Configuring PROFIBUS PA on S7-300 with CP 342-5 and Cerabar S

Overview

Integrating a single PROFIBUS PA field device (Endress+Hauser Cerabar S differential pressure transmitter) into a laboratory panel that otherwise uses 4-20 mA loop-powered devices on a National Instruments DAQ master requires an explicit understanding of the PROFIBUS physical-layer hierarchy. PROFIBUS PA cannot be wired directly to a Siemens S7-300 CPU: the host speaks PROFIBUS DP on RS-485 while the transmitter speaks PROFIBUS PA on MBP (Manchester Bus Powered, IEC 61158-2). A DP/PA coupler performs the media conversion; a CP 342-5 module on the S7-300 acts as the DP master for the lower-speed PA segment; and a GSD file lets STEP 7 recognize the Cerabar as a slave with a defined cyclic I/O map.

This article walks through the full bill of materials, bus topology, GSD installation in STEP 7 (or TIA Portal for S7-300 retrofits), CP 342-5 commissioning, DP/PA coupler selection, MBP bus termination, FieldCare parameterization, and the verification steps that confirm a clean cyclic pressure value before that value is scaled to a 4-20 mA output to the NI server's DAQ card.

Prerequisites

Before commissioning, confirm that the following hardware, software, and documentation are on the bench.

PROFIBUS Architecture: DP versus PA

PROFIBUS separates physical layer from application profile. Two physical layers matter for this project.

A DP/PA coupler terminates the DP segment, regenerates the bit stream, and presents a separate MAU for the PA segment. The coupler itself is invisible to the DP master (in the transparent variant) or appears as a DP node that owns a virtual PA proxy (in the Link variant). The S7-300 with CP 342-5 must be configured to use the coupler at a specific DP slave address, and the Cerabar S must have its own PA address (default 126) on the PA side.

Network Topology and Bill of Materials

The complete panel-level bus layout is illustrated in the following inline schematic.

For a single PA device, the simplest variant is a transparent DP/PA coupler. The S7-300 is configured to have one DP slave (the coupler) with a single PA proxy slot; the Cerabar appears as a sub-module slot beneath the coupler proxy. The Siemens TIA Portal V20 documentation confirms this configuration pattern for S7-300/400/1500 DP masters in the Configuring PROFIBUS DP (S7-300, S7-400, S7-1500) reference.

Installing the Cerabar S GSD in STEP 7

Before opening HW Config, install the manufacturer GSD. The GSD file describes the device identity, supported baud rates, slot count, and the structure of the cyclic I/O data block.

1. Download the Cerabar S GSD. For the E&H Cerabar S family, the GSD is named EH_PMC51_xxxx.gsd or eh3x15bd.gsd depending on firmware. The exact filename and revision are listed on the device's data plate and in the device's W@M product page. Siemens HW Config recognizes the same GSD file because PA profile 3.0 is vendor-neutral at the slot level.
2. In STEP 7, open HW Config. Choose Options > Install GSD File and browse to the directory containing the GSD. The .gsd, .gsg (bitmap), and any .gse extension files must be in the same folder.
3. Restart HW Config so the catalog tree refreshes. The Cerabar appears under PROFIBUS PA > Sensors > Pressure.
4. In TIA Portal, the equivalent path is Options > Manage general station description files (GSD). The device is then available in the hardware catalog under Other field devices > PROFIBUS PA.

Configuring the S7-300 Hardware in HW Config

Build the station in the following order.

1. Insert the S7-300 rail and add the power supply (PS 307, 6ES7307-1EA01-0AA0).
2. Add the CPU (e.g. CPU 315-2 DP 6ES7315-2AH14-0AB0). Set the integrated DP port as a master. The default DP address of the CPU's integrated port is 2. If the CP 342-5 is used as the DP master instead, leave the CPU port passive or unconfigured.
3. Insert CP 342-5 (6GK7342-5DA03-0XE0 or current 6GK7342-5DF00-0XE0) in slot 4. Open its properties, set Operating Mode to DP master, and assign DP address 1. Confirm baud rate: the DP side of the network must be set to 1.5 Mbit/s or 12 Mbit/s to keep the bus scan short; the DP/PA coupler will adapt to 31.25 kbit/s on the PA side automatically.
4. Drag the DP/PA Coupler from the catalog (PROFIBUS PA > Gateway > Siemens > DP/PA Coupler) onto the DP subnet. Assign DP address 3. In its properties, set the Number of connected PA devices to 1.
5. Open the coupler proxy slot, then drag the Cerabar S from the catalog into the slot. Assign PA address 126 (factory default). If the panel will host more devices later, give it address 3 or 4 to leave headroom.
6. Compile and save the configuration. STEP 7 generates a system data block (SDB) containing the bus parameters: HSA (Highest Station Address), TTR, TSL, min TSDR, max TSDR, and baud rate. Default values for 1.5 Mbit/s and HSA 126 are normally acceptable for a 1-device PA segment.

