Siemens Profibus PA Hardware: S7-300 + ET 200M Selection Guide

Overview

This technical reference validates and expands a Siemens bill of materials for a remote monitoring installation that reads 25 PROFIBUS PA instruments and 5 4..20 mA analog devices. The architecture combines a SIMATIC S7-300 CPU 314C-2PN/DP, an ET 200M distributed station fronted by an IM 153-2 High Feature interface module, an FDC 157-0 DP/PA field-device coupler, a PS307 / SITOP Modular 24 V supply tree, PROFIBUS FC Process cable, SplitConnect taps, SIMATIC PDM V8.x, STEP 7 Professional 2010 SR4, and WinCC flexible 2008 Advanced for HMI. The complete topology is one PROFIBUS DP segment that terminates at two FDC 157-0 couplers, each driving one PROFIBUS PA segment with multiple SplitConnect taps.

PROFIBUS PA is defined by IEC 61158-2 and operates on the 31.25 kbit/s Manchester Bus Powered (MBP) physical layer. The bus carries both data and 24 V device power on the same twisted pair, which is why the DP/PA coupler (FDC 157-0) supplies a regulated 31 V at up to 1000 mA to the segment while isolating the PA segment from the DP master. For an automation cell of this size the FDC 157-0 is the right component; the alternative FDC 157 (without the trailing 0) is the older, Ex-restricted revision and is generally superseded by the FDC 157-0 in non-Ex installations. An active field distributor (AFD) is only required if you need star topology with more than the linear trunk the FDC supports.

System Architecture and Topology

The control cabinet architecture is intentionally simple. The S7-300 CPU hosts a DP master on its integrated PROFIBUS interface (X2), which is wired directly to the IM 153-2 High Feature in the ET 200M. The ET 200M backplane hosts two BM DP/PA bus units; the FDC 157-0 plugs into one of those bus units, isolating the PA segment and converting it to a logical extension of the same DP line. Each PA segment is wired with PROFIBUS FC Process cable and terminated with SplitConnect terminators; instruments are connected through SplitConnect taps so that a device can be removed without breaking the trunk.

CPU Selection: S7-300 CPU 314C-2PN/DP

The 314C-2PN/DP compact CPU is a balanced choice for a 25-instrument PA plant when no PCS 7 library blocks are required. The integrated PROFIBUS DP master on X2 is used to drive the ET 200M, the FDC 157-0, and (via the PA segments) all 25 PA field devices. The two PROFINET ports are typically reserved for the HMI panel connection and the engineering station; this BOM does not include a Comfort panel, but the PN ports are useful even with WinCC flexible 2008 if you later migrate to TIA Portal / WinCC Professional.

If the project later grows above 8 DP slaves, or if the OB1 cycle needs more headroom for the PA cyclic data exchange, the next logical step is the 315-2 PN/DP. The order code suffix EH14 denotes the hardware release 14 with PROFINET V2.3 and supports iDevice, MRP, and IRT, all of which are not strictly required for this architecture but useful for future PROFINET migration of the analog devices.

Memory Card and Working Memory

The MMC is mandatory for S7-300 CPUs from the 31x series onward; the load memory is no longer built in.

Distributed I/O: ET 200M with IM 153-2 High Feature

The ET 200M in this BOM acts as the physical home for the FDC 157-0 and (optionally) the SM 331 analog module used for the five 4..20 mA instruments. The IM 153-2 High Feature variant is required because the standard IM 153-1 does not support the active bus module BM DP/PA that the FDC 157-0 needs.

The IM 153-2 High Feature supports up to 12 I/O modules, supports S7 redundancy in HF mode (with two IM 153-2 HF in parallel, requires STEP 7 redundancy block library), and supports value status / time stamping with the corresponding PROFIBus profile. None of these HF features are required for a non-redundant monitoring station, but the IM 153-2 HF is still the correct choice because the standard IM 153-1 (6ES7153-1AA03-0XB0) physically cannot host an FDC 157-0.

DP/PA Coupling: FDC 157-0 Field Device Link

The FDC 157-0 (Field Device Coupler) is the translator between the PROFIBUS DP segment at 9.6 kbit/s to 12 Mbit/s and the PROFIBUS PA segment at fixed 31.25 kbit/s. It is not a passive media converter; it has its own address on the DP segment and is configured in STEP 7 Hardware Configuration just like any DP slave. From the DP master's point of view, every PA device behind an FDC appears as a separate DP slave on the same DP line; the FDC itself occupies one DP address.

For 25 PA devices you do not need two FDCs from a current-budget point of view; the FDC 157-0 can supply up to 1000 mA, and a typical PA instrument draws 10 to 20 mA. 25 devices × 20 mA = 500 mA, well inside the 1000 mA budget. However, the original BOM lists two FDC 157-0 units, which is still a reasonable choice if you split the field devices into two physical segments (one per process area) for faster troubleshooting and a shorter bus cycle. Each FDC occupies one DP address and limits the cyclic data exchange to the PA devices on its segment.

