S7-400 CPU 414-3 PN/DP: PROFIBUS DP Node Limits and Expansion Reference

This reference consolidates the hard architectural limits for PROFIBUS DP node count, segment length, repeater usage, and master expansion on the SIMATIC S7-400 CPU 414-3 PN/DP. The CPU can host multiple PROFIBUS DP masters and can be expanded with plug-in and external communication processors (CPs) to scale from a single 32-node segment up to the practical maximum of 124 slaves per master and ten masters per station. Use the planning tables, formulas, and commissioning sequence below when targeting 200+ DP devices on a copper PROFIBUS DP network.

1. CPU 414-3 PN/DP Interface Architecture

The 6ES7414-3PN/DP (CPU 414-3 PN/DP) ships with three physical interfaces and one pluggable submodule slot. Two of the three on-board ports can operate as a PROFIBUS DP master simultaneously.

Interface 1 (X1) is a combined MPI/DP port. In STEP 7 / TIA Portal you choose either MPI or PROFIBUS DP for this port; it cannot do both. When configured as DP, it supports DPV0 only and is limited to 32 slaves and 2 KB of process I/O. Interface 2 (X2) is a PROFINET 2-port switch with Web server and is not part of the PROFIBUS count. Interface 3 (X3) is the on-board PROFIBUS DP master that becomes usable only when an IF964-DP submodule is plugged into the slot behind the door on the front of the CPU.

2. PROFIBUS DP Master Capacity Per Interface

The CPU firmware and the catalog entry jointly cap how many DP slaves each interface can poll. The numbers below are firm maximums in HW Config; STEP 7 raises a configuration warning if a DP master interface is loaded beyond its catalog limits.

Total DP slaves addressable from a CPU 414-3 PN/DP station is therefore not a single hard number; it depends on how many CP 443-5 Extended modules you install in the rack and which interfaces are enabled. Siemens documentation permits up to 10 DP masters per S7-400 station when using CP 443-5 Extended modules (subject to the rack slot count and power-budget limits of the S7-400 base unit).

2.1 The 124 vs 125 Slave Ceiling

PROFIBUS DP uses 7-bit station addresses, allowing addresses 0 through 126. Address 127 is the global broadcast address. Within a single DP master system (i.e., one PROFIBUS segment or one logical DP master domain), one address is consumed by the DP master itself, and STEP 7 reserves address 0 by default for programming devices (PG) and routing. The lowest practical slave address is therefore 1, and the highest is 126. This yields:

Max slaves per DP master = 126 - 1 (master) - 1 (PG/reserved) = 124 slaves

Some third-party DP masters accept address 0 as a slave, which would allow 125 slaves on that master. The CP 443-5 Extended and the IF964-DP both follow Siemens convention, so plan for 124 as the practical ceiling.

3. Segment Topology, Cable Lengths, and Repeater Rules

A segment is a contiguous piece of PROFIBUS cable terminated at both ends with the characteristic 220 Ω termination, with no repeater in between. Two independent constraints cap a segment, and the more restrictive one defines its boundary:

1. Node count: 32 stations (master + slaves + repeaters count as stations).
2. Cable length: depends on baud rate.

When you exceed 32 stations, or the cable length for your baud rate, you must insert a PROFIBUS repeater (e.g., 6ES7972-0AA02-0XA0). The repeater joins two segments and regenerates the signal. You may use up to 9 repeaters in series in a single PROFIBUS network, which gives a maximum of 10 segments and a maximum end-to-end bus length set by accumulated segment delays rather than DC resistance. At 12 Mbps the practical end-to-end cable length is roughly 1 000 m when repeaters are used in a balanced topology; at 187.5 kbps the network can span the full 10 km allowed by 9 repeaters and ten 1 000 m segments.

3.1 Repeater Budget Worksheet

Use the following formula to confirm a topology stays inside the PROFIBUS limits:

Total stations = Master + (Repeaters * 2) + Slaves
Stations per segment ≤ 32
Repeaters in series ≤ 9
Sum of segment lengths at the chosen baud ≤ bus-length budget

For a planned 200-slave network at 1.5 Mbps using 200 m segments:

• Devices per segment budget: 32 (1 master + 31 slaves) → ceil(200 / 31) = 7 segments
• Repeaters required: 6 (one between each pair of adjacent segments)
• Total cable length: 7 × 200 m = 1 400 m — exceeds the 200 m segment limit only if you forget to break at repeaters
• Repeater headroom remaining: 9 − 6 = 3 spare repeaters

4. Expansion Beyond the On-board Masters

The 414-3 PN/DP supports two PROFIBUS DP masters on board (IF1 as DP plus IF3 with IF964-DP), giving a ceiling of 32 + 96 = 128 slaves without adding CPs. To reach the documented 200+ slave target you need additional masters.

