BACnet B-SS (Smart Sensor) - Python example

A minimal, copy-paste-friendly example showing how to implement the BACnet B-SS (Smart Sensor) device profile in Python, using the CAS BACnet Stack. It listens on BACnet/IP (UDP 47808), answers ReadProperty, and is discoverable via Who-Is / I-Am.

This is the first Python example in the BACnet profile example series. There is no prior Python example repo to copy structurally - this one was built from the C++ edition's device model and the C# edition's documentation skeleton, with a binding mechanism new to the series: Python's standard- library ctypes, against a vendored, generated adapter (cas_bacnet_stack/), not an embedded C++ link or P/Invoke.

TUTORIAL.md - how to extend this example and how to review it for conformance. Read it when you start turning this into your own device.
docs/PICS.md - the Protocol Implementation Conformance Statement: every object, every property, and who answers it.

Versions: this document describes example v1.0.0, built and verified against the CAS BACnet Stack submodule pinned at 6.x, at Protocol_Revision 24, with the vendored Python common/ helper at v1.0.0. Running the example prints the app and API versions - if what it prints disagrees with this line, trust the program and check CHANGELOG.md.

What is the B-SS (Smart Sensor) profile?

B-SS (BACnet Smart Sensor), defined in Annex L of ANSI/ASHRAE 135, is the simplest BACnet device profile - the standard describes it as "a simple sensing device with very limited resources." It is meant for inexpensive, fixed-function sensors (temperature, humidity, occupancy, a contact, ...) that mostly just need to report what they measure when asked.

A B-SS device answers ReadProperty and is discoverable. It does not have to support WriteProperty (a smart sensor is read-only), alarming / event reporting, scheduling, or trending, and this example implements none of them on purpose.

But it is still a full BACnet device. Even the simplest profile must present the standard object model - a Device object, a Network Port object (every device needs one), and its sensor objects - and each object must expose all of its required properties. The CAS BACnet Stack generates most of those automatically (Object_Identifier, Object_Type, Status_Flags, Object_List, Protocol_*, ...); this example supplies the handful that are application-specific. The result is conformant for Protocol_Revision 24. docs/PICS.md lists every property and who answers it.

The device this example creates

Device 389001  "Rainbow"   (Vendor 389 - Chipkin Automation Systems)
    │
    ├── Analog Input 1       "Bronze"      Present_Value  21.5     (REAL, degrees Celsius; read-only)
    ├── Binary Input 1       "Emerald"     Present_Value  inactive (read-only)
    ├── Multi-State Input 1  "Hot Pink"    Present_Value  1        (state 1..3; read-only)
    └── Network Port 1       "Vermilion"   the BACnet/IP port     (required on every device)

What this example supports

The example implements exactly the capabilities below - and nothing more, which is the point of a profile example.

BIBBs (BACnet Interoperability Building Blocks)

BIBB	Description	Supported
DS-RP-B	Data Sharing - ReadProperty - B	✅
DM-DDB-B	Device Management - Dynamic Device Binding - B	✅
DM-DOB-B	Device Management - Dynamic Object Binding - B	✅

Services (executed / B-side)

Service	Notes
ReadProperty	Responds to property reads (DS-RP-B).
Who-Is / I-Am	Answers Who-Is with I-Am, and broadcasts an I-Am on start-up (DM-DDB-B).
Who-Has / I-Have	Answers Who-Has with I-Have (DM-DOB-B).

Object types

Object type	Instance	Name
Device	389001	Rainbow
Analog Input	1	Bronze
Binary Input	1	Emerald
Multi-state Input	1	Hot Pink
Network Port	1	Vermilion

Every required property of every object, and who answers it, is in docs/PICS.md.

Requires the CAS BACnet Stack (licensed product)

This example builds against the CAS BACnet Stack, which is a commercial Chipkin product - it is not free or open source, and there is no public/trial build. The stack is referenced here as the private git submodule submodules/cas-bacnet-stack (tracking the 6.x branch). Unlike the C++ edition (compiles the stack's own source straight into the executable) or the C# edition (P/Invoke against a separately built native library), the Python edition links against that same separately built native shared library (CASBACnetStack_x64_Release.dll on Windows / libCASBACnetStack_x64_Release.so on Linux) through the standard-library ctypes module - see Build the native CAS BACnet Stack library below. To get the CAS BACnet Stack (and access to build this example), contact Chipkin: https://store.chipkin.com/services/stacks/bacnet-stack or sales@chipkin.com.

