Resolving CPU555 ET200U PROFIBUS Connection Failure on TDC

Resolving CPU555 ET200U PROFIBUS Connection Failure on TDC Systems

The Siemens SIMATIC TDC (Technology and Drives Controller) platform pairs high-performance multiprocessor CPUs with dedicated communication modules such as the CP50M1 for PROFIBUS integration. When migrating a PROFIBUS-DP segment from a CPU551 + CP50M0 combination to a CPU555 + CP50M1 combination, ET 200U distributed I/O stations frequently fail to enter cyclic data exchange, even though ET 200M stations on the same CP come online without issue. This article documents the root cause, the diagnostic error codes (6012/6013), the functional status dependency of the CP50M1, and the field-proven resolution procedure to restore the DP link.

1. Problem Description

1.1 System Configuration

The affected installation consists of the following Siemens components:

1.2 Symptom Summary

• The CPU 555 does not detect the ET 200U station during DP start-up.
• The ET 200U station appears greyed out in the hardware configuration (HW Config) inside STEP 7.
• The H6 LED on the front panel of the CP50M1 lights, indicating a master-side diagnostic event.
• Replacement of the ET 200U with an ET 200M station on the same PROFIBUS segment works correctly — establishing the issue is specific to ET 200U with CPU 555.
• The same configuration on a CPU 551 + CP50M0 pair operates without error, confirming that the ET 200U itself is functional.
• STEP 7 diagnostic buffer of the DP master produces: "End of the synchronization with a DP master/IO controller on slot 2. Number of distributed stations: 0. Incoming event."

2. Diagnostic Error Codes 6012 and 6013

The runtime diagnostics surface two YTS error codes that map directly to the communication function blocks CRV (Communication Receive) and CTV (Communication Transmit) on the TDC side. These codes are emitted by the CFC library used to interface the CP50M1 with the user program.

2.1 Reading YTS Connectors on CRV and CTV

To obtain the YTS error values in CFC on the TDC engineering station, open the CRV/CTV instance and inspect the STATUS connector. For STEP 7 / D7-SYS engineering on TDC:

1. Open the S7 program containing the CP50M1 communication blocks.
2. Navigate to the chart where CRV_P and CTV_P instances are placed (suffix _P denotes the parameterized instance).
3. Right-click the STATUS output → "Monitor/Modify" and force the OS to read.
4. Record the displayed value (decimal and hex) for both CRV and CTV.
5. Compare against the Siemens Industry Online Support error code list for the TDC CP50 library.

3. Root Cause Analysis

The ET 200U is a SIMATIC S5-class distributed I/O station that uses an IM 318 interface module to attach to PROFIBUS-DP. The IM 318-8MB12 supports DP-V0 and operates at up to 1.5 Mbps with a maximum of 32 stations per segment. Because the IM 318 predates many of the cyclic extensions added to the PROFIBUS specification after the introduction of the ET 200M and ET 200S families, its GSD file (SIEM8195.GSD for the IM 318 family, or vendor-specific GSDs) must be installed in the STEP 7 hardware catalog and the station must be configured with the correct slot-based I/O mapping.

When a CPU 555 is used in place of a CPU 551, two compatibility factors come into play:

1. Functional status of the CP50M1 module: The CP50M1 must be at functional status (FS) ≥ 4 for operation with the CPU 555. Earlier firmware revisions of the CP50M1 (FS 1, 2, 3) were developed and qualified only against the CPU 551. Running an under-status CP50M1 on a CPU 555 results in silent synchronization failures where the DP master reports "0 distributed stations" even though the bus is physically healthy and the H6 LED toggles.
2. Joint CPU 555 + CPU 551 operation: Mixed operation of a CPU 555 with a CPU 551 on the same TDC rack is not approved. If the rack contains both processor types, the PROFIBUS synchronization of the CP50M1 on the CPU 555 will fail intermittently.
3. CP5100 / CP50M0 phase-out: The CP5100 and CP50M0 modules have been phased out by Siemens and cannot be used in new CPU 555 configurations. Any attempt to migrate must move to the CP50M1 at FS ≥ 4.

3.1 Diagnostic Buffer Interpretation

The diagnostic buffer entry — "End of the synchronization with a DP master/IO controller on slot 2. Number of distributed stations: 0. Incoming event." — is the DP master's own acknowledgment that the start-up sequence terminated cleanly (no bus short, no duplicate station, no parameterization error) but found zero slaves matching its expected PROFIBUS address list. This is consistent with:

• The IM 318 having a different PROFIBUS address than what is configured in HW Config.
• The IM 318 being physically absent or powered down.
• The GSD file for the IM 318 not being installed, causing HW Config to skip the station.
• The CP50M1 not being in a state where it can enumerate slaves (functional status issue).

4. CP50M1 Functional Status Requirement

The functional status of a Siemens module is a build-state identifier that bundles firmware, hardware revision, and approved interoperability. The CP50M1 (6DD1670-0AF0) ships with several functional status levels; only FS ≥ 4 is validated for CPU 555 operation.

4.1 How to Identify the Functional Status

1. Power down the TDC rack and remove the CP50M1 module.
2. Locate the type plate on the right-hand side of the module housing.
3. Read the field labeled "F-STAND" or "FS" — this prints two digits indicating the functional status (e.g., "FS:04" or "F-Stand 4").
4. Alternatively, in STEP 7 / D7-SYS, open HW Config, right-click the CP50M1 slot → "Module Information" → "Version" tab. The functional status is listed under "Functional status of module."