4.1 IF964-DP Pluggable Submodule

• Order number: 6ES7964-2AA04-0AB0
• Function: Adds PROFIBUS DP master capability to interface X3 of the S7-400 CPU
• Capacity: 96 DPV1 slaves, 6 KB I/O per direction
• Power: Drawn from the CPU; no external supply needed
• Configuration: Selected in HW Config under the CPU slot, sub-slot for IF964-DP

4.2 CP 443-5 Extended Communications Processor

• Order number: 6GK7443-5DX10-XE0
• Function: External PROFIBUS DP master or slave in the S7-400 rack
• Capacity: 124 DPV1 slaves, 6 KB I/O per direction per slave
• Slot: Occupies one S7-400 I/O slot in the central rack (UR) or expansion rack (ER) with interface module
• Firmware note: Always match the firmware release listed in the STEP 7 hardware catalog (TIA Portal V20 catalog or STEP 7 V5.x HSP). Older CP firmware blocks DPV2 functions and reduces some diagnostic detail.

5. Planning a 200-Device PROFIBUS Network

The original question was how to host nearly 200 PROFIBUS devices on a CPU 414-3 PN/DP. The table below is one valid partitioning using only the on-board masters plus CP 443-5 Extended modules. Other splits are equally valid as long as no master exceeds 124 slaves.

Headroom in this plan: Master 1 has 2 spare slave addresses; Master 2 has 6; Master 3 has 44. Adding one more CP 443-5 Extended would push the maximum to ~320 slaves per station, after which the next constraint is the rack slot count of the S7-400 base unit (CR) and power supply sizing.

6. STEP 7 / TIA Portal Configuration Procedure

Use STEP 7 V5.x (SIMATIC Manager) or TIA Portal V16+ with the matching HSP installed for the CPU 414-3 PN/DP. The TIA Portal reference page Configuring PROFIBUS DP (S7-300, S7-400, S7-1500) - STEP 7 describes the modern flow. Siemens also publishes a product finder at Siemens System Interfacing for SIMATIC S7-400 for module selection.

6.1 Prerequisites

• STEP 7 V5.6 SP2 or TIA Portal V16/V17/V18/V19/V20 with the latest HSP for the S7-400.
• CPU 414-3 PN/DP firmware version matching the catalog entry (use the menu PLC > Module Information on a connected CPU to read the firmware).
• IF964-DP submodule installed in IF3 slot if you intend to use X3.
• One CP 443-5 Extended per additional DP master beyond the two on-board ports.
• PROFIBUS DP cable (violet, type A, 220 Ω characteristic impedance) and PROFIBUS connectors with built-in termination.
• Bus terminator power supplied by the DP master or an active terminator at both ends of each segment.

6.2 Procedure (STEP 7 V5.x)

1. Open SIMATIC Manager, open the S7-400 station, and double-click Hardware to launch HW Config.
2. Insert the rack, power supply, and CPU 414-3 PN/DP from the hardware catalog.
3. Double-click the CPU and configure Interface IF1: select PROFIBUS DP Master, set the address (default 2), and choose the network. Confirm the operating mode is DP, not MPI.
4. Configure Interface IF3: set the address (e.g., 3), and from the catalog under SIMATIC 400 > CPU 414-3 PN/DP insert the IF964-DP submodule into the IF3 sub-slot. Activate DP Master.
5. For each CP 443-5 Extended, drag it into the required slot in the UR (or ER with IM 460/461) and set its PROFIBUS address and baud rate.
6. Right-click each PROFIBUS interface and choose Properties > Network Settings. Set the Highest PROFIBUS Address parameter to 126; this parameter controls the token rotation and poll cycle. A higher value slows the bus; setting it lower than the highest slave address causes bus faults.
7. On each DP master, add the slave GSD files (Options > Install GSD File) or select Siemens catalog slaves (ET 200SP, ET 200S, SINAMICS drives, etc.). Assign addresses 1 through N, skipping 0 (PG).
8. Configure I/O slots on each slave and download the hardware configuration to the CPU.

