Troubleshooting SIMATIC PDM LifeList Scanning and DP/PA Coupler Configuration for PROFIBUS PA Devices

In SIMATIC PDM, failures during LifeList scanning of PROFIBUS PA devices via a DP/PA Coupler occur when the DP bus baud rate is set incorrectly, missing device EDDs prevent identification, or incorrect coupling hardware is specified. When using a standalone DP/PA Coupler (FDC 157), the PROFIBUS-DP master bus speed must be set strictly to 45.45 kbps. To resolve detection failures, verify bus speed, ensure the device EDD is integrated via the Device Integration Manager, and set the transmitter address using PDM LifeList.

PROFIBUS PA Coupling Topologies: DP/PA Coupler vs. DP/PA Link

In process automation plants running SIMATIC PCS 7, PROFIBUS-PA (IEC 61158-2) networks provide power and two-way digital communication to field instruments (pressure, temperature, flow transmitters, and valve positioners) in hazardous and non-hazardous areas. Connecting these Manchester-encoded 31.25 kbps field devices to an RS-485 PROFIBUS-DP network can be accomplished through two fundamentally different physical layer architectures:
1. Standalone DP/PA Coupler (e.g., FDC 157-0 MLFB 6ES7157-0AC85-0XA0): Operates as a transparent physical layer converter without memory or protocol processing. It translates RS-485 electrical signals into Manchester Coded Bus Powered (MBP) signals. Because it contains no internal buffer, the entire PROFIBUS-DP bus must operate at 45.45 kbps. Attempting to run the DP master at 1.5 Mbps, 187.5 kbps, or 9.6 kbps will cause total communication failure.
2. DP/PA Link (IM 153-2 MLFB 6ES7153-2BA10-0XB0 + FDC 157 Couplers): Operates as a modular gateway and DP slave. The DP side operates at full high speeds up to 12 Mbps, while the IM 153-2 acts as an independent master on the lower PA segment at 31.25 kbps.

When an engineer opens SIMATIC PDM LifeList to scan an attached PROFIBUS PA instrument (such as a PR electronics 5350 temperature transmitter) and encounters detection errors or invalid baud rate warnings (e.g., 9200 / 9600 baud), the root cause is almost invariably a baud rate mismatch between the DP master and the transparent coupler, or an unassigned factory default address (Address 126).

Coupler and Link Architectural Comparison Table

Review the hardware and networking requirements between standalone couplers and DP/PA links:

Parameter / FeatureStandalone DP/PA Coupler (6ES7157-0AC85-0XA0)Modular DP/PA Link (6ES7153-2BA10-0XB0 + FDC 157)PROFIBUS-DP Baud RateStrictly fixed at 45.45 kbpsConfigurable up to 12 Mbps (Standard: 1.5 Mbps)Protocol TransparencyFully transparent (repeater behavior)Gateway / Slave proxy (isolates DP from PA)PA Segment Speed31.25 kbps MBP31.25 kbps MBPMax Number of PA DevicesUp to 31 devices (power limited, e.g., 1000 mA)Up to 64 devices across multiple coupler segmentsHW Config RepresentationNot explicitly parameterized; devices on DP busIM 153-2 parameterized; PA devices placed under linkPDM LifeList AccessDirect scan via CP 5611 / CP 5711 at 45.45 kbpsRouted scan via CPU / IM 153-2 slave addressRedundancy SupportCoupler redundancy supported with active busActive bus redundancy on both DP and PA sides

Procurement notice: Always verify bus coupler part numbers and intrinsically safe barrier ratings against authorized Siemens equipment schedules.