What's in this repository

This is a self-contained project. It ships:

main.py - the example device. Run it directly with python3 main.py.
common/ - the shared Python helper (UDP, callbacks, CLI) vendored in.
cas_bacnet_stack/ - the vendored Python adapter (copied from the submodule's adapters/python/ - see cas_bacnet_stack/README.md).
docs/PICS.md - the conformance statement.
docs/objects.json - the input to the objects-and-properties generator.
submodules/cas-bacnet-stack/ - the CAS BACnet Stack as a git submodule (private; requires a license - see above). Its adapters/python/ is the source cas_bacnet_stack/ is copied from; its source/ and MSVC/CMake build produce the separate native shared library this example loads at run time.

Prerequisites

Python 3.8+ (python3 --version). No third-party packages - standard library only (ctypes, socket, argparse, signal; msvcrt on Windows or termios/tty/select on POSIX for interactive keys).
Git (to fetch the stack submodule).
To build the native library (see below): on Windows, Visual Studio 2022 Build Tools (MSBuild) targeting the stack's projects/msvs/BuildCASBACnetStack.sln; on Linux, a C++17 toolchain (g++/clang++).

Build

git clone --recursive https://github.com/chipkin/BACnetProfileExample-B-SS-Python.git
cd BACnetProfileExample-B-SS-Python

Already cloned without --recursive? Run git submodule update --init --recursive first.

There is no compile step for main.py itself - Python is interpreted. What you still have to build is the native CAS BACnet Stack library the ctypes adapter loads at run time (below); Python never builds that for you.

Build the native CAS BACnet Stack library

The Python adapter's bind() sets up ctypes bindings against an already- open library handle; main.py opens it with ctypes.CDLL("CASBACnetStack_x64_Release.dll") (Windows) / ctypes.CDLL("libCASBACnetStack_x64_Release.so") (Linux), preferring a copy next to main.py if one exists there, else falling back to the OS's own library search path. There is no bundled copy and no PyPI package. Build it once from the stack submodule, then copy it next to main.py.

Windows - the stack's MSVC solution has a ReleaseDll|x64 configuration that builds exactly this DLL, under exactly this name:

& "C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\MSBuild\Current\Bin\MSBuild.exe" `
  submodules\cas-bacnet-stack\projects\msvs\BuildCASBACnetStack.sln `
  /p:Configuration=ReleaseDll /p:Platform=x64

copy submodules\cas-bacnet-stack\bin\CASBACnetStack_x64_Release.dll .

(Adjust the MSBuild path to your Visual Studio / Build Tools install; vswhere or the "Developer Command Prompt for VS 2022" both resolve it for you if you have Visual Studio Installer set up.)

Linux - the stack does not currently ship a ready-made shared-library CMake target (its CMake adapters/cpp/ builds a static adapter; its Linux projects/linux/ build is for the gtest suite). Build one directly from the stack's own sources with g++:

g++ -shared -fPIC -std=c++17 -O2 \
    -I submodules/cas-bacnet-stack/source \
    submodules/cas-bacnet-stack/source/*.cpp \
    -o libCASBACnetStack_x64_Release.so

Either way, the DLL/.so must end up in the same directory as main.py - that is the first place main.py's load logic looks.

Why `property_buffer_helper.py` is here but unused

cas_bacnet_stack/ vendors all three of the adapter's files for parity with its expected set, but property_buffer_helper.py packs buffer-shaped Send* calls (ReadProperty/WriteProperty/CreateObject as a client) that this read-only B-SS example never makes - a Smart Sensor only ever answers ReadProperty, it never initiates one. It is imported nowhere; that is a documented, deliberate choice, not an oversight. See cas_bacnet_stack/README.md.