5. Compatibility Matrix

Joint operation of CPU 555 and CPU 551 in the same TDC rack is not approved. If the migration is staged — for example, adding a CPU 555 alongside an existing CPU 551 — the PROFIBUS segments handled by CP modules attached to the CPU 555 will fail to enumerate ET 200U slaves regardless of GSD correctness.

6. Step-by-Step Resolution Procedure

6.1 Prerequisites

• STEP 7 V5.5 SP4 or later (for D7-SYS engineering).
• D7-SYS add-on for TDC configuration.
• GSD file for IM 318-8MB12 installed under Siemens\Automation\S7\data\gsd\.
• Access to the TDC rack with CPU 555, CP50M1, and the ET 200U IM 318 in place.
• PG/PC connected via MPI/ PROFIBUS or Ethernet to the TDC.

6.2 Procedure

1. Verify CP50M1 functional status. Open HW Config, locate the CP50M1, right-click → Module Information → Version. Confirm FS ≥ 4. If lower, the module must be replaced; firmware-only upgrades of the CP50M1 are not generally field-recoverable — exchange the module.
2. Verify rack homogeneity. Ensure no CPU 551 is installed in the same TDC rack. If a CPU 551 must coexist during a transition, dedicate the CP50M1 to a CPU 555-only rack and re-home the ET 200U PROFIBUS segment.
3. Install the IM 318 GSD file. In HW Config: Options → Install GSD File. Browse to the IM 318 GSD (commonly SIEM8195.GSD or a vendor-specific GSD). Restart HW Config so the device appears in the catalog under "PROFIBUS-DP / Other Field Devices."
4. Insert the ET 200U station. Drag the IM 318-8MB12 from the catalog onto the DP master system attached to the CP50M1. Assign the same PROFIBUS address (1–125) that is set on the rotary switches of the physical IM 318.
5. Insert the AO and DI modules in their configured slots. Drag the 6ES5470-8MA12 (AO) and 6ES5421-8MA12 (DI) into the slots below the IM 318 in HW Config. Match the slot order to the physical mounting on the ET 200U backplane.
6. Compile and download the hardware configuration. Station → Save and Compile, then PLC → Download to Target. The CPU 555 will perform a DP start-up; observe the diagnostic buffer for the synchronization message.
7. Enable CRV and CTV function blocks. In the CFC chart, set the EN input of CRV_P and CTV_P to TRUE (or to a logic condition that evaluates to TRUE during normal operation). The most common implementation error is leaving EN tied to a never-true condition, which produces exactly the 6012/6013 codes observed.
8. Cross-check the PROFIBUS address on the IM 318. Power-cycle the ET 200U and read the rotary address switches. Confirm the address matches HW Config. The IM 318-8MB12 uses two BCD rotary switches (x1 and x10) for addresses 1–99. Address 0 is invalid for DP slaves.
9. Check bus termination and shielding. Verify that PROFIBUS connectors have terminating resistors enabled only at the physical ends of the segment. The CP50M1 has an internal terminator that can be switched on or off via a DIL switch on the module — disable it if the CP50M1 is not at the physical end of the bus.
10. Recompile the S7 program and download to the TDC. Trigger a CPU restart (STOP → RUN) so the CP50M1 reinitializes its PROFIBUS stack.

7. Verification

After applying the procedure, perform the following checks to confirm the ET 200U station is in cyclic data exchange:

8. ET 200U Configuration Notes

The ET 200U is a member of the SIMATIC S5-era distributed I/O family and has several configuration peculiarities that affect its behavior on a modern DP master.

8.1 Slow-Mode Operation

Per the ET 200U documentation, when the IM 318 is configured for "slow mode" the bus reaction time tI/O bus is prolonged. This mode is enabled via a parameter bit in the GSD file and is typically selected when the connected I/O modules are of the slow S5-100U family. With an AO 470 and DI 421, slow mode is usually not required — but if parameterization errors are reported (e.g., diagnosis frames indicating parameterization fault), enabling slow mode is a valid workaround.

Reference: Siemens S5/505 ET 200 Distributed I/O System Manual (PDF)

8.2 GSD Installation Path

The IM 318 GSD must reside in the STEP 7 GSD directory:

After copying the .GSD and any accompanying .BMP icon files, run HW Config → Options → Install GSD File → Browse. The IM 318 will appear under "PROFIBUS-DP → Other Field Devices → ET 200U." If the GSD is missing or unrecognized, the HW Config will silently omit the station and the DP master will report zero distributed stations.

8.3 Slot Mapping for IM 318-8MB12

Slot ordering must mirror the physical mounting order on the ET 200U backplane. Reordering slots will not prevent start-up but will misalign the process image and produce apparently "shifted" I/O data.

9. ET 200M as Drop-In Alternative

If the ET 200U cannot be brought online after exhausting the procedure above, the ET 200M is a functionally equivalent modern replacement. ET 200M uses an IM 153 interface module (e.g., 6ES7153-1AA03-0XB0) and accepts S7-300 I/O modules, which are pin-compatible with many S5-100U footprints when wired through a Siemens termination panel.

For new deployments, Siemens recommends the ET 200SP or ET 200AL families. The ET 200SP can be equipped with its own CPU (e.g., CPU 1510SP-1 PN or CPU 1512SP-1 PN), supporting up to 65 modules of the S7-1500 I/O family.

References:

• ET 200SP with own CPU module — TIA Portal documentation
• Siemens SIMATIC ET 200 AL Series Field I/O — Mouser catalog

10. Extended Troubleshooting Matrix

11. Frequently Asked Questions