PROFIBUS Daisy Chain Topology with Siemens IM 153-1 and S7-400

Overview

PROFIBUS DP (IEC 61158 / EN 50170) uses an RS-485 balanced differential physical layer that physically implements a linear bus, also called a daisy chain. Every node taps the same shielded twisted-pair cable; there are no point-to-point spurs in a correctly designed segment. This is fundamentally different from Ethernet, where stars and unmanaged taps are common. When integrating a Siemens S7-400 (e.g., CPU 414, CPU 416, CPU 417) with ET 200 distributed I/O over PROFIBUS, two architectural questions dominate the field-engineering design:

1. Do field devices (motor drives, smart transmitters, boiler controllers, third-party packages) require an IM 153-1 interface module, or can they connect directly to the PROFIBUS master?
2. Can multiple field packages share a single PROFIBUS segment terminated at one IM, or must each package get its own IM?

This reference answers both questions with the rules, electrical constraints, address limits, and commissioning checks a control engineer must apply. It assumes an S7-400 station acting as PROFIBUS DP Class 1 master and a Siemens ET 200 (IM 153-1) acting as a DP slave on the same shielded copper segment.

Physical Layer and Segment Limits

PROFIBUS DP segment parameters are fixed by the RS-485 transceiver specification documented in the PROFIBUS standard reference and the PI (PROFIBUS & PROFINET International) installation guidelines:

Why the Bus Is a Daisy Chain by Nature

RS-485 is a multi-drop bus, not a star. The signal travels along a single transmission line; each device is bridged onto that line through a bus connector that exposes two pairs of terminals (A1/B1 and A2/B2). The incoming line lands on one pair, the outgoing line continues from the other pair, and the device electronics tap the line through isolation transformers. The official Siemens PROFIBUS bus connector manual illustrates this clearly: the connector labeled A1/B1 always receives the cable coming from the previous station, and A2/B2 feeds the cable going to the next station.

Consequences for field design:

• No stubs: A T-branch with cable length is a stub and must be avoided. The exception is the very short active stub inside the bus connector itself, which is part of the device's transceiver and is not counted as a stub.
• One linear cable run: All devices must lie along a single cable path. If physical layout forces branches, use a PROFIBUS repeater (e.g., 6ES7972-0AA01-0XA0) or a DP/DP coupler (6ES7158-0AD01-0XA0) to extend or segment the network.
• Termination at exactly two ends: The cable must be terminated at both physical ends of the segment. Every station in the middle must have its terminator OFF.

Siemens IM 153-1: Function and Limits

The IM 153-1 is the interface module for an ET 200S distributed I/O station. It connects the ET 200S backplane bus (where up to 8 power, electronic, or motor-starter modules are plugged) to the PROFIBUS DP master via the RS-485 segment.

Key engineering constraints of the IM 153-1:

• The IM 153-1 consumes one PROFIBUS address from the segment budget of 126 stations.
• It supports up to 8 I/O modules on its backplane. This is the ET 200S slot limit, not a PROFIBUS limit.
• The modules on the backplane cannot exist standalone; without the IM, they have no communication path to the master. They are not slaves.
• The IM must be the physical termination of the PROFIBUS segment only if it sits at one end of the linear run. In the middle, termination is OFF.

Does Each Field Package Need Its Own IM?

The short answer: No, not for PROFIBUS topology reasons. The deciding factor is whether the field device already has a native PROFIBUS DP interface.

Case 1 - Field devices with native PROFIBUS interface

If a motor drive (e.g., Siemens SINAMICS G120 with CU240S PN/DP), a smart pressure transmitter (e.g., Endress+Hauser Cerabar / Deltabar with PROFIBUS PA or DP variant), or a boiler controller exposes a PROFIBUS DP port, it is a slave on its own. You connect it directly to the daisy chain segment:

1. Set a unique DP address on the device (1-125) using its local keypad or DIL switch.
2. Plug the incoming cable into A1/B1 and the outgoing into A2/B2 of its bus connector.
3. Set termination ON only at the two physical ends of the segment; OFF for every device in between.
4. In STEP 7 / TIA Portal, install the device's GSD file (e.g., SIEM8115.GSD for SIMOCODE, EH__0F38.gsd for Endress+Hauser) and configure its I/O slots in HW Config.

No IM 153-1 is required. The device is a peer of the S7-400 on the same DP segment.

Case 2 - Field devices without PROFIBUS interface (conventional I/O)

If a field package contains only discrete sensors, 4-20 mA loops, RTDs, or low-power contactors that do not speak PROFIBUS, then those signals must be marshalled onto the bus. The standard Siemens solution is the ET 200S station built around an IM 153-1, with the appropriate electronic modules (e.g., 6ES7131-4BF00-0AA0 digital input, 6ES7132-4BD30-0AA0 digital output, 6ES7134-4GD00-0AA0 analog input).

The question then becomes: should every package have its own IM, or can multiple packages share one IM?

• One IM per package is the safer rule. Each package can then have its own DP address, its own diagnostics, and its own hot-swap capability. The IM 153-1 supports module-level diagnostics, and STEP 7 presents each IM as a separate DP slave in the diagnostic buffer.
• Sharing one IM is technically allowed if the packages can be physically clustered. The IM 153-1 backplane supports up to 8 modules, and the cables between the field junction boxes and the IM can be as long as the IM's terminal-specific limits allow (typically tens of metres for standard digital I/O). However, this collapses all packages into one PROFIBUS station: one address, one diagnostic event per package, and one point of failure.

