Designing a deterministic RS-485 multi-node protocol

Why a custom RS-485 protocol?

When we built the EOD ROV for the Bangladesh Army, off-the-shelf buses couldn't give us deterministic timing across motors, sensors, and the robotic arm at once. So we designed a master-slave broadcast protocol on the RS-485 physical layer.

• Deterministic polling windows per node
• CRC-checked broadcast frames
• Graceful degradation when a node drops

This work became our Best Paper at ICCIT 2024. In these notes I'll unpack the design decisions and what I'd do differently next time.