Device Capacity Limits and Redundant Network Topologies for SIMATIC CN 4100 in PCS 7

In SIMATIC PCS 7, a single SIMATIC CN 4100 communication node supports up to 70 concurrent Modbus/TCP slave devices. It connects to the plant architecture via standard Industrial Ethernet switches on the Plant Bus rather than requiring direct point-to-point wiring to CP 443-1 communication processors. In redundant S7-400H configurations, high availability mandates connecting interface X1 to Switch 1 and interface X2 to Switch 2 across separate network rings. If a plant requires interfacing more than 70 Modbus devices, multiple CN 4100 nodes can be deployed on the same automation station by balancing controller communication load in HW Config.

Executive Summary

Integrating distributed field devices, package units, and third-party PLCs via Modbus/TCP into SIMATIC PCS 7 requires careful consideration of communication capacity, controller processing load, and network availability. The SIMATIC CN 4100 serves as the primary high-density industrial protocol gateway for PCS 7 V9.0 and V9.1 architectures. However, systems engineers frequently encounter design uncertainties regarding device connection thresholds, physical network attachment rules, and redundancy topologies.

Key design questions include the maximum number of Modbus TCP devices supported per gateway, the physical connection topology (whether CN 4100 must wire directly into controller CP 443-1 modules or standard SCALANCE plant switches), and the scaling strategy when a facility exceeds 70 field devices. This guide provides exact engineering capacity limits, outlines high-availability dual-switch cabling rules, and details how to deploy multiple CN 4100 gateways while maintaining deterministic controller cycle times.

Architecture and Fundamental Mechanics

The SIMATIC CN 4100 interfaces with PCS 7 controllers and external Ethernet networks through decoupled communication layers:

Device Connection Limits and Polling Engines:
- A single SIMATIC CN 4100 hardware appliance hosts an autonomous communication processor capable of maintaining up to 70 concurrent Modbus/TCP connections (acting as Modbus Client querying 70 distinct slave IP addresses or Unit IDs).
- Internal polling cycles are managed independently by the CN 4100 operating system, shielding the S7-400 controller from Modbus TCP socket overhead.
Plant Bus Network Attachment Architecture:
- The CN 4100 features multiple physical Gigabit Ethernet interfaces organized into logical subnets:
- Field Network (Interfaces X1/X2): Connects to the third-party Modbus TCP field network.
- Plant Network (Interfaces X3/X4): Connects to the PCS 7 Plant Bus.
- CN 4100 does NOT require dedicated direct-attach cabling to S7-400 CP 443-1 modules. Instead, it connects directly into standard Industrial Ethernet switches (e.g., SCALANCE X-200 or X-300) on the Plant Bus, communicating with CPU 410-5H over PROFINET IO or S7 Communication (PUT/GET).
High-Availability Redundancy Topologies:
- In fault-tolerant S7-400H deployments, deploying redundant CN 4100 modules requires avoiding common-mode failures.
- Wiring both redundant CN 4100 interfaces into a single physical switch creates an unacceptable single point of failure (switch power or backplane loss severs both gateway paths).
- High-availability topology requires connecting CN 4100 Module 1 to Switch A (Ring 1) and Module 2 to Switch B (Ring 2).
Scaling Beyond 70 Devices (Multiple CN 4100 Gateways):
- When an installation exceeds 70 Modbus devices, multiple CN 4100 gateways can be assigned to a single S7-400 / CPU 410-5H station.
- In STEP 7 HW Config, the communication load parameter must be adjusted (configurable between 5% and 50%, with 20% being the default) to prevent communication telegram processing from starving cyclic control tasks in OB35.

Step-by-Step Implementation Procedure

Follow these steps to architect and configure high-capacity CN 4100 networks in PCS 7:

Determine Device Quantities and Gateway Count:
- Audit the total number of third-party Modbus TCP devices in the plant.
- If the total count is between 1 and 70 devices: specify 1 CN 4100 node (or 2 nodes if hardware redundancy is mandated).
- If the total count exceeds 70 devices: divide device polling groups across multiple CN 4100 nodes (e.g., 120 devices = 2 CN 4100 gateways).
Establish Redundant Switch Cabling:
- Install two SCALANCE industrial Ethernet switches to form redundant Plant Bus rings.
- Connect Interface X3 of CN 4100 Node 1 to Switch A; connect Interface X4 to Switch B.
- Connect Interface X3 of CN 4100 Node 2 to Switch B; connect Interface X4 to Switch A.
- Ensure Media Redundancy Protocol (MRP) is configured on the switches.
Configure S7 Controller Communication Load in HW Config:
- Open HW Config for the CPU 410-5H station in SIMATIC Manager.
- Double-click the CPU to open 'Object Properties' > switch to the 'Cycle/Clock Memory' tab.
- In the 'Scan cycle load from communication' field, verify that the value is set appropriately (recommended: 20% to 30% when interfacing multiple CN 4100 gateways).
- Save and compile HW Config.
Configure CNET Project for Distributed Devices:
- Launch the Siemens CNET configuration tool.
- Configure Modbus TCP client connections, allocating IP addresses, slave IDs, and register offset tables across the configured CN 4100 nodes.
- Balance polling frequencies (e.g., slow monitoring tags at 1000 ms, critical control tags at 200 ms) to optimize network bandwidth.
Download Configurations and Commission:
- Download the CNET configuration files to each CN 4100 gateway.
- Download the updated S7 program and HW Config to the controller in RUN mode.

Verification

In CNET online diagnostics: confirm that all configured Modbus TCP devices report 'Connected' and show active transaction counters with zero frame timeouts.
In SIMATIC Manager, monitor CPU cycle time (Ctrl + D > 'Cycle Time' tab): verify that scan cycles remain stable within configured OB scan boundaries.
Test network redundancy: disconnect the Ethernet uplink on Switch A; verify that communication with third-party devices continues without interruption via Switch B.
In WinCC Runtime: confirm that process variables from all Modbus devices update continuously across operator faceplates.

Limitations and Operational Precautions

Ensure that third-party Modbus TCP slave devices can sustain the configured polling rates; low-cost OEM PLCs frequently stall if queried faster than their scan cycle allows. Connecting both redundant gateway ports to the same physical switch degrades system availability to a single-switch failure boundary. Hardware components, communication licenses, and CNET software packages should be verified through an authorized Siemens distributor or the Siemens Industry Mall.

Engineer FAQ

What is the maximum number of Modbus TCP devices one CN 4100 can handle?

A single SIMATIC CN 4100 supports up to 70 concurrent Modbus/TCP devices.

Can I connect more than one CN 4100 to a single S7-400H controller?

Yes. Multiple CN 4100 gateways can be integrated with a single controller by configuring appropriate communication load settings in HW Config.

Must the CN 4100 wire directly to a CP 443-1 communication processor?

No. The CN 4100 connects directly into standard Industrial Ethernet switches on the Plant Bus.

Why shouldn't redundant CN 4100 ports connect to the same switch?

Connecting both redundant ports to a single switch creates a single point of failure; if that switch loses power, the entire redundant communication path is lost.