Profibus DP Network: S7-400 Master with S7-300 Slaves and HMI on a Single Bus

1. Overview

A common point of confusion when commissioning a Siemens PROFIBUS DP field network is whether more than one "master" can coexist on the same copper segment. The configuration in question is a classic plant layout:

• One S7-400 CPU acting as the DP Class 1 master (DPM1), e.g., CPU 414-3 PN/DP or CPU 416-3 with an integrated DP interface.
• Two S7-300 CPUs configured as DP slaves (e.g., CPU 315-2 DP / CPU 317-2 DP), exposing their I/O or data records to the master.
• One HMI panel (e.g., SIMATIC Comfort Panel TP700, MP277, or OP/TD class) connected to the same PROFIBUS cable.

This configuration is fully supported by PROFIBUS DP as defined in IEC 61158 / IEC 61784 and by Siemens engineering tooling (STEP 7 V5.x and TIA Portal). The HMI does not need to be treated as a DP master in this scenario — it uses the S7 / PG-OP communication channel, which is a parallel protocol that travels on the same PROFIBUS segment but does not arbitrate for DP slaves.

2. PROFIBUS DP Architecture Refresher

PROFIBUS DP (Decentralized Periphery) is optimized for high-speed cyclic exchange of I/O data between a controller and distributed field devices. The relevant roles are:

Media access on PROFIBUS uses a hybrid scheme. Active stations (masters) form a logical token ring. The token is passed in ascending station address order. A master that holds the token may send telegrams; when the token hold time expires, it passes the token to the next active station. Passive stations (slaves) never hold the token — they only respond when addressed by a master.

2.1 Limits and Baud Rates

3. Active Nodes vs. "Masters" — What the Token Sees

A frequent mistake is to call every "active" device on a PROFIBUS segment a master. PROFIBUS uses two terms that are easy to confuse:

• Active station (master-capable node): A device that participates in the token ring. It can send unsolicited telegrams when it holds the token.
• DP master: A specific active station that has been configured to poll DP slaves cyclically. There are two classes: DPM1 and DPM2.

The HMI panel, although it is an active node on the bus, is not acting as a DP master in the typical configuration. It uses S7 Communication (S7 functions read/write S7-400 DBs/Merkers) and PG/OP services. Those services are routed over the same RS-485 cable but are not part of the DP cyclic data exchange. So the HMI is an active node, but not a DPM1 or DPM2 in the DP sense.

4. HMI Classification on PROFIBUS DP

How the HMI is connected determines its master class:

For the topology in question (1 × S7-400 + 2 × S7-300 slaves + 1 × HMI), the HMI is best configured for S7 Communication only. The S7-400 is the sole DPM1; the two S7-300s are slaves; the HMI is an S7 client. This keeps the bus profile predictable and avoids DPM2 handshake overhead.

5. Supported Topology: S7-400 Master + 2× S7-300 Slaves + HMI

The full topology is therefore:

• Bus segment with PROFIBUS cable (typically violet, 2-wire, shielded).
• Terminating resistors ON at the two physical end nodes (typically the S7-400 and the HMI if they sit at the cable ends; otherwise use PROFIBUS connector with switchable terminator).
• Station addresses: S7-400 = 2 (DPM1), S7-300 #1 = 3 (slave), S7-300 #2 = 4 (slave), HMI = 5 (active, S7 client).
• Baud rate: 1.5 Mbit/s is the conservative default for a small mixed-master/slave network; 12 Mbit/s is achievable if all four nodes and the cable plant support it.