Siemens provides this same configuration flow in the TIA Portal V20 reference for PROFIBUS DP, with the S7-300 added as a "legacy" station if the project was migrated from STEP 7 V5.5.

CP 342-5 Module Configuration Details

The CP 342-5 runs its own protocol stack and exchanges data with the CPU via I/O areas rather than cyclic S7 communication. In the CP properties dialog:

• Mode: DP master (mode 1). The CP token-masters the segment, the CPU's integrated port should be removed from the DP network or set to passive.
• Diagnostic addresses: Accept default base of 2048 for diagnostics (e.g. PE 2048).
• Slot 0 / 1 / 2 / 3: Configure one input slot of 5 bytes (4 bytes IEEE-754 floating-point + 1 status byte) and one output slot of 1 byte. The default PA Profile 3.0 mapping places the process variable (PV) in the first 4 bytes of the input and the status in byte 5.
• Time-out: 1500 ms. PA is intrinsically slow; if you set the time-out to the DP default of 100 ms the master will report slave diagnostic "station not ready".
• Ident number: Match the GSD. The Cerabar S GSD ident number is 0x0E0F (or vendor-specific). STEP 7 enforces matching identifiers before bus start.

For the FC blocks that move data between the CP and the CPU, install the CP 342-5 / CP 343-5 function blocks from the SIMATIC NET library:

• FC1 DP_SEND — writes CPU-side output area to the CP for transmission to slaves
• FC2 DP_RECV — reads the input area assembled by the CP from the slaves

Call DP_RECV in OB1 with a 6-byte receive area:

CALL "DP_RECV" (FC2)
  CPLADDR   := W#16#100   // logical base address of CP 342-5
  RECV      := P#DB100.DBX0.0 BYTE 6
  NDR       := M10.0
  ERROR     := MW12
  STATUS    := MW14
  DPSTATUS  := MB16

The receive area contains 4 bytes PV (REAL), 1 byte status, 1 byte reserved. Byte 5 carries the PA Profile 3.0 quality code: 0x80 = good, 0x40 = uncertain, 0x00 = bad.

DP/PA Coupler Power, Wiring, and Bus Termination

The DP/PA coupler has two sides:

• DP side (9-pin D-sub): Pin 3 RxD/TxD-P (red), pin 8 RxD/TxD-N (green), pin 5/6 ground/shield. The 220 Ω + 390 Ω termination network is internal to a Siemens DP connector and is enabled by sliding the switch in the connector housing.
• PA side (terminals PA+ and PA-): Accepts the blue IEC 61158-2 cable. The coupler supplies 24 VDC at up to 400 mA to the PA trunk; the Cerabar S draws approximately 10-20 mA.

Wiring rules per AutomationDirect H2-PBCM Installation Manual and IEC 61158-2:

1. Enable termination only at the two physical ends of each segment. For the DP side, that means the connector on the S7-300 end and the connector inside the DP/PA coupler. The middle connector on the CPU's PG port must be set to "OFF" unless the S7-300 is at a bus end.
2. For the PA side, attach one active terminator at the DP/PA coupler output and one at the end of the cable run near the Cerabar S. The terminator is R = 100 Ω + C = 1 µF across PA+/PA-, not the simple 220 Ω resistor used on DP.
3. Use the blue cable (Siemens 6XV1830-3FH10) for the entire PA run. Maintain ≥ 200 mm separation from VFD power cables and ≥ 100 mm from any cable carrying > 2 A.
4. Ground the cable shield at the cabinet entry gland on one side only (the coupler end) to avoid ground loops.

Cerabar S PA Address and FieldCare Setup

Change the PA address from the default 126 to a project-unique value using either the local pushbuttons on the display module or via FieldCare over the PA segment. The display module lives in the head of the Cerabar and accepts a 4-digit numeric tag plus the bus address.

