Communication Issue: S7-224/226 with Radio Modems via RS485

You've successfully established communication between S7-224 and S7-226 CPUs using PROFIBUS. However, replacing the PROFIBUS cable with RS485 radio modems is causing communication failure. Here's a breakdown of potential issues and solutions:

Key Considerations & Troubleshooting Steps:

• RS485 vs. PROFIBUS: PROFIBUS and RS485 are different communication standards. Direct replacement isn't possible without configuration changes.
• Modem Configuration: Ensure the radio modems are correctly configured for RS485 communication, including baud rate, parity, and data bits. These *must* match the S7 CPU settings.
• S7 CPU Configuration: You'll need to configure the S7 CPUs for serial communication (RS485) instead of PROFIBUS. This involves setting the correct interface type and parameters within the CPU's configuration.
• Addressing: Verify correct addressing is configured for both CPUs when using serial communication.
• Termination: Ensure proper RS485 termination resistors (typically 120 ohms) are installed at both ends of the RS485 cable.
• Wiring: Double-check the RS485 wiring. A, B, and Ground must be connected correctly.

Resources:

Refer to Siemens documentation for detailed configuration instructions:

• S7-200 Communication - Overview
• How to configure the serial communication (RS485) on SIMATIC S7-200/300/400

If problems persist, please provide the following information for more targeted assistance:

• Modem make and model
• S7 CPU configuration details (communication settings)
• RS485 cable length