5.1 Logical View (token ring perspective)

   ┌──────────────────────────────────────────────┐
   │  PROFIBUS DP segment (RS-485)                │
   │                                              │
   │  Station 2  Station 3  Station 4  Station 5 │
   │  (S7-400)   (S7-300#1) (S7-300#2)  (HMI)    │
   │  DPM1       Slave      Slave      S7 client  │
   │  (active)   (passive)  (passive)  (active)  │
   │     ▲           ▲          ▲          ▲     │
   │     └── token ──┘          │          │     │
   │        ring: 2 → 5 ─────────┴──────────┘     │
   └──────────────────────────────────────────────┘

The token is passed only between active stations (2 and 5). Slaves at 3 and 4 are not in the token ring; they respond only when addressed by 2. Even though the S7-300 CPUs in slave mode can be configured as active nodes, when operated in DP slave mode the DP interface is set to passive for token purposes.

6. Prerequisites

Before commissioning, verify the following:

1. STEP 7 V5.5 / V5.6 or TIA Portal V16+ installed with HSP for the S7-400 and S7-300 CPUs in use.
2. GSD files for any third-party PROFIBUS device. Siemens devices are integrated; no GSD import is required for ET200 or S7-300 in slave mode.
3. PROFIBUS cable of correct impedance (150 Ω characteristic), shielding clamped at both ends, separation from VFD power cables by ≥ 200 mm.
4. PROFIBUS connectors with switchable terminating resistor (6ES7972-0BA12-0XA0 for 90° outlet, 6ES7972-0BB12-0XA0 for 35° outlet, etc.).
5. Station addresses set uniquely on each device either via DIP switches on the CPU or via software (TIA / STEP 7 hardware configuration).
6. Bus terminator ON at the two physical end stations only; OFF on all intermediate stations.

7. Step-by-Step Configuration

7.1 Insert the S7-400 as DPM1

1. In the project tree, add the S7-400 station (e.g., CPU 416-3 PN/DP, order number 6ES7416-3ES06-0AB0, firmware V6.x or later).
2. Open the DP interface properties → set Operating mode = DP master.
3. Assign Station address = 2.
4. Set baud rate: 1.5 Mbit/s (recommended) or 12 Mbit/s if cabling quality is verified.
5. Open the DP master system and prepare slots for the two slaves.

7.2 Add the S7-300 Slaves

1. Insert the first S7-300 station (e.g., CPU 315-2 DP, 6ES7315-2AH14-0AB0).
2. Set its DP interface to Operating mode = DP slave.
3. Assign Station address = 3.
4. Configure the slot for DP master exchange: choose between MS (master-slave) and DS (direct slave-to-slave) communication. Use MS for the S7-300 to publish inputs/outputs to the S7-400.
5. Define the I/O slot count (e.g., 16 bytes in / 16 bytes out) to match the S7-400's I/O mapping.
6. Repeat for the second S7-300 with Station address = 4.

7.3 Add the HMI as S7 Client (Active Node, Not a DP Master)

1. Insert the HMI station (e.g., TP700 Comfort, 6AV2124-0GC01-0AX0).
2. Open the HMI's Connections editor and add a new connection.
3. Set the Connection type = S7 Communication (or "SIMATIC S7 300/400" depending on TIA version).
4. Set the PROFIBUS address = 5.
5. Point the connection to the S7-400 station (address 2), rack 0, slot 2 (CPU slot).
6. Do not place the HMI inside the DP master system in the S7-400 hardware config — that would force it into a DPM1 or DPM2 role.

7.4 Compile and Download

1. Compile the S7-400 hardware configuration and download to the CPU.
2. Compile the S7-300 hardware configurations and download to each CPU.
3. Compile the HMI project and transfer to the panel.
4. Power cycle the bus if you changed station addresses via DIP switches (a CPU restart is required for address changes to take effect on some S7-300 DP interfaces).

8. Bus Parameter Tuning

When multiple active nodes share a bus, the token rotation time (T_TR) becomes the critical timing budget. If you leave the bus parameters at "automatic" in STEP 7, the tool calculates the timing from the discovered topology — but it can over- or under-estimate the real load.

8.1 How to Set "Standard" in STEP 7

1. Open DP > Properties > Network Settings on the DP master interface of the S7-400.
2. Click Bus Parameters → Profile → select "Standard".
3. Confirm the recomputed T_TR and HSA values.
4. Recompile and download.