6.3 Procedure (TIA Portal)

1. Add the CPU 414-3 PN/DP to the device configuration. Open Device view.
2. Select the PROFIBUS interface X1, choose Add new subnet, set the address, and confirm DP master mode.
3. Select PROFIBUS interface X3, ensure the IF964-DP submodule is mounted in the catalog under the CPU sub-slot, and activate DP master.
4. Drag CP 443-5 Extended modules into free slots, configure the DP subnet, and assign PROFIBUS addresses.
5. In Network view, drag slaves from the catalog or install their GSDML files. Connect each slave to the correct DP master system.
6. Compile and download to the CPU.

7. Parameter Reference

8. Diagnostics and Verification

8.1 Online Diagnostics in STEP 7

1. In HW Config select PLC > Online > Diagnostics (or Station > Online > Diagnose Hardware in TIA Portal).
2. For each DP master, expand the slaves tree and verify every slave reports OK and the configured I/O is consistent.
3. Open Module Information for any slave reporting a fault and read the diagnostic buffer. Common PROFIBUS fault codes:

8.2 Verification Checklist

• Each PROFIBUS segment has exactly two terminators powered (typically one at each end).
• Shield continuity is preserved across each segment with low-impedance grounding at one point.
• Repeater count is ≤ 9 in series on any single DP master subnet.
• DP master address is non-zero.
• Highest PROFIBUS Address parameter on every DP master ≥ highest configured slave address.
• All slave addresses are unique across the whole DP master system (no address collisions between two CPs).
• CPU diagnostic buffer contains no DP master startup errors.
• SF / BF LEDs on the CPU are off.
• CP 443-5 Extended module LEDs show RUN green, BUSF off.

9. Field Commissioning Sequence for a 200-Slave Plan

1. Power up the S7-400 station with the CP modules installed but no DP cable connected.
2. Download the project to the CPU. Confirm IF1 (as DP), IF3 (with IF964-DP), and all CP 443-5 modules report RUN.
3. Build segment 1 with the DP master termination enabled and one slave at a time. Verify each slave via Online > Diagnose Hardware.
4. Insert a repeater at the planned boundary and continue to segment 2. Repeat the slave-by-slave verification.
5. Repeat for each segment up to the limit of 9 repeaters in series.
6. Once all slaves are connected, run a cyclic read/write test from the user program on representative I/O points and verify OB1, OB82 (diagnostic interrupt), and OB86 (rack failure) behave as configured.
7. Capture the diagnostic buffer at end of commissioning as a baseline.

10. Field-Proven Caveats and Edge Cases

• Repeater count vs. segment count: 9 repeaters in series implies 10 segments. If your plant layout requires a star topology with optical fiber, use an OLM (Optical Link Module) or an OBT (Optical Bus Terminal) instead of a copper repeater; these do not consume DP address space.
• Address 0 reuse: Setting the DP master address to 0 is rejected by STEP 7 for the same routing reasons address 0 is reserved for the PG. Plan around it.
• CP 443-5 Extended vs. Basic: The CP 443-5 Basic (older variant) supports 32 slaves only. The Extended variant is required to reach 124 slaves per master. Confirm the suffix -5DX10 or later in the order number.
• CP 443-5 Extended as DP slave: The CP can be configured as a DP slave on a higher-level bus while acting as DP master on its lower-level bus. This is useful when nesting a remote S7-400 station under a higher-level master but consumes a slave address on the upper bus.
• PROFINET vs. PROFIBUS migration: The PROFINET port (X2) on the CPU 414-3 PN/DP cannot carry PROFIBUS traffic. For PROFINET-based I/O migration off PROFIBUS, use IE/PB Link PN IO (6GK1411-5AB20) as a gateway, and remember that the IE/PB Link itself consumes one DP master slot address and one PROFINET device slot.
• GSD version drift: When replacing a third-party DP slave, install the matching GSD file revision. A newer GSD may declare additional diagnostic bytes that the CPU cannot interpret, leading to OB82 floods. Pin the GSD revision in the project.
• Power supply headroom: The CP 443-5 Extended typically draws 1.0 A from 5 V; ten CPs would draw 10 A. The PS 405/PS 407 power supplies in S7-400 stations cap out well below this; use an ER with its own PS or split the masters across stations.

11. Quick Decision Matrix: How Many Masters Do You Need?

12. Frequently Asked Questions