Integrating Third-Party PLCs into SIMATIC PCS 7 via PROFINET and Modbus TCP

To connect third-party controllers (such as Rockwell ControlLogix or Schneider Modicon) with a SIMATIC PCS 7 controller, the two officially supported industrial methods are the Siemens PN/PN Coupler (6ES7158-3AD10-0XA0) for cyclic PROFINET I/O exchange and Open Modbus/TCP for client/server communication. The PN/PN Coupler provides deterministic, galvanically isolated cyclic data transfer between separate PROFINET networks without sharing engineering projects. Alternatively, Modbus/TCP blocks (Modbus/TCP PN-CPU) communicate over Industrial Ethernet without extra hardware. Direct PROFINET I-Device coupling is not officially validated across third-party controllers in PCS 7.

Architectural Challenges of Heterogeneous Controller Integration

In modern chemical and manufacturing plants, package units (compressor skids, refrigeration chillers, centrifuges) are frequently delivered with third-party PLCs (e.g. Allen-Bradley ControlLogix, Omron, or Schneider Electric Modicon). Connecting these standalone systems into a SIMATIC PCS 7 DCS requires high-integrity data exchange without compromising DCS deterministic cycles or cybersecurity boundaries.

Attempting direct PROFINET master-slave coupling is often constrained because S7-400 and CPU 410-5H controllers do not support acting as PROFINET I-Devices within PCS 7, and sharing a common PROFINET subnet across disparate engineering tools (STEP 7 vs Studio 5000) creates device name and IP management conflicts.

PROFINET PN/PN Coupler: Deterministic Galvanic Isolation

The SIMATIC PN/PN Coupler (6ES7158-3AD10-0XA0) is the primary hardware solution for interconnecting two independent PROFINET IO systems:

Decoupled Network Boundaries: Side X1 connects to the PCS 7 PROFINET network; Side X2 connects to the third-party PLC PROFINET network. Each side possesses independent IP addresses, subnet masks, and device names.
Galvanic and Cyber Isolation: Network packets, broadcast storms, or IP collisions cannot cross the internal ASIC barrier; only configured user data bytes (up to 1440 bytes in each direction) are exchanged in memory.
Configuration Workflow in PCS 7:
- In PCS 7 HW Config, insert the PN/PN Coupler from the hardware catalog.
- Define input and output module slots matching the third-party configuration (e.g. 16 Words Input, 16 Words Output).
- In the third-party engineering software (e.g. Studio 5000), import the Siemens GSDML file and configure identical module slots.
- In PCS 7 CFC, interconnect input/output addresses directly or disable module driver generation for the coupler.

Open Modbus/TCP Integration via On-Board PN Ports and CPs

For skid packages that lack PROFINET interfaces or communicate over standard Industrial Ethernet TCP/IP, Open Modbus/TCP is the universal software standard:

Software Package: Siemens provides certified Modbus/TCP communication blocks for PCS 7:
Modbus/TCP PN-CPU (6AV6676-6MB20-3AX0) for CPU on-board PROFINET ports (CPU 410-5H, CPU 414-3 PN/DP).
Modbus/TCP CP (6AV6676-6MB00-0AX0) for external communication processors (CP 443-1 6GK7443-1EX30-0XE0).
Operation: The PCS 7 controller can function as a Modbus TCP Client (querying registers from third-party PLCs) or as a Modbus TCP Server (allowing third-party PLCs to read/write holding registers). Connection parameters are defined via DB configuration without requiring HW Config hardware changes.

Alternative Communication Protocols: OUC and OPC UA

Other communication strategies include:

Open User Communication (OUC): Native TCP/UDP communication using S7 standard blocks (TCON, TSEND, TRCV, TDISCON). Requires custom telegram framing and checksum handling in application logic.
OPC UA Architecture: For supervisory data exchange, expose data via an OPC UA server (e.g. OpenPCS 7 station or third-party OPC gateway) communicating at the OS/SCADA level rather than controller-to-controller.

Comparison Matrix of Heterogeneous Controller Coupling Methods

The following table compares integration methods between SIMATIC PCS 7 and third-party controllers:

Integration TechnologyPhysical InterfaceUpdate SpeedEngineering Boundary SeparationRequired Siemens Package / MLFBPROFINET PN/PN CouplerDual RJ45 PROFINET PortsDeterministic (<5 ms)Complete (Galvanic & IP isolation)Hardware: 6ES7158-3AD10-0XA0Open Modbus/TCP (PN-CPU)On-board PN InterfaceCyclic (50–200 ms)Shared TCP/IP network subnetSoftware License: 6AV6676-6MB20-3AX0Open Modbus/TCP (CP 443-1)Dedicated CP 443-1 ModuleCyclic (50–200 ms)Isolated physical network portHardware: 6GK7443-1EX30-0XE0 + Lic: 6AV6676-6MB00-0AX0Open User Communication (OUC)Industrial EthernetVariableShared TCP/IP network subnetNative S7 System Function Blocks (Free)Supervisory OPC UATerminal Bus EthernetNon-real-time (0.5–1s)Client/Server OS levelOpenPCS 7 Software: 6ES7658-0CX58-0YB0

[!NOTE]
Commercial Accuracy Notice: Hardware PN/PN Couplers require physical hardware procurement (6ES7158-3AD10-0XA0). Modbus/TCP > communication requires runtime software block licenses per controller. Direct I/O memory reads via PN/PN Coupler consume no > AS Process Objects unless mapped to APL monitoring blocks.

Verification

Confirm reliable communication between PCS 7 and the third-party PLC:

Hardware LED Inspection: On the PN/PN Coupler, verify that both X1 and X2 link/activity LEDs are green and that the BF (Bus Fault) and SF (System Fault) LEDs remain extinguished.
Data Consistency Test: Write a known numeric pattern (e.g. 16#1234) from the third-party PLC into the output word. Open the corresponding CFC input address in PCS 7 and verify that the value reflects instantaneously without byte swapping.
Cable Disconnect Recovery: Unplug the third-party Ethernet cable. Confirm that the PCS 7 controller reports a localized fault without crashing the primary PCS 7 PROFINET IO ring.

Limitations and Operational Precautions

Adhere to these operational boundaries when connecting third-party systems:

Byte Order (Endianness): Siemens S7 controllers use Big-Endian byte ordering, while Rockwell and many third-party PLCs use Little-Endian. Swap high/low bytes on multi-byte integers or floating-point values if necessary.
Disable Module Drivers for Couplers: In HW Config properties for the PN/PN Coupler, uncheck 'Generate module drivers' to prevent CFC compilation errors.
Network Isolation: Never connect third-party PLC subnets directly to the PCS 7 Plant Bus without firewalling or routing segmentation.

Engineer FAQ

Can I configure a CPU 410-5H as a PROFINET I-Device for a third-party PLC?

No. In SIMATIC PCS 7, the CPU 410-5H does not support acting as a PROFINET I-Device. Use a PN/PN Coupler to bridge the networks.

Does using a PN/PN Coupler require the third-party vendor to share their project file?

No. You only need to agree upon the byte layout (e.g. 32 bytes IN, 32 bytes OUT). Each side configures their half of the coupler independently using GSDML.

Do I need an extra software license to use a PN/PN Coupler in PCS 7?

No. The PN/PN Coupler is standard hardware configured via HW Config; it requires no runtime software licenses.

Why are floating point numbers corrupted when read from a Modicon or Rockwell PLC?

Rockwell and Modicon controllers often transmit 32-bit floats in Little-Endian or word-swapped format. Use a custom SCL conversion function to swap the word order before converting to REAL.