CP 342-5 PROFIBUS Loss After IM153-2 Added: Troubleshooting Guide

1. Problem Summary and Affected Hardware

An established S7 communication between a SIMATIC PCS 7 automation station and a remote S7-300 station fails the moment a new PROFIBUS DP node is added to the same physical segment. The reported system:

• Master station: PCS 7 AS, CPU 416-3 DP (typical article numbers 6ES7416-3XR05-0AB0, 6ES7416-3XS07-0AB0, or 6ES7416-3ER05-0AB0) on the integrated DP master port (X1).
• Remote partner: S7-300 station with CPU 314C-2DP (typical 6ES7314-6CG03-0AB0 or 6ES7314-6CH04-0AB0).
• Bridge module: CP 342-5 (typical 6GK7342-5DA03-0XE0, 6GK7342-5DA02-0XE0, or the FO variant 6GK7342-5DF00-0XE0) configured as a DP slave in the S7-300 rack at PROFIBUS address 17.
• New device: ET 200S distributed I/O station with IM 153-2 (typical 6ES7153-2BA10-0XB0, 6ES7153-2BA02-0XB0, or 6ES7153-2BB00-0XB0) at PROFIBUS address 54.
• Symptom: NetPro shows the FDL connection as "Is being set up"; no data is exchanged with the S7-300 station.
• Recovery: removing the IM 153-2 from the bus (power off or disconnect the bus connector) restores the FDL connection in a few seconds.

Because the link is fully operational before the new slave appears and is restored the instant it disappears, a hardware defect on the CP 342-5, on the CPU 314C-2DP, or on the S7-400 is ruled out. The disturbance is therefore caused by the addition of the IM 153-2 in one of three categories: physical-layer damage (bad connector, doubled terminator, open shield), bus-parameter or configuration incompatibility exposed by the new participant, or an S7-connection or CPU-resource issue that surfaces only when the new device is on the bus at the same time as the FDL connection is being established.

The IM 153-2 in the centre of the diagram is the change that coincided with the failure. Every check in this guide starts from that single fact: the new device is the trigger.

2. What "Is Being Set Up" in NetPro Really Means

The NetPro status text "Is being set up" (German: "Wird aufgebaut") is a generic S7-connection establishment state. It does not identify the cause; it only states that the connection establishment is in progress and the partner has not yet acknowledged. The standard S7 connection states in NetPro/STEP 7 are:

For an FDL connection through the CP 342-5, the S7-400 issues an FDL_SDA or FDL_SDN telegram to the configured slave address. If the CP 342-5 does not reply with a Data Acknowledge within the configured retry / slot-time budget, the connection stays at "Is being set up" indefinitely. The S7-400 will keep retrying, so a permanent state of "Is being set up" is a strong signal that the partner is not on the bus at the expected PROFIBUS address, or that the bus itself is too unstable to deliver the response.

3. Root-Cause Matrix

The following table ranks the most common causes observed in PCS 7 installations that match the symptom above. Probability is based on field frequency:

4. PROFIBUS Physical-Layer Diagnostics

Because the failure appears immediately when the new node is physically connected, the physical layer is the first thing to check. Apply the sequence below.

4.1 Connector and Termination Inspection

Open every PROFIBUS connector on the segment that was touched during the IM 153-2 installation. Verify:

1. The termination switch is ON only at the two end nodes of the bus segment and OFF on all intermediate nodes. With the IM 153-2 inserted mid-segment, the IM 153-2 connector must have the switch OFF.
2. The shield clamp is engaged and the drain wire is making contact with the cable shield (terminal 3 / shield clamp).
3. The cable screen is continuous across any T-piece or new connector; do not "snip" the shield at a connector.
4. Incoming and outgoing cores are on the correct terminals: green on pin 3 (B-line), red on pin 8 (A-line) for the older convention; for the 9-pin sub-D used in PROFIBUS, A is pin 3 and B is pin 5 (note that the connector pin assignment differs from the wire colour convention).

4.2 Resistance Measurement

Power down the entire segment. Measure the resistance between pins 3 (B) and 8 (A) of the 9-pin sub-D at one end of the segment:

4.3 Bus Segment Length and Stub Budget

Compare the longest stub length to the table below. If the new IM 153-2 was added via a long stub, the bus may remain functional at low baud rates but fail at higher rates:

4.4 Live Bus Scan with CP 5611 / CP 5613

With a CP 5611 (PCI) or CP 5613 (PCIe) installed in an engineering PG/PC, run Set PG/PC Interface → CP5611(PROFIBUS), then start SIMATIC Manager → Options → PROFIBUS Diagnostics or the modern PRONETA tool from Siemens. The tool will report all active slaves, their address, the bus timing, signal quality per slave, and any repeated telegram errors. PRONETA is available on the Siemens Industry Online Support site.