Power Supply Architecture

The control cabinet needs two distinct 24 V rails: a clean, regulated 24 V for the S7-300 / ET 200M / FDC 157-0 logic, and a separate SITOP rail for the analog instruments and any third-party loads. Using a separate SITOP rail for the analog instruments prevents ground loops and noise from corrupting the 4..20 mA signals.

Power budget (24 V DC cabinet rail):

1. CPU 314C-2PN/DP: typically 0.6 A @ 24 V (from datasheet).
2. IM 153-2 HF: 0.6 A @ 24 V typical, peak 1.0 A.
3. FDC 157-0: 0.6 A @ 24 V plus the PA segment current (up to 1.0 A on the PA side).
4. SM 331 (1KF02) AI8: 0.2 A @ 24 V (transducer feed excluded).
5. SplitConnect Tap 6GK1905-0AA00: negligible (~10 mA each, for active terminator electronics if used).
6. Total logic rail budget (worst case): 0.6 + 1.0 + 0.6 + 0.2 = 2.4 A.
7. Available headroom on PS307 (10 A): 10 - 2.4 = 7.6 A, sufficient.

The PA segment supply current is provided by the FDC 157-0 itself, not by the 24 V rail directly. The FDC accepts 24 V on its ET 200M backplane feed and generates the regulated 31 V needed for MBP. Therefore the SITOP Modular in the BOM is intended for the non-PA loads (analog instruments, relays, fans, lights), not for the FDC. This is a common point of confusion that should be flagged during cabinet design review.

PROFIBUS PA Network Components

PROFIBUS PA requires two terminators per segment, one at each end of the trunk. With a linear topology, the SplitConnect Terminator (6GK1905-0AE00) is the correct component to terminate the remote end of the trunk; the FDC 157-0 already contains the termination for the cabinet end internally. If you use the SplitConnect Tap on both ends, you also need a terminator block. The 6GK1905-0AA00 tap itself is a passive junction with no internal termination.

PROFIBUS PA Trunk Length Calculation

PROFIBUS PA (31.25 kbit/s) supports a maximum trunk length of 1900 m using Type A cable (PROFIBUS FC Process) when no repeaters are used and the cable parameters meet the IEC 61158-2 specification (impedance 100 Ω ± 20%, loop resistance ≤ 44 Ω/km, attenuation ≤ 3 dB/km at 39 kHz). The original BOM lists 1250 m of FC Process cable, which is comfortably within the 1900 m budget and leaves ~650 m for installation tolerances and spare.

Spur length (from the trunk tap to the field device) is limited by the IEC 61158-2 MBP rules:

For 25 devices you must keep each spur ≤ 1 m, which means physical layout must be a daisy-chain rather than a star. If the field layout forces star topology, the correct alternative is an AFD (Active Field Distributor, 6ES7157-0AG00-0XA0), which repeats the bus and removes the spur-length restriction.

Engineering Software Stack

Use the TIA Selection Tool to validate the entire BOM before placing the purchase order; the tool flags invalid combinations (e.g., wrong bus unit for the FDC, missing active bus module) and exports an HWCN export that STEP 7 can import.

SIMATIC PDM Integration for PA Device Commissioning

SIMATIC PDM is the Siemens Device Manager and is mandatory for proper commissioning of PROFIBUS PA instruments. The integration steps with STEP 7 V5.5 are documented in detail in the Siemens support article Configuring field devices with SIMATIC PDM V5.2+SP1 and Hardware Configuration in PCS 7 V6.0 (the procedure for non-PCS 7 STEP 7 V5.5 is identical). The lifecycle steps for each PA device are:

1. Import the device EDD: PDM > Device Integration Manager > install the Endress+Hauser EDD package (or import the GSD if no EDD is available).
2. Insert the device in HW Config: right-click on the FDC 157-0 > "Insert PROFIBUS PA Slave" > choose the device from the catalog. The device gets a PROFIBUS address automatically; you can edit it.
3. Open the device in PDM: right-click > "Open Object" (or the "Open in PDM" toolbar button). PDM connects to the live device over PROFIBUS.
4. Assign the tag: set TAG, descriptor, message, and unit parameters under "Identification".
5. Parameterize: configure measurement range, damping, output scaling, and HART-style device variables.
6. Download: "Download to device" writes the parameters into the PA instrument's non-volatile memory.
7. Diagnostic: use "Online > Diagnostics" to read device status, last calibration date, simulation mode, and write-protected parameters.

When a device is replaced, PDM retains the parameter set as the "plant view" assignment, so a new physical device can be brought into service with a single "Load to PG" / "Download to device" cycle. This is one of the strongest arguments for using PDM in a plant with 25 PA devices.

PROFIBUS PA Network Configuration in STEP 7

Configuration of the PA segment in STEP 7 V5.5 follows a few non-obvious rules; the official procedure is summarized in How do you configure a PROFIBUS PA network in STEP 7?. Key points:

• PA slaves are always connected to the DP/PA coupler (FDC 157-0) inside HW Config; you cannot attach them directly to a DP line.
• The FDC 157-0 occupies one DP address and is configured with the GSD file SI08157.GSD (or later SIEM8157.GSD depending on STEP 7 release).
• Each PA device is configured individually under the FDC node as a sub-slave, with its own address (1..125).
• The bus cycle time on a PA segment is dominated by the number of devices and the number of cyclic input/output bytes per device; with 25 PA devices at 4 bytes input + 2 bytes output each, the typical acyclic + cyclic cycle is 1..2 seconds. If the application requires faster update, reduce the number of devices per FDC.

