Resolving Siemens CP 342-5 Replacement Issues on S7-300 PROFIBUS

Problem Overview

A common field scenario involves replacing a Siemens CP 342-5 communications processor in an S7-300 station where a CPU 315-2DP drives AC motor inverters over PROFIBUS DP. After the spare module is installed, the hardware configuration is re-downloaded into the CPU and SF/BF faults clear, yet the connected drives refuse to run. The spare module is identical in catalog number but carries a different hardware revision (for example the original is labeled x/3, x/4, x/5 and the replacement is labeled x/5, x/6, x/7). Because no SF/BF LED is on and the CPU diagnostic buffer is empty, the fault appears to be "logical" rather than electrical.

This article isolates the most common root causes that match the symptoms described, namely: protocol-type mismatch (CP 342-5 versus CP 343-5), backplane-logon mismatch (CP 342-2 versus CP 343-2 P family), firmware defects on the CP 342-5 (6GK7342-5DA02-0XE0), and STEP 7 hardware catalog/version conflicts. It also documents the firmware update path to V5.7 and the verification procedure that confirms the replacement module is operational.

Identifying the Affected CP Family

Siemens designates two visually and electrically similar S7-300 communications processors that are frequently confused in the field. Confirming the exact catalog number (MLFB / order number) is the first step in any replacement diagnosis.

Hardware Revision Field Format

The Siemens hardware revision label printed on the front of the CP module follows the format ES/x/y/z where each digit is incremented when a hardware change is released. Example strings observed on 6GK7342-5DA02-0XE0 modules include:

• HW Rev 3 / FW 5.x — original production lots
• HW Rev 4 / FW 5.x — minor PCB revision
• HW Rev 5 / FW 5.x — pre-V5.7 firmware baseline
• HW Rev 6 / FW 5.7 — current production, shipped with V5.7
• HW Rev 7 / FW 5.7 — latest variant, ESD improvements

The revision alone is not a defect indicator. Any of the above revisions is functional with firmware V5.7 loaded, but revisions 3 through 5 will retain the older firmware unless explicitly updated. A change of revision between the old and new module is therefore not, by itself, a cause of the "drive will not operate" symptom.

Root Cause 1 — Protocol Stack Mismatch (CP 342-5 vs CP 343-5)

The CP 342-5 implements the PROFIBUS DP master/slave service classes required to exchange cyclic process data with SINAMICS, MICROMASTER, SIMOVERT, and SIMOREG drives. The CP 343-5, by contrast, is restricted to PROFIBUS FMS messaging and cannot generate DP-V0 or DP-V1 I/O telegrams. When a CP 343-5 is installed in a slot that is configured for a CP 342-5, the CPU completes initialization because the backplane handshake matches, but the drive network never sees a DP master, so slaves enter the wait-for-parameterization state and the CPU never receives any process-image updates from the drives.

Diagnostic steps:

1. Read the order number on the front of the new module and compare it byte-for-byte against the configured order number in the STEP 7 hardware catalog (HW Config → right-click the CP → Object Properties → Order Number).
2. Open SIMATIC Manager → Options → Module Information → Diagnostic Buffer. Look for the entry Module incorrect / module does not exist (event ID 0xE2B1) or Parameter assignment error (0xE2B4) — these entries appear only when the configured MLFB does not match the inserted module.
3. If the CPU reports no error and the CP is recognized as "OK," the protocol-stack mismatch (CP 343-5 in a DP slot) is the prime suspect. In that case re-reading the order number is mandatory because the front labels differ only in the third functional block (-5DA for DP, -5FA for FMS).

Root Cause 2 — Backplane Logon Mismatch (CP 342-2 vs CP 343-2 P)

Although this article focuses on the CP 342-5, the same symptom (fault cleared after download, drives still inoperative) also occurs when an AS-i master CP 342-2 is replaced by a CP 343-2 P. Siemens explicitly documents that a CP 343-2 P cannot substitute a CP 342-2 because the two AS-i masters log on to the CPU differently on the backplane bus. The slot-oriented registration mechanism used by the CP 342-2 is incompatible with the registration used by the CP 343-2 P, so the replacement module will not exchange AS-i process data with the CPU even though the hardware configuration downloads successfully and the diagnostic buffer remains empty.

