MyStat as a BACnet Server

| Overview

The MyStat is part of 75F’s vertically integrated suite of intelligent building solutions, designed to provide smart zone-level control, runtime monitoring, and an enhanced user experience for commercial building environments. The device is built to operate both within a 75F ecosystem connected to a CCU and as part of an external BACnet-based Building Management System (BMS), enabling greater interoperability and deployment flexibility.

The MyStat supports BACnet MS/TP (Master-Slave Token Passing) communication over the RS-485 bus, allowing external BACnet clients to monitor device parameters, read runtime status, and write supported control values through standardized BACnet objects. Firmware continuously manages synchronization between the local MyStat interface and the BACnet network to ensure consistent operation across local and remote interactions. In addition to BACnet integration, the device can also function independently in standalone mode using locally configured profiles and runtime settings. 

| BACnet Architecture 

The MyStat supports BACnet MS/TP (Master-Slave Token Passing) communication over the RS-485 bus, enabling integration with BACnet-compatible Building Management Systems (BMS). As a BACnet server device, the MyStat exposes supported runtime parameters and control points as BACnet objects, allowing external BACnet clients to monitor device status and perform supported control operations. 

The device can operate both within a 75F ecosystem connected to a CCU and in third-party BACnet environments, providing flexibility for different building integration requirements. Through the BACnet network, authorized BACnet clients can access supported operational data such as temperature values, occupancy status, setpoints, and runtime control parameters based on the configured BACnet object mapping. 

Firmware continuously manages communication handling and synchronization between the local MyStat interface and the BACnet network to maintain consistent operation across local and remote interactions during runtime. 

| BACnet Object Mapping 

The MyStat exposes supported runtime parameters, sensor values, status points, and configurable control settings as BACnet objects over the BACnet MS/TP network. These BACnet objects allow external BACnet clients and Building Management Systems (BMS) to monitor device operation, retrieve runtime data, and perform supported control actions through standardized BACnet communication.  

The BACnet object mapping includes sensor readings, occupancy status, relay states, operational modes, setpoints, fan configuration, HVAC profile configuration, and other runtime control parameters required for HVAC monitoring and control applications. Depending on the object type and intended functionality, objects may support read-only monitoring or read/write control access. 

The following BACnet objects are supported by the MyStat device: 

Analog Inputs (AI) 

Binary Inputs (BI)

Binary Values (BV) 

Multi-state Inputs (MI) 

Analog Values (AV) 

Multi-state Values (MV) 

| Read/Write Behaviour 

The MyStat supports BACnet read and write operations for supported BACnet objects exposed through the BACnet MS/TP network. Runtime values updated locally through the MyStat interface are reflected on the BACnet network during normal communication updates, allowing external BACnet clients and Building Management Systems (BMS) to maintain synchronized runtime visibility. 

When a BACnet client writes supported values to the device, the updated parameters are synchronized with the local MyStat interface during the communication polling cycle. Runtime operation and associated control behaviour are updated accordingly based on the active profile and current device operation. Related system functionalities dependent on the updated parameter also respond as part of normal runtime execution. 

BACnet write operations continue to follow existing installer lock, user lock, and configurable property restrictions configured on the device. Operational limits and parameter boundaries are controlled through the respective property configurations.  

BACnet-written values are retained across device reboot and power cycle conditions, ensuring configuration persistence during normal operation. Users should ensure that writable temperature-related parameters are configured appropriately based on the intended HVAC operation, as runtime relationships such as deadband dependencies should be considered during configuration. 

| Runtime Workflow 

During device startup in BACnet mode, the MyStat initializes BACnet communication, registers supported BACnet objects and establishes communication over the BACnet MS/TP network. Once initialization is completed, the device becomes available for BACnet client communication, monitoring, and supported control operations. 

During normal runtime operation, BACnet communication is managed continuously as part of the device operation. Depending on deployment requirements and system configuration, communication updates may operate through periodic polling or Change of Value (COV)-based communication. Runtime synchronization is maintained between the local MyStat interface and the BACnet network to ensure consistent operational behaviour across local and remote interactions. 

The MyStat supports operation within both a 75F ecosystem connected to a CCU and third-party BACnet environments, providing flexibility for different building integration requirements while maintaining consistent BACnet communication handling during runtime. 

| Fault Handling & Recovery 

The MyStat is designed to maintain stable runtime operation during BACnet communication abnormalities and unsupported communication scenarios. If an invalid BACnet write operation or unsupported object access is attempted, the device retains the previously accepted valid value to prevent unintended runtime behaviour or configuration changes. 

During RS-485 disconnection or BACnet bus interruption conditions, BACnet communication and external data exchange are interrupted until communication is restored. Communication failure may be observed externally through loss of expected BACnet updates or control behaviour. 

BACnet communication faults do not interrupt standalone HVAC runtime operation. The MyStat continues operating using the active runtime profile and previously retained configuration values, ensuring uninterrupted local device functionality during BACnet communication loss conditions. 

Following communication restoration, the device automatically re-establishes BACnet communication and resumes normal runtime synchronization with the BACnet network. During BACnet communication interruption conditions, the communication icon on the device display starts blinking to indicate the communication loss state. 

For communication-level diagnostics and troubleshooting activities, serial terminal tools such as Tera Term may be used for runtime log monitoring and debugging purposes. 

| Debugging & Diagnostics

The MyStat supports BACnet MS/TP communication over the RS-485 network and can communicate with 75F CCUs, third-party BACnet devices, or BACnet diagnostic tools connected through a laptop or external BACnet client. During commissioning and troubleshooting activities, communication can be validated through BACnet device discovery, successful object polling, and verification of runtime value updates across the BACnet network. 

Standard RS-485 troubleshooting methods such as verification of device addressing, baud-rate configuration, network polarity, termination, and physical wiring inspection are recommended during communication debugging activities. BACnet object read and write operations may also be used to validate runtime communication behaviour and confirm proper data exchange between the MyStat and connected BACnet devices. 

For communication monitoring and runtime verification, BACnet diagnostic applications such as YABE, BACnet Explorer, Wireshark with BACnet decoding support, and USB-to-RS485 adapters may be used to monitor BACnet traffic, validate BACnet object accessibility, and confirm runtime data transmission over the RS-485 network. Serial terminal applications such as Tera Term may also be used during firmware debugging and communication verification activities. 

| Known Limitations 

Writable BACnet objects support value updates only within the supported operational range and configured parameter limits of the device. BACnet write operations outside the supported range are not applied to runtime operation. 

Compatibility and operational behaviour may vary depending on the configuration and implementation of the third-party BACnet Building Management System (BMS) environment.

| Summary 

The MyStat supports BACnet MS/TP communication over the RS-485 bus, enabling integration with BACnet-compatible Building Management Systems (BMS) for monitoring and control applications. Through standardized BACnet object mapping, external BACnet clients can access supported runtime parameters, monitor device status, and perform supported control operations. 

The device is designed to operate both within a 75F ecosystem connected to a CCU and in third-party BACnet environments while maintaining standalone runtime HVAC functionality. Runtime synchronization between the local MyStat interface and the BACnet network ensures consistent operational behaviour across local and remote interactions. 

The BACnet implementation supports stable runtime communication handling, reboot recovery behavior, and continued standalone operation during BACnet communication interruption conditions, providing flexible integration capability for BACnet-supported building automation deployments.