For greenhouse automation, I wanted a protocol that survives long cable runs and noisy environments. Modbus RTU over RS485 is still one of the best options for this type of deployment.
I split the system into three layers: sensing, control logic, and actuation. Sensor values are exposed as holding registers, while relay and valve commands are written through a small command map. Keeping register addresses documented in a table prevented integration mistakes later.
The control loop is intentionally conservative. I use hysteresis bands instead of single thresholds to avoid relay chatter, and I enforce minimum on/off intervals for pumps and fans.
A dedicated heartbeat register tells me if each device is alive. If heartbeat updates stop, the controller falls back to a safe default profile instead of trying to continue with stale readings.