RS-485 is a long-standing differential wired communication heavily relied upon in many industrial applications from factory and building automation, motor drives, medical devices, and grid infrastructure to name a few. The standard is prized for its ability to create long distance wired communication networks across multiple communication nodes while maintaining the robustness required for harsh industrial operating environments. With bus lengths up to 1.2 km (approx. 4000ft) cabling quickly becomes one of the costliest aspects of the system, with cabling requiring at minimum two data lines, neutral, and power cables; it becomes quickly apparent that cost saving measures would be beneficial to end users.
TI’s Powerbus is one way to meet this need as it allows power and data to share the same cabling through On-Off Keying (OOK) modulated RS-485. However, Powerbus and Classical RS-485 are not directly compatible so typically they cannot be mixed and matched. This is problematic for some end users that could benefit from Powerbus, but in some of their systems they don’t control the design of every communication node, so they can’t directly implement Powerbus. This application note aims to guide end users how to design one board that can be placed either in Classical RS-485 systems or systems using Powerbus.
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In long distance RS-485 systems the need to find cost effective solutions without risking performance is paramount to a successful system design. TI’s Powerbus devices, the THVD8000 and THVD8010, tackle this issue by using OOK modulation of RS-485 and an external coupling network to allow data and power to share the same bus. However due to the nature of Classical RS-485 against Powerbus neither device can properly understand each other preventing communication between Classical RS-485 and Powerbus.
This presents a problem in large systems where the designer may not have control of every communication node connected to the bus and can cause the designer to fall back on Classical RS-485 to ensure compatibility in all of their systems. However combined solutions can exist to allow one board to be designed for multiple systems – Powerbus or not. This note will first cover what Powerbus is and how it works. Next it will delve into the differences between Powerbus and Classical RS-485 analyzing where the incompatibility lies. Finally, a look into a joint solution that can be applied into varied systems.