Run

python3 main.py --port 47821 --deviceID 12345

Expected output:

BACnet B-SS (Smart Sensor) Example - Python v1.0.0
CAS BACnet Stack v6.0.21.0
FYI: listening on <your-ip>:47808 (broadcast <your-subnet-broadcast>)
FYI: Device 389001 ("Rainbow") ready. Vendor ID 389. Press 'h' for help.

The device listens on UDP 47808 (BACnet/IP) by default. Allow that port through your firewall. To use a different port, pass --port.

Command-line options

Option	Default	Meaning
`--port <n>`	`47808`	UDP port to listen on (BACnet/IP).
`--deviceID <n>`	`389001`	The device's BACnet instance number (BACnet requires this to be configurable).
`--help`, `-h`	-	Show usage and exit (via `argparse`).
`--version`	-	Print the example and stack versions, then exit.

Interactive commands

While the example runs in a real console (not redirected - see TUTORIAL.md for what that means), these keys are available:

Key	Action
`h`	Show the version information and this command list.
`q`	Quit.
up arrow	Increase Analog Input 1 (`Bronze`) by 1.1.
down arrow	Decrease Analog Input 1 (`Bronze`) by 1.1.

The up/down keys change the live Present_Value of the analog input, so a client re-reading it sees the new value. Ctrl+C also shuts the device down cleanly (an interactive console is not required for that).

Verify

Use a BACnet client such as the CAS BACnet Explorer:

Discover - send a Who-Is. The device replies with I-Am from instance 389001 (vendor 389). It also broadcasts an I-Am at start-up.
Browse the object model - the device shows five objects: the Device (Rainbow), the three sensors, and the Network Port (Vermilion). Reading the Device's Object_List returns all five.
Read the Device - ReadProperty 389001 -> Object_Name returns "Rainbow"; Protocol_Revision returns 24; Description returns the profile description string.
Read a sensor - ReadProperty Analog Input 1 -> Present_Value returns 21.5; Units returns degrees-Celsius; Out_Of_Service returns false; Object_Name returns "Bronze". Repeat for Binary Input 1 ("Emerald", has Polarity) and Multi-State Input 1 ("Hot Pink", has Number_Of_States = 3). Every required property of every object is readable.
Confirm the profile boundary - a WriteProperty to any object is rejected. That is correct: a B-SS Smart Sensor is read-only.

For a property-by-property review against the conformance statement, see TUTORIAL.md.

How the Python binding differs from the C++/C# editions

If you have read either sibling example, the object model and callback shapes are identical - this section is only about what is mechanically different because Python talks to the stack through ctypes instead of an embedded link or P/Invoke. See common/CHANGELOG.md for the full list; the highlights:

An explicit load step, like C++. main.py calls ctypes.CDLL(libname) then bacnet.bind(library) itself, before any BACnetStack_* call - closer to the C++ edition's LoadBACnetFunctions() than the C# edition's implicit first-P/Invoke-call resolution. A missing/wrong-architecture library fails as OSError at that explicit point, with a clear error message.
ctypes POINTER out-parameters, indexed like a list. Every Get*Property callback receives ctypes.POINTER objects it writes with value[0] = ... or for i in range(n): value[i] = ... - the same idea as C#'s raw pointers, different syntax, no managed-buffer marshaling layer.
A separately built native library, the same one the C# edition uses - there is no "compile the stack into the executable" mode for Python (unlike the C++ edition's default SOURCE link mode). You build the native shared library once, explicitly (see above), and it has to be found next to main.py at run time.
Callbacks must be rooted against CPython's reference counting, not a GC pause: an unrooted ctypes.CFUNCTYPE object can be freed as soon as nothing references it, which can be the very next line, not "eventually". Every callback is appended to a module-level list for exactly that reason - see the comments in common/cas_example_helper.py and main.py.

The BACnet profile example series

The CAS BACnet Stack supports every standardized device profile in ASHRAE 135-2024 Annex L, and there is one example repository per profile. Pick the profile your device claims, then the language you build in. "Ask" means the example hasn't been built yet for that language - contact Chipkin if you need one.