1. Power up the PA segment. The Cerabar S powers on, displays the boot screen, and shows the current PA address (default 126).
2. From the display menu: Setup > Communication > PROFIBUS address. Change from 126 to the slot address you assigned in HW Config (for example, 3).
3. To verify with FieldCare, connect the engineering PC to the DP/PA coupler or the DP bus using a PROFIBUS PC adapter cable (e.g. 6GK1571-1AA00). Launch FieldCare, scan the PA segment. The Cerabar appears as a DTM (e.g. "Cerabar S PMC51 PA"). Open the device DTM to verify the live PV, the engineering units (typically mbar or Pa), the low-flow cutoff, and the PA profile revision.
4. Set the desired units and lower/upper range values (LRV/URV). These map into the cyclic input data as a normalized 0-100% value; the scaling is local to the device, the master only sees the linearized 0-100% PV plus a status byte.

Siemens ships FieldCare with the device DTMs pre-installed; if the Cerabar DTM is not visible, run the Endress+Hauser Device Library installer (DTM Library Setup) to register the device family.

Scaling PA Process Value to a 4-20 mA Output

The CP 342-5 input area contains a 4-byte IEEE-754 REAL. Because PA Profile 3.0 transmits a normalized 0-100% PV, the Cerabar has already linearized and ranged the value. To drive the existing NI DAQ card, scale the PA PV to a 0-27648 integer that maps to a 0-20 mA span on the SM 334 (6ES7334-0CE01-0AA0) or SM 332 (6ES7332-5HD01-0AB0) analog output.

// OB1 - read PA PV from CP 342-5 receive buffer, scale to 0-27648
L DB100.DBD 0     // PA PV as REAL (already linearized 0-100%)
L 27648.0
*R
T MD 200          // scaled REAL
ROUND
T MW 220          // integer to write to analog output via PQW

Write MW220 to the SM 334 PQW using the standard peripheral-write instructions. Set the analog output channel to "4-wire current, 4-20 mA" in the hardware configuration. If the panel uses a 0-10 V output to the DAQ card, divide by 2.7648 and round to 0-27648 for the 0-10 V SM 332 range.

For status hand-off, copy the status byte from DB100.DBB 4 into a flag (e.g. M30.0) so that LabVIEW on the NI host can detect a "bad" or "uncertain" PA quality and either zero the loop or freeze the last good value.

Verification Procedure

1. Bus scan: Open STEP 7 > PLC > PROFIBUS > Diagnose. Confirm the CP 342-5 reports "Bus OK" and the DP/PA coupler is in "Data Exchange".
2. Slaves online: In the diagnostic view, the Cerabar S must show green with state "Data Exchange". If it shows yellow, the bus is connected but a parameter check has failed; if red, the slave is offline.
3. Live value: Open VAT (Variable Table) and monitor DB100.DBD 0. Apply a known pressure to the Cerabar HART module port (or use the built-in simulation mode in FieldCare, Simulation > Process Variable > Set Value 50.0%). The 4-byte REAL should respond within one DP cycle (~ 1 s on a 1.5 Mbit/s bus).
4. Loop continuity: Disconnect the 4-20 mA output and read the loop with a Fluke 789 or equivalent. Confirm 4 mA at 0% pressure and 20 mA at 100% pressure. Confirm reverse action if the controller is configured for reverse-acting.
5. PA quality byte: In VAT, monitor DB100.DBB 4. With a healthy value it should read 0x80 (good). If it reads 0x40 (uncertain) during normal operation, check the engineering units and the LRV/URV settings in the Cerabar DTM.
6. End-to-end: In LabVIEW on the NI host, read the analog input channel. Compare against the pressure applied to the Cerabar. Acceptable tolerance is 0.1% of the URV plus the D/A drift of the SM 334.

Troubleshooting Matrix

Notes for S7-1500 / TIA Portal Users

Siemens has shifted the configuration reference for PROFIBUS DP to TIA Portal V20. The S7-300 family is still supported as a "legacy" station within a TIA project, and the same SDB-based bus parameters are calculated by the configurator. The S7-1500 ET 200MP PROFIBUS CM (6GK1543-1AX02) or the S7-1500 built-in DP port can act as a master for the same DP/PA coupler. If the panel is later modernized to an S7-1500, the Cerabar PA configuration in TIA Portal imports directly from the GSD; the ladder remains, but OB1 cyclic I/O calls are replaced with distributed I/O (PROFIBUS PROFINET-style addressing), eliminating the CP 342-5 function blocks. The Rockwell PanelView Plus side of the same architecture is covered in the Rockwell 2711-UM005 PanelView PROFIBUS DP manual, useful for a future migration to a PanelView Plus 7 with a PROFIBUS DP option card.

Frequently Asked Questions