The PROFIBUS DP segment between the CPU and the IM 153-2 runs at 1.5 Mbit/s by default (the highest baud that all devices in this BOM support), giving a DP cycle well below 50 ms. The PA cycle is independent of the DP cycle and is the dominant latency for the operator screens.

4..20 mA Device Integration via SM 331

The five 4..20 mA instruments that are not PROFIBUS PA are brought into the S7-300 by an SM 331 analog input module. The original BOM lists the 6ES7331-1KF02-0AB0, which is an 8-channel, 13-bit resolution analog input module with two-wire transducer feed.

For only five 4..20 mA channels the SM 331 is over-provisioned; an alternative is the 6ES7331-1KF01-0AB0 (8 AI, 12-bit) if 13-bit is not required, or the 6ES7331-7KB02-0AB0 (2 AI, 9/12/14-bit) if you need only two channels and want the highest precision per channel. The original BOM choice is correct and gives you three spare channels.

Engineering Network Stack: WinCC flexible 2008 vs WinCC Professional / TIA Portal

WinCC flexible 2008 Advanced is supported with STEP 7 V5.5 and runs on Comfort Panels, Panels, and IPC-based Runtime. The licensing model is per seat (developer license) with a runtime license per panel. For a remote monitoring station the minimum configuration is:

• WinCC flexible 2008 Advanced Engineering (6AV6613-0AA51-3CA5): one developer seat.
• WinCC flexible /Archives option (6AV6618-7ED01-3AB0): enables tag and alarm logging on the panel or on a PC runtime.
• Runtime license: per-panel runtime license (e.g., 6AV6613-1AA51-3CA5 for a 10" Comfort panel).

The official communication documentation for WinCC flexible 2008 with S7-300 is in two parts (Communication Part 1 and Communication Part 2) on the Siemens support portal. Both documents must be read before commissioning because the protocol selection (PROFINET, PROFIBUS, MPI/DP) and the connection resource configuration in the S7-300 are interlinked.

If a later migration to TIA Portal / WinCC Professional is anticipated, consider keeping the project in STEP 7 V5.5 format for now; WinCC flexible 2008 projects can be migrated to TIA Portal with the WinCC flexible migration tool. A direct TIA Portal commissioning from day one is also valid and recommended for new projects, but it requires WinCC Professional (not WinCC flexible) and a TIA Portal STEP 7 license.

Verification and Commissioning Checklist

1. Mechanical installation: verify the ET 200M rail length matches the number of bus modules; the 483 mm rail (6ES7195-1GA00-0XA0) fits five active bus modules. Confirm the BM DP/PA (6ES7195-7HF80-0XA0) is plugged into the correct slot for the FDC.
2. 24 V wiring: verify the PS307 (10 A) feeds only the S7-300 logic. The SITOP Modular 20/40 feeds the analog instruments and any non-PA load.
3. PROFIBUS DP wiring: check the two PROFIBUS bus connectors (6ES7972-0BA12-0XA0): one at the CPU end (terminated, switch ON), one at the IM 153-2 end (terminated, switch ON). Measure 220 Ω between pins 3 and 8 with the device powered down — you should see ~110 Ω if exactly two terminators are active, or an open circuit if none.
4. PROFIBUS PA wiring: measure ~100 Ω between the two cores of the PA trunk with all devices disconnected. The FDC 157-0 already terminates one end internally; the SplitConnect Terminator (6GK1905-0AE00) is required at the far end.
5. Address plan: confirm no duplicate PROFIBUS addresses on the DP segment. CPU is master only (does not consume an address). IM 153-2 = DP address 3 (typical). FDC 157-0 #1 = DP address 4. FDC 157-0 #2 = DP address 5. PA devices = DP addresses 6..30 on each segment.
6. STEP 7 HW Config: download the HW configuration. Resolve any GSD/EDD warnings before testing cyclic IO.
7. PDM commissioning: for each PA device, load the EDD, assign the tag, parameterize the device, and download. Verify the value status (good / uncertain / bad) under PDM Online > Diagnostics.
8. SM 331 calibration: with all five 4..20 mA instruments disconnected, perform the SM 331 "Module Calibration" step in STEP 7 HW Config (right-click the module > "Calibrate"). This removes the module-specific offset.
9. WinCC flexible screens: download the project to the HMI. Verify each tag is updating with the expected value range. Simulate an alarm and confirm the alarm log is populated.
10. Long-term stability: run the bus for 24 hours, then check the CPU diagnostic buffer for any transient PROFIBUS errors. Even a single DP station failure on a clean bus indicates a wiring or termination problem.

Common Pitfalls and Configuration Corrections

Source Part Numbers and Default Quantities Summary

FAQ