Configuring DP Bus Speed and Addressing for Standalone Couplers

To establish communication and perform LifeList scanning with a standalone coupler:

Verify PROFIBUS-DP Master Network Settings in HW Config:
* Open HW Config for the automation station.
* Double-click the PROFIBUS-DP master system line to open Properties - PROFIBUS.
* Under the Network Settings tab, check the transmission rate.
* If a standalone DP/PA coupler is connected directly to the CPU's DP port or CP 443-5, set the transmission rate strictly to 45.45 kbps.
* If high-speed DP devices (ET 200M, drives) share the bus, you cannot use a standalone coupler; you must install an IM 153-2 DP/PA Link to isolate the segments.
Execute LifeList Scan in SIMATIC PDM:
* Open SIMATIC PDM LifeList.
* In the connection options, select the local engineering communication interface (e.g., CP 5711 USB adapter MLFB 6GK1571-1AA00).
* Select protocol PROFIBUS-DP with baud rate 45.45 kbps.
* Start the LifeList scan across addresses 0 to 126.
Address Assignment for Factory Instruments:
* New transmitters from PR electronics, Siemens, or Endress+Hauser ship with default address 126.
* Devices at address 126 cannot enter cyclic process data exchange.
* In LifeList, right-click the detected device at address 126, select Assign Address, and assign a unique technological address between 1 and 125.
* Cycle 24 V DC power to the PA coupler if the instrument requires a power-on reboot to latch the new address.

Integrating Field Device Descriptions (EDD) via Device Integration Manager

Even when physical communication is healthy, SIMATIC PDM cannot configure or display instrument parameters if the Electronic Device Description (EDD) is missing:
1. Open the Device Integration Manager (DIM) utility from the Siemens PDM program group.
2. Download the certified EDD for the specific field instrument (e.g., PR electronics 5350 Profibus-PA EDD) from the device manufacturer or Siemens Industry Online Support.
3. In DIM, select Source > File > Open Source Path and browse to the extracted EDD folder.
4. Select the detected device profile, verify device revision compatibility with PDM V6.0 / V8.x, and click Install.
5. Once installation finishes, open SIMATIC Manager, select Options > SIMATIC PDM > Device Catalog > Update Device Catalog.
6. In HW Config or the PDM Process Device Plant View, insert the transmitter from the updated catalog.

Verification

Validate successful field device integration:
1. Run SIMATIC PDM LifeList: The PR 5350 transmitter must be discovered at its newly assigned operating address (e.g., Address 24).
2. Double-click the device in LifeList: SIMATIC PDM must open the device faceplate/parameter window without device description errors.
3. Read online parameters: Verify that live process values (temperature, sensor resistance, loop status) update cyclically in PDM.
4. Download HW Config to the controller and verify that the DP master establishes cyclic data exchange without bus fault (BF) LEDs.

Limitations and Operational Precautions

Operating an entire PROFIBUS-DP line at 45.45 kbps drastically reduces cyclic throughput and increases response times; never connect high-density remote I/O racks to a 45.45 kbps bus. Use a DP/PA Link instead.
Ensure total PA segment current does not exceed the coupler output limit (typically 400 mA for intrinsically safe Ex couplers 6ES7157-0AD82-0XA0 or 1000 mA for standard couplers 6ES7157-0AC85-0XA0).
Always verify that passive bus terminators (RC network: 100 ohm + 1 uF) are installed at both physical extremities of the PA trunk line.

Engineer FAQ

Why can't I set the PROFIBUS bus speed to 9.2 kbps for PA communication?

9200 baud is not a standard PROFIBUS speed. Standard PROFIBUS-DP speeds are 9.6 kbps, 19.2 kbps, 45.45 kbps, 93.75 kbps, 187.5 kbps, 500 kbps, 1.5 Mbps, up to 12 Mbps. For transparent PA couplers, 45.45 kbps is mandatory.

Can multiple DP/PA couplers be connected to one DP/PA Link?

Yes. Up to five FDC 157 couplers can be mounted behind a redundant pair of IM 153-2 link modules, providing separate PA branch segments.

What happens if two new PA instruments are connected at address 126 simultaneously?

Both devices will respond to address arbitration simultaneously, causing packet collisions and making both devices invisible in LifeList. Connect and address new instruments one at a time.