OK".

17. FAQ