5. Bus-Parameter and Topology Analysis

If the physical layer is healthy, the next step is to inspect the bus parameters. In STEP 7 / PCS 7, open HW Config → double-click the PROFIBUS subnet → Properties → Bus Parameters. The PCS 7 default profile is DP at 1.5 Mbit/s. When you insert a new device without loading the configuration to that device, the master may keep the old profile while the new device tries to negotiate a different one.

6. GSD File and Master Configuration Check

The IM 153-2 exists in multiple firmware versions; the GSD file must match. Article numbers you may encounter:

• 6ES7153-2AA02 — early version, not recommended for PCS 7
• 6ES7153-2BA01 / 6ES7153-2BA02 — common version 1
• 6ES7153-2BA10 — current "standard"
• 6ES7153-2BA70 — high-feature
• 6ES7153-2BB00 — "BA" profile with F-modules
• 6ES7153-2NA00 — IM 153-2 for redundant configuration

Install the GSD file that matches the article number on the device label. With the GSD imported, drag the IM 153-2 from the catalog (folder PROFIBUS DP → ET 200S) onto PROFIBUS address 54. Configure the I/O modules to match the actual sub-modules on the rail; mismatches generate diagnostics in the AS during start-up and may hold the bus in "is being set up" while the master retries parameterisation. The matching device manual is available in the Siemens Industry Online Support portal.

After configuring, perform a full Station → Save and Compile and download the new HW Config to the CPU 416-3 DP. The configuration change to the bus parameters is only active after the master has been reloaded.

7. S7-Connection and CPU Resource Audit

7.1 CP 342-5 Connection Limits

The CP 342-5 supports a maximum of 16 S7 connections (or 32 with the latest firmware revision of the 6GK7342-5DF00 / 5DA03). Once this limit is reached, new connections stay at "Is being set up" because the CP cannot allocate a free job. Open the CP 342-5 diagnostic buffer in STEP 7 and check for the entry "No more resources" (SFB / SFC error W#16#80C3 / DS 0xC3).

7.2 CPU 314C-2DP Connection Limits

The CPU 314C-2DP supports a maximum of 16 S7 connections. The on-board DP port counts as one, and each FDL connection through the CP 342-5 counts as another. The diagnostic buffer entry "No connection resources" indicates exhaustion. CPU 314C-2DP operating-mode documentation is part of the S7-300 CPU 31xC manual set on the Siemens Industry Online Support portal.

7.3 CPU 416-3 DP Connection Limits

Depending on the order number, the CPU 416-3 supports between 64 and several hundred S7 connections; the resource budget is normally not the limiting factor in PCS 7 unless the AS is also serving as the WinCC server and the connection limit is shared between OS / BATCH / route. Reference the S7-400 CPU Specifications manual for the exact budget of each article number.

7.4 Verifying the Connection Resources

Open the S7-300 station in NetPro, select the CP 342-5 in the rack, and review the connection table on the right-hand pane. If a previously working connection is missing from the downloaded configuration, re-run NetPro → Save and Compile → Download to target station for the S7-300 station. Many "Is being set up" cases are in fact caused by the partner not having the same connection list loaded.

8. Step-by-Step Diagnostic Procedure

Apply the steps in this order; stop as soon as the connection comes up.

1. Open the diagnostic buffer of the CP 342-5 and of the CPU 416-3 DP. Look for entries around the time the IM 153-2 was added. Typical entries: "Station failure" (event ID 0x0103), "Bus fault" (event ID 0x0200), "No connection resource" (event ID 0x0330).
2. Power off the IM 153-2 only (leave the cable connected). Check whether the FDL connection comes up. If yes, the issue is on the IM 153-2 side (GSD, address, module configuration). If no, the issue is at the new connector (wiring, termination).
3. With a CP 5611 / CP 5613 connected, run PRONETA. Confirm whether node 17 (CP 342-5) is visible after the IM 153-2 is plugged. If node 17 is missing from the scan, the new device is killing the bus.
4. Replace the new PROFIBUS connector on the IM 153-2 cable with a known-good one (for example 6ES7972-0BA12-0XA0). Verify termination OFF if the IM 153-2 is mid-segment.
5. Reduce the PROFIBUS baud rate to 500 kbit/s temporarily and re-test. If the connection comes up at 500 kbit/s, the issue is a signal-quality or stub-length problem and the segment must be rebuilt as a line (no stubs) or a diagnostic repeater (for example 6ES7158-0AD01-0XA0) must be inserted.
6. Compare the IM 153-2 DIP switch to the configured address 54. If the IM 153-2 is delivered with a different address (for example, 3 by default on some variants), change it.
7. Reload the S7-300 station (CPU 314C-2DP and CP 342-5) with the current NetPro connection list.
8. Reload the AS 416-3 DP with the updated HW Config and NetPro.
9. Recycle power on the IM 153-2 and watch the S7 connection state in NetPro. The state should transition from "Is being set up" to "Established" within a few seconds.