Reference: Siemens Support entry "Replacing an AS-i master CP342-2 by a CP343-2" (ID 19423255). The same principle applies to the CP 342-5 / CP 343-5 pair on the PROFIBUS side: the backplane handshake succeeds, but the higher-layer service dispatcher (DP versus FMS) differs.

Root Cause 3 — CP 342-5 Firmware Bug (Repeated Download Issue)

Siemens documented a defect on the CP 342-5 (6GK7342-5DA02-0XE0) where a CP that had its configuration downloaded once would operate normally, but a repeated download of the hardware configuration caused the CP to remain in STOP mode sporadically. This is described in Siemens Support entry ID 19726245, with the corrected firmware available in entry ID 24104930.

Step-by-Step: Firmware Update to V5.7 on a Replacement CP 342-5

Use the following procedure to bring any 6GK7342-5DA02-0XE0 module — whether old or replacement — to the current firmware level. STEP 7 V5.5 SP4 or higher is required for the update file format.

Prerequisites

• STEP 7 V5.5 SP4 or later (V5.6 / V5.7 recommended) installed on the programming device
• Ethernet (TCP/IP) or MPI/DP connection between the programming device and the S7-300 station
• Online access to the target CPU (PG/PC interface set to the correct subnet)
• CP 342-5 firmware update file from Siemens Support ID 24104930 (.upd file for V5.7)
• Backup of the STEP 7 project (S7 program archive)

Procedure

1. Open the project in SIMATIC Manager and go online with the target station.
2. Select the CP 342-5 in HW Config and confirm the order number is exactly 6GK7342-5DA02-0XE0.
3. Right-click the CP → PLC → Firmware Update.
4. In the "Firmware Update" dialog, browse to the V5.7 .upd file downloaded from entry 24104930.
5. Confirm the dialog showing Current version: x.x.x.x → Target version: 5.7.0.0.
6. Click Execute Update. The CP will reset, the RUN LED will flash, and the update will take 60 – 120 seconds.
7. After the CP re-enters RUN, close the dialog and verify the firmware in Module Information → Firmware. It must report V5.7.
8. Re-download the hardware configuration to the CPU (target system → PLC → Download to Target Station).

Verification After Update

1. The CP RUN LED must be steady green and the SF/BF LEDs off.
2. In SIMATIC Manager, open Module Information → Diagnostic Buffer for the CP — no STOP or parameter-assignment entries should be present.
3. From the CP diagnostic buffer confirm DP master status: entry DP master has entered RUN.
4. Read the diagnostic buffer of each connected drive (SINAMICS or MICROMASTER) — it must report DP-V0 cyclic data exchange active or equivalent.

Step-by-Step: Replacing a CP 342-5 in the Field

1. Document the existing CP order number, hardware revision, and firmware version before removal.
2. Disconnect the PROFIBUS DP connector from the front of the CP.
3. Power down the S7-300 rack (or use hot-swap only if the rack configuration and PS 307 power budget permit; CP 342-5 supports module replacement without rack power-down on most S7-300 stations, but verify the CPU diagnostic buffer afterwards).
4. Remove the old CP and insert the spare.
5. Reconnect the PROFIBUS connector (verify the terminating resistor setting on the last node).
6. Power up the rack and observe the CP RUN LED sequence: flashing during backplane registration → steady green.
7. Download the hardware configuration to the CPU.
8. If the CP RUN LED does not reach steady green, repeat the firmware update to V5.7 procedure above.
9. Cycle the drive enable logic and verify that each inverter reports DP cyclic data exchange.

Diagnostic Buffer Event Reference

PROFIBUS DP vs FMS — Quick Discrimination Matrix

Troubleshooting Matrix

Verification Checklist

• Inserted CP order number exactly matches HW Config (6GK7342-5DA02-0XE0)
• CP firmware version = V5.7 (confirmed via Module Information)
• CP RUN LED steady green; SF and BF LEDs off
• CP diagnostic buffer shows Module has entered RUN (0x4301) and DP master has entered RUN
• CPU diagnostic buffer free of 0xE2B1, 0xE2B4, 0xE2B5 entries
• Each drive reports active cyclic DP communication
• PROFIBUS terminator enabled only on the first and last physical node of the segment
• Drive enable (control word 1, bit 0..3 = 0x047E → 0x047F) toggles from PLC
• Setpoint words from CPU arrive at drive (status word feedback consistent with sent control word)
• Field cabling shield bonded at one end to PE; PROFIBUS connector backshell grounded

Frequently Asked Questions