Resolving Siemens RED_IN 8004H Error in ET 200M Redundancy

Problem Description

A SIMATIC S7-300 or S7-400 controller is configured with redundant I/O at an ET 200M station using the Siemens REDIO library. After OB100 startup, the application code calls RED_IN (function block FB 451 in the standard REDIO V1.x library) and the block returns RET_VAL = 8004H (hexadecimal, decimal 32772) for the redundant input channel. Concurrent evaluation of RED_INIT in OB100 reports a return value of 1. The redundancy layer does not select between the two redundant input channels and the process image remains frozen or returns invalid data. This article documents the field-proven diagnostic path for that fault combination on ET 200M (6ES7 153-1AA03 / 6ES7 153-2BA02 / 6ES7 153-2BA10 / 6ES7 153-2BA22 IM 153-x heads) with SM 321 (6ES7 321-…) digital input and SM 322 (6ES7 322-…) digital output modules.

ET 200M I/O Redundancy Architecture

ET 200M supports channel-granular redundancy on the I/O level using two physically separated PROFIBUS-DP segments running into the same station. Each IM 153-x interface module terminates one PROFIBUS line; both IM 153-x heads are loaded with identical configuration. The user program sits on either an S7-300 (CPU 315-2 DP, CPU 317-2 DP/PN, CPU 319-3 PN/DP) or an S7-400 (CPU 41x, CPU 41xH, CPU 41xF/H) controller. The redundancy runtime is realized entirely in the REDIO library blocks, not in the CPU operating system.

Two shared RED_IO data blocks (one for inputs, one for outputs) carry the channel mapping. The library expects the user to instantiate exactly one RED_IO DB per redundant pair and not to read or write the internal structure from user code.

Decoding RET_VAL 8004H from RED_IN

RET_VAL of RED_IN is a 16-bit WORD. The high byte (bits 15..8) contains the binary event source; the low byte (bits 7..0) carries the module-relative error index. The 8004H pattern is decoded as follows:

When RED_IN returns 8004H, the block has executed without raising a fatal error, but the returned input IN_VALUE is either the last good value or a configured substitute value. The partner module is reporting a condition that the library cannot classify — typically a transient diagnostic condition that has not yet been confirmed, or a partner module that is reachable over PROFIBUS but not yet configured for redundancy.

Decoding RED_INIT RET_VAL = 1 in OB100

The return value of RED_INIT is structured differently from the cyclic blocks. Possible values per the Siemens manual entry on FB 450 / FC 100:

A RET_VAL of 1 from RED_INIT is the root cause that cascades into the 8004H you observe on RED_IN: if the redundancy was not initialized, no healthy partner channel exists, so the cyclic block has no valid value to switch to and reports 8004H (partner temporarily unavailable). The two error codes are causally linked, not independent.

Root Cause Matrix

The following matrix maps the most common root causes observed in field service to the symptoms you would see in RED_INIT and RED_IN.

Module Pairing Requirements (ET 200M)

Not every SM 321 / SM 322 module can be paired. The approved pairings — strictly enforced by the library — are listed in the Siemens ET 200M Redundancy manual. The most common pairings in service are:

Modules that cannot be used in a redundant pair include SM 323 (combined DI/DO), SM 331 analog input, SM 332 analog output, SM 334, and SM 365 (the latter is the explicit non-redundant variant). If any of these are present in the redundant slot, RED_INIT returns 2 and RED_IN returns 8004H.

Step-by-Step Diagnostic Procedure

1. Capture the snapshot. Connect STEP 7 to the CPU. Open Online → Accessible Nodes and read both PROFIBUS segments. Save the online view of the redundant station so you can compare against HW Config.
2. Verify slot symmetry. In HW Config, open the station and confirm that every slot 0 through 7 in IM 153-x (head 1) has a counterpart slot in IM 153-x (head 2) with the same order number and the same input/output range. The slot indices must be identical in both heads.
3. Verify base addresses are in the process image. For every module in the redundant group: Module Properties → Addresses. The Start address must be inside the process image (PI) range configured for OB1. The default PI size is 0; many users leave it at 0 which excludes I/O from OB1. Either increase the PI size in CPU properties or assign the redundant modules inside the configured PI.
4. Open the RED_IO data block and verify the channel map. The RED_IO DB contains one UDT entry per redundant channel with fields: CH_BASE_ADDR (input byte), CH_PARTNER_ADDR (partner input byte), CH_LEN (length in bits), and CH_FLAGS. Each CH_BASE_ADDR must equal the configured start address of the master module; each CH_PARTNER_ADDR must equal the configured start address of the partner module.
5. Confirm no user code touches the RED_IO DB. In STEP 7, use Options → Reference Data → Display and search for the RED_IO DB number. Any read/write to its internal structure from user blocks other than the REDIO FBs is a violation and will corrupt the redundancy state machine.
6. Confirm OB70, OB82, OB86 are loaded. Without OB 70 (I/O redundancy error), OB 82 (diagnostic interrupt), and OB 86 (rack failure), the CPU cannot inform the library about module faults and the partner module will appear "temporarily unavailable." Load empty OBs as a minimum if the application does not use them.
7. Re-run a controlled restart. Perform a STOP → cold restart. Observe the RED_INIT return value in OB100. If it is still 1, the address map in the RED_IO DB does not match HW Config — fix and re-run.
8. After successful init, observe RED_IN cyclically. If 8004H persists with a healthy partner, escalate to a partner-channel diagnostic pull using RED_DIAG and read the structured DIAG output to identify which sub-condition is active (external fault, parameter error, etc.).