Controllers (Annex L.4)

Profile	C++	Node.js	C#	Rust	Python	Go
B-SS Smart Sensor	B-SS-CPP	B-SS-Node	B-SS-CS	B-SS-Rust	B-SS-Python	B-SS-Go
B-SA Smart Actuator	B-SA-CPP	Ask	Ask	Ask	Ask	Ask
B-ASC Application Specific Controller	B-ASC-CPP	B-ASC-Node	Ask	Ask	Ask	Ask
B-AAC Advanced Application Controller	B-AAC-CPP	Ask	Ask	Ask	Ask	Ask
B-BC Building Controller	B-BC-CPP	Ask	Ask	Ask	Ask	Ask

Life safety controllers (Annex L.5)

Profile	C++	Node.js	C#	Rust	Python	Go
B-LSC Life Safety Controller	B-LSC-CPP 🚧	Ask	Ask	Ask	Ask	Ask
B-ALSC Advanced Life Safety Controller	B-ALSC-CPP	Ask	Ask	Ask	Ask	Ask

Access control controllers (Annex L.6)

Profile	C++	Node.js	C#	Rust	Python	Go
B-ACC Access Control Controller	B-ACC-CPP	Ask	Ask	Ask	Ask	Ask
B-AACC Advanced Access Control Controller	B-AACC-CPP	Ask	Ask	Ask	Ask	Ask

Lighting controllers (Annex L.11)

Profile	C++	Node.js	C#	Rust	Python	Go
B-LD Lighting Device	B-LD-CPP	Ask	Ask	Ask	Ask	Ask
B-LS Lighting Supervisor	B-LS-CPP	Ask	Ask	Ask	Ask	Ask

Elevator controllers (Annex L.13)

Profile	C++	Node.js	C#	Rust	Python	Go
B-EM Elevator Monitor	B-EM-CPP	Ask	Ask	Ask	Ask	Ask
B-EC Elevator Controller	B-EC-CPP	Ask	Ask	Ask	Ask	Ask
B-AEC Advanced Elevator Controller	B-AEC-CPP	Ask	Ask	Ask	Ask	Ask

Authentication and authorization (Annex L.14)

Profile	C++	Node.js	C#	Rust	Python	Go
B-AS Authorization Server	B-AS-CPP	Ask	Ask	Ask	Ask	Ask

Miscellaneous (Annex L.7, combinable with any one family)

Profile	C++	Node.js	C#	Rust	Python	Go
B-BBMD Broadcast Management Device	B-BBMD-CPP	Ask	Ask	Ask	Ask	Ask
B-ACDC Access Control Door Controller	B-ACDC-CPP	Ask	Ask	Ask	Ask	Ask
B-ACCR Access Control Credential Reader	B-ACCR-CPP	Ask	Ask	Ask	Ask	Ask
B-RTR Router	B-RTR-CPP	Ask	Ask	Ask	Ask	Ask
B-GW Gateway	B-GW-CPP	Ask	Ask	Ask	Ask	Ask
B-DAP Device Address Proxy	B-DAP-CPP	Ask	Ask	Ask	Ask	Ask
B-SCHUB BACnet/SC Hub	B-SCHUB-CPP	Ask	Ask	Ask	Ask	Ask
B-GENERAL General device (Annex L.8)	(satisfied by every example above)	—	—	—	—	—

Operator interfaces and workstations (Annex L.1–L.3, L.9–L.10, L.12)

Client-side profiles.

