Dual PLC Control: S7-1500 & S7-400 Ethernet for Smart Monitoring

Many manufacturing facilities still rely on proven Siemens S7-400 PLCs that lack native Ethernet ports. These controllers were designed when MPI and PROFIBUS DP were the standard fieldbuses. Today, the need to connect them to modern S7-1500 CPUs, SCADA systems, and MES platforms creates a significant integration challenge. Replacing the entire control system is often cost-prohibitive and causes extended downtime. A more practical approach is to retrofit the existing hardware with an Ethernet communication module that transparently converts the serial MPI/DP protocol to TCP/IP.

The Challenge: Bridging Legacy and Modern PLCs

Consider a plastic injection molding plant with 32 production lines, each controlled by a Siemens S7-400 CPU 416-2DP. The original design used MPI at 187.5 kbps for HMI communication and limited data exchange. When the plant wanted to implement a new S7-1500-based auxiliary system for robots, central feeding, and mold temperature control, the lack of Ethernet on the S7-400 became a critical bottleneck. Additionally, the existing 38 HMI panels (various brands) connected via MPI could not be easily replaced due to high costs and re-engineering effort.

The injection cycle time had been reduced from 6 seconds to 3.6 seconds, demanding faster data sampling for quality traceability. The old MPI network could only provide a 2-second sampling period, which was insufficient for real-time monitoring and defect detection.

The Solution: MPI/DP to Ethernet Converter Module

A dedicated Ethernet communication module designed for S7-300/400 PLCs can be plugged directly into the 9-pin MPI or DP port of the CPU. This compact device (typically 71mm x 25mm x 96mm) requires no external power supply and does not occupy any rack slot. It transparently converts the MPI/DP protocol to standard TCP/IP, enabling seamless integration with Ethernet-based devices.

Key features of such a module include:

• Serial-to-Ethernet conversion: Transforms MPI/DP signals to TCP/IP with full transparency.
• Extended 9-pin female connector: Allows simultaneous connection of the original HMI panel, so existing touchscreens (e.g., Weinview, Kunlun Tongtai, Pro-face, Siemens) continue to work without modification.
• Multi-protocol support: Compatible with S7 communication, Modbus TCP, and OPC UA data models.
• Plug-and-play: No changes to the PLC program; retains original MPI address and baud rate.
• High speed: 10/100 Mbps auto-negotiation, with end-to-end latency typically under 5 ms.
• Robust isolation: Magnetic coupler isolation plus TVS protection, tested to withstand 2 kV electrical fast transients with zero packet loss.

System Architecture and Device List

The upgraded system consists of three layers:

Implementation Steps

Hardware Installation: The retrofit requires only about 3 minutes of downtime per machine. Simply unplug the existing MPI connector from the CPU, insert the Ethernet module onto the CPU port, and then plug the original MPI connector into the module’s extension port. Connect an Ethernet cable from the module’s RJ45 port to the industrial switch. The HMI remains connected through the module, keeping its MPI address unchanged.

Parameter Configuration: Using the module’s configuration software, scan the network to discover all modules and assign IP addresses within the same subnet as the S7-1500 (e.g., 192.168.10.100-131/24). In TIA Portal, create S7 connections to each module’s IP. For the HMIs, change the driver to “Siemens Ethernet (ISO-on-TCP)” and enter the corresponding module IP; the existing tag database remains intact.

Data Mapping: Establish a 2 KB shared memory area between the S7-400 and S7-1500 using the module’s built-in PLC-to-PLC data exchange function. For example, DB1.DBW0-DBW99 can carry real-time temperature, pressure, and injection speed from S7-400 to S7-1500, while DB2.DBW0-DBW31 can transmit robot pick-up signals and mold temperature setpoints from S7-1500 to S7-400. The refresh cycle is typically 20 ms, well within the 3.6-second injection cycle.

SCADA and MES Integration: WinCC connects to all 32 modules via Kepware’s Siemens TCP/IP Ethernet driver, handling up to 12,000 OPC tags with a 500 ms sampling rate. The MES retrieves critical quality data through RESTful API from WinCC, enabling per-cavity traceability.

Performance Results

Lessons Learned and Future Outlook

This project demonstrates that legacy equipment does not necessarily mean obsolete. By retrofitting S7-400 PLCs with Ethernet modules, the plant achieved a cost-effective digital transformation. The existing investment in controllers, I/O, and HMIs was preserved, while gaining the benefits of high-speed networking, remote access, and IIoT readiness.

The next phase involves extending the Modbus TCP capability of the modules to connect hot runner temperature controllers directly, aiming for a “one network to the edge” architecture. Eventually, over 900 auxiliary devices could be integrated into a Time-Sensitive Networking (TSN) infrastructure, paving the way for a fully digitalized injection molding factory.

For engineers facing similar challenges, the key takeaway is that upgrading communication interfaces can unlock significant performance improvements without a complete system overhaul. The combination of S7-1500 and S7-400 over Ethernet enables advanced control strategies, better data visibility, and higher overall equipment effectiveness.

Note: The technical details and performance figures mentioned are based on real-world case studies. Specific module names and brands have been omitted to maintain vendor neutrality. Always verify compatibility and performance with your specific hardware and network environment.