| Overview
MyStat supports BACnet MS/TP (Master-Slave/Token-Passing) communication over an RS-485 interface for integration with a Building Management System (BMS).
A BACnet client can use the BACnet MS/TP network to read supported sensor values, device status, configuration parameters, and HVAC control points from MyStat. Supported writable BACnet object properties can be written by the BACnet client. The availability and read/write access of BACnet objects and properties depend on the supported object types and device configuration.
| RS-485 Interface
MyStat provides an RS-485 interface for BACnet MS/TP communication through the following terminals
| Terminal | Function |
|---|---|
| A− | RS-485 communication |
| B+ | RS-485 communication |
| GND | Signal reference |
Connect MyStat to the BACnet MS/TP network in accordance with the BMS network design and applicable RS-485 wiring requirements.
| BACnet MS/TP Communication Settings
BACnet MS/TP communication parameters are configured through the MyStat Comm Options page. MyStat communicates with BACnet-compatible systems using BACnet MS/TP over RS-485.
The communication parameters configured on MyStat must match the corresponding parameters configured for the BACnet MS/TP network.
| Parameter | Default Value | Description |
|---|---|---|
| Protocol | BACnet | Communication protocol used by MyStat. |
| Address | 5 | Unique BACnet device identifier. |
| Baud Rate | 9600 bps | Supported rates: 9600, 19200, 38400, 76800, and 115200 bps. |
| Parity | None | Serial communication parity setting. |
| Stop Bits | 1 | Number of stop bits used for serial communication. |
| Max Master | 32 | Maximum master address configured for the BACnet MS/TP network. |
| Max Info Frames | 3 | Maximum number of information frames that can be transmitted before passing the token. |
Note: The Device ID must be unique within the BACnet system.
| 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)
| Object Name | Instance |
|---|---|
| Room Temperature | AI:0 |
| Humidity | AI:1 |
| CO2 | AI:2 |
| Pressure | AI:3 |
| Universal Input 5 | AI:4 |
| Ext Sensor Bus Temperature 1 | AI:5 |
| Ext Sensor Bus Temperature 2 | AI:6 |
| Ext Sensor Bus Temperature 3 | AI:7 |
| Ext Sensor Bus Temperature 4 | AI:8 |
| Ext Sensor Bus Humidity 1 | AI:9 |
| Ext Sensor Bus Humidity 2 | AI:10 |
| Ext Sensor Bus Humidity 3 | AI:11 |
| Ext Sensor Bus Humidity 4 | AI:12 |
| Ext Sensor Bus CO2 1 | AI:13 |
| Ext Sensor Bus CO2 2 | AI:14 |
| Ext Sensor Bus CO2 3 | AI:15 |
| Ext Sensor Bus CO2 4 | AI:16 |
| Ext Sensor Bus Pressure 1 | AI:17 |
| Ext Sensor Bus Pressure 2 | AI:18 |
| Ext Sensor Bus Pressure 3 | AI:19 |
| Ext Sensor Bus Pressure 4 | AI:20 |
Binary Inputs (BI)
| Object Name | Instance |
|---|---|
| Occupancy Status | BI:0 |
| Relay 1 | BI:1 |
| Relay 2 | BI:2 |
| Relay 3 | BI:3 |
| Relay 4 | BI:4 |
| Occupancy Status | BI:5 |
| Ext Sensor Bus Occupancy 1 | BI:6 |
| Ext Sensor Bus Occupancy 2 | BI:7 |
| Ext Sensor Bus Occupancy 3 | BI:8 |
| Ext Sensor Bus Occupancy 4 | BI:9 |
Binary Values (BV)
| Object Name | Instance |
|---|---|
| Relay 1 Enable | BV:0 |
| Relay 2 Enable | BV:1 |
| Relay 3 Enable | BV:2 |
| Relay 4 Enable | BV:3 |
| Relay 5 Enable | BV:4 |
| Analog Out 1 Enable | BV:5 |
| Analog Out 2 Enable | BV:6 |
| Analog Out 1 Pulse Enable | BV:7 |
| Analog Out 2 Pulse Enable | BV:8 |
| Enable Force Occupied | BV:9 |
| Enable Auto Away | BV:10 |
| Unoccupied Mode | BV:11 |
| Show Centigrade | BV:12 |
| Enable Onboard Temp/Humi Sensor | BV:13 |
| Enable Onboard CO2/Occu Sensors | BV:14 |
| Display Desired Temp | BV:15 |
| Reboot the Device | BV:16 |
Multi-state Inputs (MI)
| Object Name | Instance |
|---|---|
| Operating Mode | MI:0 |
| Active Conditioning Mode | MI:1 |
| Active Fan Speed | MI:2 |
| Occupancy State | MI:3 |
Analog Values (AV)
| Object Name | Instance |
|---|---|
| Desired Set Temperature Heating | AV:0 |
| Desired Set Temperature Cooling | AV:1 |
| Max Set Temperature Heating | AV:2 |
| Min Set Temperature Heating | AV:3 |
| Max Set Temperature Cooling | AV:4 |
| Min Set Temperature Cooling | AV:5 |
| Temperature Offset | AV:6 |
| Heating Deadband | AV:7 |
| Cooling Deadband | AV:8 |
| Min Humidity Setpoint | AV:9 |
| Max Humidity Setpoint | AV:10 |
| Analog Out 1 Min Setting | AV:11 |
| Analog Out 2 Min Setting | AV:12 |
| Analog Out 1 Max Setting | AV:13 |
| Analog Out 2 Max Setting | AV:14 |
| Zone CO2 Dampper Opening Rate | AV:15 |
| Zone CO2 Threshold | AV:16 |
| Zone CO2 Target | AV:17 |
| Proportional Constant | AV:18 |
| Integral Constant | AV:19 |
| Proportional Temperature Range | AV:20 |
| Integration Time | AV:21 |
| Unoccupied Setback | AV:22 |
| Heating Deadband Multiplier | AV:23 |
| Relay Activation Hysteresis | AV:24 |
| Analog Fan Speed Multiplier | AV:25 |
| Humidity Hysteresis | AV:26 |
| Forced Occupied Time | AV:27 |