Profile	C++	Node.js	C#	Rust	Python	Go
B-OD Operator Display	B-OD-CPP	Ask	Ask	Ask	Ask	Ask
B-OWS Operator Workstation	planned	—	—	—	—	—
B-AWS Advanced Operator Workstation	planned	—	—	—	—	—
B-XAWS Extended Advanced Operator Workstation	planned	—	—	—	—	—
B-LSAP Life Safety Annunciator Panel	planned	—	—	—	—	—
B-LSWS Life Safety Workstation	planned	—	—	—	—	—
B-ALSWS Advanced Life Safety Workstation	planned	—	—	—	—	—
B-ACSD Access Control Security Display	planned	—	—	—	—	—
B-ACWS Access Control Workstation	planned	—	—	—	—	—
B-AACWS Advanced Access Control Workstation	planned	—	—	—	—	—
B-LOD Lighting Operator Display	planned	—	—	—	—	—
B-ALWS Advanced Lighting Workstation	planned	—	—	—	—	—
B-LCS Lighting Control Station	planned	—	—	—	—	—
B-ALCS Advanced Lighting Control Station	planned	—	—	—	—	—
B-ED Elevator Display	planned	—	—	—	—	—
B-EWS Elevator Workstation	planned	—	—	—	—	—
B-AEWS Advanced Elevator Workstation	planned	—	—	—	—	—

🚧 = in progress. "Ask" = not yet built for that language; contact Chipkin if you need it. Profile definitions: ANSI/ASHRAE 135-2024 Annex L. BIBB definitions: Annex K. Get the stack: https://store.chipkin.com/services/stacks/bacnet-stack.

References

ANSI/ASHRAE Standard 135 (BACnet) - the protocol standard. Object model (Clause 12), services (Clause 15), BACnet/IP (Annex J), device profiles (Annex L). Purchase / preview via the ASHRAE store.
What is BACnet? - Chipkin's introduction: https://docs.chipkin.com/protocols/bacnet/.
CAS BACnet Stack - product page and documentation: https://store.chipkin.com/services/stacks/bacnet-stack.
CAS BACnet Explorer - client for testing this device: https://store.chipkin.com/products/tools/cas-bacnet-explorer.
Shared helper used by this example - common/README.md.
Vendored adapter used by this example - cas_bacnet_stack/README.md.

See also TUTORIAL.md, docs/PICS.md, CHANGELOG.md, and AGENTS.md.

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BACnet B-SS (Smart Sensor) - Python example

What is the B-SS (Smart Sensor) profile?

The device this example creates

What this example supports

BIBBs (BACnet Interoperability Building Blocks)

Services (executed / B-side)

Object types

Requires the CAS BACnet Stack (licensed product)

What's in this repository

Prerequisites

Build

Build the native CAS BACnet Stack library

Why `property_buffer_helper.py` is here but unused

Run

Command-line options

Interactive commands

Verify

How the Python binding differs from the C++/C# editions

The BACnet profile example series

Controllers (Annex L.4)

Life safety controllers (Annex L.5)

Access control controllers (Annex L.6)

Lighting controllers (Annex L.11)

Elevator controllers (Annex L.13)

Authentication and authorization (Annex L.14)

Miscellaneous (Annex L.7, combinable with any one family)

Operator interfaces and workstations (Annex L.1–L.3, L.9–L.10, L.12)

References

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cas_bacnet_stack		cas_bacnet_stack
common		common
docs		docs
submodules		submodules
.gitignore		.gitignore
.gitmodules		.gitmodules
AGENTS.md		AGENTS.md
CHANGELOG.md		CHANGELOG.md
LICENSE		LICENSE
README.md		README.md
TUTORIAL.md		TUTORIAL.md
main.py		main.py

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BACnet B-SS (Smart Sensor) - Python example

What is the B-SS (Smart Sensor) profile?

The device this example creates

What this example supports

BIBBs (BACnet Interoperability Building Blocks)

Services (executed / B-side)

Object types

Requires the CAS BACnet Stack (licensed product)

What's in this repository

Prerequisites

Build

Build the native CAS BACnet Stack library

Why property_buffer_helper.py is here but unused

Run

Command-line options

Interactive commands

Verify

How the Python binding differs from the C++/C# editions

The BACnet profile example series

Controllers (Annex L.4)

Life safety controllers (Annex L.5)

Access control controllers (Annex L.6)

Lighting controllers (Annex L.11)

Elevator controllers (Annex L.13)

Authentication and authorization (Annex L.14)

Miscellaneous (Annex L.7, combinable with any one family)

Operator interfaces and workstations (Annex L.1–L.3, L.9–L.10, L.12)

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Why `property_buffer_helper.py` is here but unused