Field practice is to give each control package its own IM and its own DP address. The cost of an IM 153-1 (low hundreds of EUR) is much lower than the troubleshooting time when a single shared IM loses its backplane and takes down the whole package.

Designing the Daisy Chain for an S7-400 with Multiple Packages

Consider an S7-400 with CPU 416-3 (order number 6ES7416-3ES07-0AB0) acting as PROFIBUS master and four control packages (P1..P4), each with conventional I/O. The recommended topology is:

S7-400 CPU [Term ON] --- IM153-1 (P1, addr 3) --- IM153-1 (P2, addr 4) --- IM153-1 (P3, addr 5) --- [Term ON]

Rules applied:

1. CPU 416-3 has an integrated PROFIBUS DP interface (X2). It is the physical left end of the segment. Termination ON.
2. Each IM 153-1 sits in the middle of the daisy chain. Termination OFF.
3. The last device (P3) is the right end. Termination ON.
4. Maximum station count so far: 1 (CPU) + 3 (IMs) = 4 devices, well within the 32/segment limit.
5. If any package requires 12 Mbaud and the total segment length from CPU to P3 exceeds 100 m, split the segment with a repeater at 100 m and continue the chain.

Adding PROFIBUS-PA field instruments

If P4 contains PROFIBUS PA devices (process automation, intrinsically safe, 31.25 kbit/s), you need a DP/PA coupler (Siemens 6ES7157-0AC00-0XA0) or a DP/PA link (6ES7157-0AA00-0XA0) to bridge from DP at 1.5 Mbaud or higher to PA at 31.25 kbaud. The PA segment then has its own RS-485-IS rules and address range of 1-126 on the PA side, but the link itself consumes one address on the DP side.

Wiring Procedure with Siemens 6ES7972-0BA12 / -0BB12 Connectors

1. Strip the PROFIBUS cable (Siemens 6XV1830-0EH10, violet jacket) and clamp the red core to B1 and the green core to A1 on the incoming side. The shield clamp lands on the connector's grounding tab.
2. Route the outgoing cable to A2/B2 on the same connector. Tighten the shield clamps.
3. Set the slide switch on the connector:

• ON if this connector is the first or last station in the segment.
• OFF for every station in the middle.

4. Repeat for each IM and each end device (drive, transmitter with PROFIBUS).
5. Verify continuity with a PROFIBUS cable tester (e.g., Softing BC-700-PB or Siemens BT200) before powering the slaves.

Commissioning and Verification Steps

1. Power up the S7-400 first, then the IM 153-1 stations, then the third-party DP slaves.
2. In STEP 7 (V5.7 or TIA Portal V18), open HW Config / Device Configuration. The PROFIBUS subnet should show the configured stations with their DP addresses.
3. Download the configuration to the CPU. The CPU performs DP bus initialization and polls each slave.
4. Open Online & Diagnostics on the PROFIBUS subnet. Verify that every configured slave reports status "OK" and that no "Slave not found" or "Diagnostic fault" events appear.
5. Force a single I/O point on each IM and read it back in a VAT table to confirm end-to-end exchange.
6. Use the PROFIBUS diagnostic repeater (6ES7972-0AB01-0XA0) or the BT200 to scan the segment for reflections and signal quality. The bus must show clean edges with no more than 0.3 V of residual voltage at the terminator.

Troubleshooting Matrix

When to Use a Repeater Instead of an IM

Use a PROFIBUS repeater (Siemens 6ES7972-0AA01-0XA0) when:

• The segment exceeds 100 m at 12 Mbaud, 200 m at 1.5 Mbaud, or 1000 m at 187.5 kbaud.
• You need more than 31 stations on the same DP subnet. A repeater counts as one station on its input segment and regenerates the segment, allowing another 31 stations downstream.
• You need galvanic isolation between two clusters of field devices.

Use a DP/DP coupler (6ES7158-0AD01-0XA0) when two masters need to exchange data without one being a slave of the other (e.g., a DCS talking to a redundant PLC package on the same bus segment).

Standards and Reference Documents

• PI Installation Guide - PROFIBUS Interconnection Technology: official wiring topology and connector guidance.
• Siemens PROFIBUS bus connector manual: A1/B1 and A2/B2 pinout, termination switch positions.
• PROFIBUS overview: RS-485 limits and segment capacity.
• Endress+Hauser PROFIBUS and IIoT overview: field-device integration perspective.
• IEC 61158 (all parts) - Industrial communication networks, fieldbus specifications.
• EN 50170 Volume 2 - General purpose field communication system for PROFIBUS.

Engineering Checklist Before Going On-Site

• [ ] Confirm PROFIBUS cable type A (150 Ω) is installed from CPU to last station.
• [ ] Confirm exactly two terminators are ON, one at each physical end.
• [ ] Each DP address is unique within 1-125; document the assignment.
• [ ] Each device's baud rate capability covers the configured DP baud rate.
• [ ] Each IM 153-1 GSD file (SIEM816F.GSD or current revision) is installed in STEP 7.
• [ ] Total segment length is within budget for the chosen baud rate.
• [ ] Shield is continuous and grounded at both ends of each cable section.
• [ ] Spare connectors and PROFIBUS terminators are on site for the first commissioning shift.