IO Modules - Network - Modbus RTU wiring guidelines for iC controllers

Introduction

The document addresses the core issue of correct Modbus RTU wiring for iC controllers, particularly when it comes to the type of cable to use and the importance of cable shielding. This is relevant as improper wiring can lead to inefficient device operation or even damage to the system.

Good Practice:

1. Cable Type: Use a 3-conductor cable with an impedance of 120 Ohms. Recommended types include Unitronic Bus LD and Belden RS485. The type of cable used can significantly impact the performance and reliability of your Modbus RTU network.

2. Wiring Style: Always wire in series, avoiding branch lines. This ensures a continuous, uninterrupted signal path, crucial for reliable data transmission.

3. Termination: Connect a 120 Ohm termination at the front (START) and back (END) of the bus. This prevents signal reflection, which can cause data corruption.

4. Number of Devices: iSMA devices' hardware supports up to 32UL (Unit Load). So if the RS485 transceiver of connected devices consumes 1 UL, you can connect up to 32 devices, but if single RS485 transceiver consumes 1/2UL or 1/4 UL (like in iSMA devices), you can connect respectively up to 64 or 128 devices without repeaters. For optimal performance, it is still recommended to limit the number to a maximum of 50 devices

5. Modbus Speed: The length of the cable and the rate at which data is polled can affect the Modbus speed. Shorter cables allow for a higher baud rate, while longer cables require a lower baud rate. Faster polling reduces the number of bus participants, while slower polling increases them.

6. Cable Length: Maximum length of the bus depends on the baud rate used. A lower baud rate, such as 9600, allows for the use of longer cable lengths up to the maximum of 1200 meters, as it is less susceptible to signal loss. A higher baud rate such as 115200 requires a significantly shorter cable than the maximum of 1200 meters. This is due to the increased likelihood of signal degradation over longer distances at higher speeds.

7. Cable Shielding: Even when the distance between the terminals of the components is not greater than 5 cm, shielding is still recommended to protect the integrity of data transmission by reducing electrical noise. Additionally, the shielding should be earthed directly after the cable enters the cabinet. This practice helps in mitigating the effects of electromagnetic interference from devices within the enclosure that generate electromagnetic radiation, such as relays, contactors, and transformers.

8. Internal Control Cabinet Wiring: Regular wires can be used within the internal control cabinet. However, ensure that there are no Variable Frequency Drive (VFD) devices or similar devices nearby to prevent interference.

9. Routing Cables in Separate Paths: Whenever possible, communication and power cables should be routed in separate channels or conduits.