| Auto Away Time | AV:28 |
| Auto Away Setback Temperature | AV:29 |
| FCU Aux Heating 1 Activate | AV:30 |
| FCU Two-Pipe Heating Threshold | AV:31 |
| FCU Two-Pipe Cooling Threshold | AV:32 |
| FCU Water Valve Sampling On Time | AV:33 |
| FCU Water Valve Sampling Wait Time | AV:34 |
| FCU Water Valve Sampling During Loop Deadband On Time | AV:35 |
| FCU Water Valve Sampling During Loop Deadband Wait Time | AV:36 |
| Fail-Safe Time | AV:37 |
| Cooling Stage 1 Fan Output | AV:38 |
| Cooling Stage 2 Fan Output | AV:39 |
| Heating Stage 1 Fan Output | AV:40 |
| Heating Stage 2 Fan Output | AV:41 |
| Linear Fan Speed Level Low | AV:42 |
| Linear Fan Speed Level High | AV:43 |
| Staged Fan Speed Level Low | AV:44 |
| Staged Fan Speed Level High | AV:45 |
| Analog Out Fan Default | AV:46 |
| Fan Runtime Post Conditioning | AV:47 |
| Staged Fan Analog Out at Recirculation | AV:48 |
| Installer Lock Pin | AV:49 |
| User Lock Pin | AV:50 |
| Stage Up Timer | AV:51 |
| Stage Down Timer | AV:52 |
Multi-state Values (MV)
| Object Name | Instance |
|---|---|
| Conditioning Mode | MV:0 |
| Fan Speed | MV:1 |
| Temperature Mode | MV:2 |
| HVAC Profile | MV:3 |
| Relay 1 Mapping | MV:4 |
| Relay 2 Mapping | MV:5 |
| Relay 3 Mapping | MV:6 |
| Relay 4 Mapping | MV:7 |
| Relay 5 Mapping | MV:8 |
| Analog Out 1 Mapping | MV:9 |
| Analog Out 2 Mapping | MV:10 |
| Universal In1 Mapping | MV:11 |
| Read/Write Behaviour
MyStat supports BACnet read and write operations for supported BACnet objects available through the BACnet MS/TP network. Values changed through the MyStat interface are reflected on the BACnet network during normal communication. Values written by a BACnet client are applied to MyStat and reflected on the local interface during normal device operation. The acceptable write range depends on the BACnet object and its configured properties. BACnet-written values are retained across device reboots and power cycles.
For temperature-related writes, ensure that the values are within the applicable temperature limits and that related parameters, such as heating and cooling deadbands, are configured appropriately for the intended HVAC operation.
Refer to the BACnet_Mapping_MyStat__ reference sheet for object-specific ranges
| BACnet MS/TP Communication Diagnostics
For diagnostic purposes, MyStat can be connected to a computer using a USB-to-RS-485 adapter and verified using YABE (Yet Another BACnet Explorer).
MyStat to USB-to-RS-485 Adapter
Connect the MyStat RS-485 terminals to the USB-to-RS-485 adapter as shown below.
| MyStat | USB-to-RS-485 Adapter |
|---|---|
| A− | A+ / A−* |
| B+ | B+ / B−* |
| GND | GND |
Note : RS-485 terminal labeling may vary by adapter manufacturer. Verify the adapter labeling before connecting the device.
Verify BACnet Communication
- Connect the MyStat RS-485 port to a USB-to-RS-485 adapter, and connect the adapter to the PC.
- Open YABE and select the COM port assigned to the USB-to-RS-485 adapter ( To identify the COM port, open Device Manager → Ports (COM & LPT) ) Configure the BACnet MS/TP connection using the communication parameters configured on MyStat.
| Parameter | Default | Supported | Description |
|---|---|---|---|
| Port | — | Available COM ports | COM port assigned to the USB-to-RS-485 adapter. |
| Baud | 9600 bps | 9600 bps, 19200 bps, 38400 bps, 76800 bps, 115200 bps | Communication speed. Must match the MyStat BACnet MS/TP baud rate. |
| Source Address | 5 | 0–127 | BACnet MS/TP MAC address used by YABE. For MyStat communication, set this to the same address configured as the Device ID in MyStat → Comm Options. |
| Max Master | 32 | 1–127 | Highest MS/TP master address used for master discovery and Poll For Master operation. |
| Max Info Frames | 3 | 1–255 | Maximum number of information frames transmitted while the device holds the token. |
- Start the connection and discover MyStat on the BACnet network.
- Open the discovered MyStat device and verify that the expected BACnet objects are available.
- Select the required objects, verify their values and properties, write supported values where applicable, and confirm the updates.
| Known Limitations
- BACnet write operations are limited to supported writable objects and their configured operational ranges.
- BACnet temperature writes are limited to the supported temperature range and applicable Heating/Cooling Min/Max limits.
- Compatibility and operational behavior may vary depending on the configuration and implementation of the third-party BACnet BMS.
- Schedules cannot be configured locally when the device operates as part of a wireless mesh network.
- When transitioning from a wireless mesh network to BACnet or Standalone operation, schedules are not automatically transferred or created. Required schedules must be configured manually.
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