9. Alternative: DP/DP Coupler for Master-to-Master Isolation

If a hard separation between two independent DP master systems is required — for example, the HMI must act as a DPM1 in a different subnet, or the plant has two independent S7-400 controllers that need to exchange a small amount of data — the recommended hardware is a DP/DP coupler (Siemens 6ES7158-0AD01-0XA0, or Phoenix Contact IBS IL DP/LK).

9.1 When to Use a DP/DP Coupler

• Two physically separate PROFIBUS DP segments that need to share ≤ 244 bytes of I/O.
• A retrofit where an existing master cannot be re-engineered to be a slave.
• Plant zones with different baud rates (e.g., 12 Mbit/s on the fast side, 1.5 Mbit/s on the field side).

9.2 Coupler Configuration

1. Mount the DP/DP coupler with two PROFIBUS connectors — one per segment.
2. Configure each side as a DP slave from the perspective of its master. The coupler has its own GSD file ("SIEM8170.GSD" or similar, depending on the version).
3. Define the I/O length on each side (e.g., 16 bytes in / 16 bytes out). The data is mirrored from one side to the other.
4. Each segment keeps its own token ring, baud rate, and station address space.

10. Verification

After download, verify the configuration with these checks:

10.1 Quick S7-400 Diagnostic Step

Use the diagnostic function in STEP 7 to dump the slave status:

// SCL / STL snippet for an OB1 diagnostic request
// Calls SFC 13 "DPNRM_DG" to read the diagnostic data of slave 3
CALL "DPNRM_DG"
  REQ    := TRUE                       // One-shot trigger
  LADDR  := W#16#100                    // Diagnostic address of slave 3 (assign in HW config)
  RETVAL := MW100                      // Return code
  RECORD := P#DB100.DBX0.0 BYTE 32      // Buffer (32 bytes diagnostic telegram)
  BUSY   := M101.0                      // Set while reading

NOP 0;

RETVAL = 0 and BUSY = 0 indicate the call completed. Decode the diagnostic buffer per the PROFIBUS DP standard to confirm the slave reports a healthy state and correct I/O length.

11. Troubleshooting Matrix

12. Field-Proven Cautions

• Address planning is the single most common source of failure. Sketch the bus and assign addresses in a table before you start. The S7-400 default is 2, S7-300 default is 2 — they will collide if you forget to change one of them.
• Terminating resistors must be ON at exactly two nodes — the two physical ends of the segment. A terminator in the middle kills the signal integrity of the entire bus.
• Shield the cable and ground both ends to a low-impedance earth bar. PROFIBUS cable shielding done wrong is a top cause of intermittent DPM1 errors at 12 Mbit/s.
• Keep PROFIBUS cable away from VFD output cables by at least 200 mm, or use a grounded metal divider. Common-mode noise from PWM output stages is the most common EMI source.
• Do not exceed the segment cable budget. At 1.5 Mbit/s the maximum trunk length (excluding stubs) is 200 m. Add a repeater (Siemens 6GK1500-0AA10) to extend the network — repeaters count as a station on each side but do not consume a DP address.

13. References (Official Documentation)

• PROFIBUS technology overview, PROFIBUS Nutzerorganisation (PNO): https://www.profibus.com/technology/description/
• Siemens SIMATIC S7-400 Automation System, Operating Instructions: https://support.industry.siemens.com/cs/ww/en/view/1117740
• Siemens SIMATIC S7-300 CPU 31xC and CPU 31x, Operating Instructions: https://support.industry.siemens.com/cs/ww/en/view/12996906
• PROFIBUS DP/DP Coupler, manual entry 6ES7158-0AD01: https://support.industry.siemens.com/cs/ww/en/view/1137565
• STEP 7 V5.5 / S7-300/400 Programming and Operating Manual, Bus Parameters chapter: https://support.industry.siemens.com/cs/ww/en/view/45531107