Hardware Wiring and PROFIBUS Configuration Checks

A software-only review is not sufficient. The 8004H / RED_INIT = 1 combination is often triggered by the partner module simply not being reachable on its PROFIBUS line:

• Confirm both IM 153-x heads have the same PROFIBUS address as configured (rotary switch on the IM). A common mistake is setting both heads to the same address; the master must be address N, the partner must be address N+1 (or per project-specific convention), and the addresses must not collide with any other node.
• Verify both PROFIBUS cables are terminated only at the ends of each segment, not in the middle. The IM 153-x has a switchable termination resistor that must be OFF except on the physical end nodes.
• Check the PROFIBUS diagnostics buffer of the CPU (via STEP 7 PLC → Diagnostic Buffer). Look for Station Failure events on the partner line. Bus errors on the partner line will surface as 8004H because the partner module becomes unreachable from the CPU's view.
• Confirm the partner IM 153-x is configured in HW Config under the same DP master system as the master IM 153-x, or under a separate DP master system tied to the same CPU. The library expects both heads to be reachable from the same CPU; a partner head on a different DP master system (e.g. on a different CPU) is not supported.

Programming Pattern for Robust Diagnostic Capture

Add the following STL snippet to OB1 to capture RED_IN return values and the active channel status into a flag word that can be read from the HMI. This makes intermittent 8004H events visible in the operator interface instead of disappearing into the cyclic scan.

// OB1 - cyclic segment
// Capture RED_IN return value per redundant channel
      CALL  FB 451 , DB RED_IN_DB1
       IN_BASE_ADDR  := 0        // base address of master DI module
       IN_PRT_ADDR   := 4        // base address of partner DI module
       IN_LEN        := 16       // number of bits (16 DI)
       IN_VALUE      := MW 100   // process image word
       RET_VAL       := MW 110   // RET_VAL from RED_IN
       STATUS        := MW 112   // current redundancy status

// Latch any non-zero RET_VAL for HMI display
      L   MW 110
      L   0
      <>I
      JC  LATCH
      L   MW 110
      T   MW 114                 // last-error latched word
LATCH: NOP 0

When a non-zero RET_VAL appears, write MW 114 to a data word exposed on the HMI so the operator can see the last error code after the event has passed. Pair this with an SFC 51 "RDSYSST" call against SSL ID W#16#0131 to read the module diagnostic snapshot of the partner module at the moment of the event.

Verification After the Fix

1. Force the partner module offline by removing its PROFIBUS connector. RED_IN must return a warning (1..7FFFH) but not 8004H after a short transition (≤ 100 ms). RED_OUT must continue to drive the master module.
2. Force the master module offline. The library must switch to the partner module and RED_IN must continue to return the partner value with RET_VAL = 0 (or a warning code, not 8004H).
3. Reconnect both modules. After one full PROFIBUS reconfiguration cycle, RED_IN must return to RET_VAL = 0 with both channels healthy.
4. Run OB100 cold restart 10 times consecutively. RED_INIT must return 0 on every restart; any non-zero return indicates a residual address-map issue.

Common Pitfalls

• HW Config uploaded after a hardware swap changed the slot order. A technician replaced a module and inserted it in slot 3 instead of slot 2. The RED_IO DB still references slot 2. The library initializes but cannot reach the partner — RET_VAL 1.
• Two different module order numbers were quoted on the purchase order. For example, a 6ES7 321-1BH01-0AA0 (16 DI) and a 6ES7 321-1BH02-0AA0 (16 DI, different firmware) are not a valid redundant pair even though the catalog description is similar.
• The CPU process image is configured to size 0. STEP 7 default for some older S7-300 CPUs is 0 input bytes / 0 output bytes. With PI size 0, the cyclic OB1 cannot read the inputs and the library initialization will fail.
• STEP 7 version mismatch. The project was edited in STEP 7 V5.5 SP4 but the station was commissioned with a project exported from TIA Portal V13 with mixed-port PROFINET/PROFIBUS representation. The RED_IO DB layout differs.
• Missing OBs after a full program download. The original project contained OB70, OB82, OB86. The new program was downloaded without the "Include system blocks" option, wiping them. The library cannot report diagnostic events, and any transient fault surfaces as 8004H.

Field-Proven Engineering Notes

In the field, the combination RED_INIT = 1 and RED_IN = 8004H is overwhelmingly caused by an address-map mismatch between the RED_IO DB and the actual module addresses. The library does not perform an explicit version or order-number check during the cyclic scan — it only performs it during RED_INIT. If RED_INIT returns 1, the library internally marks the partner channel as "temporarily unavailable" and the cyclic RED_IN then returns 8004H because that is the closest matching state it can express. Therefore the corrective action is to fix RED_INIT, not RED_IN.

For ET 200M stations, Siemens has shipped multiple REDIO library revisions (V1.0, V1.1, V1.2, V2.0, V2.1). The return value semantics are largely consistent across revisions, but the permissible module list has grown. If you are running an older revision and the module you intend to use is missing from the approved list, upgrade the library and re-import the RED_IO UDT to gain access to the new module types.

References to Official Documentation

For the complete list of approved module pairings, error-code semantics, and OB interactions, refer to the Siemens ET 200M Redundancy manual on the Siemens Industry Online Support portal: Siemens Industry Online Support. Search for entry ID 22613304 (REDIO library manual) and 18648552 (ET 200M system manual) for the authoritative source. The CPU-specific manual for the controller in use (e.g. S7-300 CPU 31x Technological Functions or S7-400 CPU 41x Manual) provides the process-image configuration details required to keep redundant modules inside the PI.