HVAC Applications

HVAC controllers are being unified in order to control a rooftop unit, chiller control unit, air-handling unit and more. HVAC controllers use standard Ethernet to interface with higher-level building automation systems such as the building management system, as well as to daisy-chain multiple HVAC controllers. In order to maintain network connectivity when any of the HVAC controllers are powered off, electromechanical relays short the Ethernet signals at the input and output ports.

HVAC controllers have multiple analog, digital or field bus interfaces to communicate or control multiple sensors that measure parameters like temperature, humidity, and pressure (see HVAC Controller Interface). The sensors could be analog output with loop power, or support 0- to 10-W/4- to 20-mA output with separate power. HVAC controllers can also connect to actuators like dampers, fans and stepper motor drives through communication interfaces or analog connections. Having SPE connectivity from the controller to the sensors and actuators would simplify installation with only two wires and enable access to devices at the edge.

SPE-Based Sensing illustrates one example implementation using a humidity sensor, where the I2C interface connects to a microcontroller (MCU) with built-in media access control (MAC). SPE PHY (10BASE-T1L or 10BASE-T1S) interfaces to the built-in MAC of the MCU, while PoDL with a DC/DC converter powers the entire circuit. This architecture offers multiple benefits, including standardization of sensor connectors, reusable hardware, sensor and hardware diagnostics and calibration.

Having multiple SPE ports within a HVAC controller to interface various sensors and actuators would require an application-specific integrated circuit to implement Ethernet switch functionality, which is readily available.