9. Corrective Actions and Re-Commissioning

Once the cause is identified, apply the corrective action that matches.

9.1 Physical-Layer Fix

• Strip and re-terminate the bus cable where the IM 153-2 was inserted.
• Use only 9-pin sub-D PROFIBUS connectors with integrated termination switch (Siemens 6ES7972-0BA12-0XA0, 6ES7972-0BB12-0XA0, 6ES7972-0BA42-0XA0 for 35° cable, 6ES7972-0BA52-0XA0 for vertical cable entry). Avoid using "FastConnect" connectors without a proper strain relief.
• Re-measure the segment resistance (see §4.2).

9.2 Topology Fix

If stubs are too long for the configured baud rate, rebuild the bus as a true line: use PROFIBUS FC standard cable (6XV1830-0EH10) and route the cable directly from the previous device to the next without T-pieces. Where a T-piece is unavoidable, insert a Diagnostic Repeater (6ES7158-0AD01-0XA0) or the DP/PA coupler (6ES7157-0AC00-0XA0) for PA segments.

9.3 Bus-Parameter Fix

Open HW Config → PROFIBUS subnet → Properties → "Use the same bus parameters for all nodes" (or set the profile to DP for every DP node). Click Recalculate to have STEP 7 compute conservative parameters (Tslot, Max Tsdr) for the new segment length. Refer to the SIMATIC S7-300 / S7-400 / ET 200 Commissioning manuals on the Siemens Industry Online Support portal for parameter details.

9.4 GSD Fix

Re-import the GSD from the latest Siemens support package, delete the old IM 153-2 instance, drag a new instance from the catalog, and re-assign the modules. Re-compile and download.

9.5 S7-Connection Fix

If the FDL connection through the CP 342-5 is still stuck after the physical and bus-parameter layer is fixed, open the FDL connection in NetPro → Properties → verify:

• Local endpoint = CP 342-5 in the S7-300 rack at the configured slot (slot 4 by default for CP 342-5 in HW Config).
• Partner endpoint = CP 342-5 (because FDL terminates at the CP, not the CPU).
• FDL address = 17 (the configured PROFIBUS address of the CP 342-5).
• Connection name matches on both stations; if you used two different names, STEP 7 may not link them.

Re-compile NetPro and download to both stations. PCS 7-specific connection commissioning is described in the SIMATIC PCS 7 Process Control System Commissioning manual on the Siemens Industry Online Support portal.

10. Verification and Acceptance Test

After the corrective action, run the following acceptance test:

1. Open NetPro. Confirm that the FDL connection is in the state "Established" (green).
2. Watch the diagnostic buffer of the CP 342-5 for 10 minutes. There should be no new "Bus fault" or "Station failure" entries.
3. Force the IM 153-2 power off and on (or pull the IM and re-insert). The FDL connection should remain established — the loss of the IM 153-2 should not affect the FDL connection to the CP 342-5.
4. Run the bus diagnostics tool (PRONETA or the legacy PROFIBUS Diagnostics) and capture a bus health report. Telegram errors per slave should be zero; retries should be in the single digits per million telegrams.
5. Cycle the AS and the S7-300 station; verify that the connection comes up on the first pass without manual intervention.

11. Preventive Measures for Future Expansion

• Document the bus topology (cable lengths, connector types, termination positions) before any expansion. The bus topology should be in the PCS 7 plant documentation and reviewed whenever a device is added or moved.
• Run the bus-parameter recalculation tool in HW Config every time the segment length or device count changes.
• Maintain a bus-spare kit: two PROFIBUS connectors (with and without PG socket), one FC standard cable reel, one FC stripping tool, one BT200 or BT-200-H tester.
• Use Diagnostic Repeaters (6ES7158-0AD01-0XA0) on long segments to localise future faults and to electrically isolate disturbed sub-segments.
• Use PRONETA for periodic bus health checks; archive the reports with the plant maintenance records.
• Train the maintenance crew to read the diagnostic buffer of the CP 342-5 and the CPU; the entries are explicit and remove the guesswork from "Is being set up" cases.

12. Frequently Asked Questions