The best way to build your small BACstac device

by Svetlana Lyons December 15, 2015

The best way to build your small BACstac device

One important trend in the market has been the growing emphasis on integrated systems. By enabling products from different vendors to interact with one another, interoperable building systems based on the BACnet open standard improve building management, increase operational efficiency and flexibility, and hold down service and expansion costs. The power of today’s BACnet-based systems continues to increase as the number of interoperable devices increases.

Another trend is the variaty and modest cost of different platforms such as Atmega or ARM. More and more companies are developing their own small BACstac devices and it is not a trivial task.

For the developers and anyone who wants to know more technical details here is in depth information why you want to use uBACstac.

1. UBACstac is well-structured and highly portable code.

  The same library source code is run unmodified on a wide range of architectures, from 8-bit ATMega to 32-bit ARM, to POSIX (Linux) OS.

  There is a well-defined and documented interface for communication with underlying hardware or operating system facilities.

  Porting to a new hardware platform is as simple as it can be. For example, to implement MS/TP only timer ISR and UART routines for initialization and sending/receiving a byte are required. A typical implementation is under 5KB of liberally commented source code.

2. The uBACstac design is very flexible. Being just a couple of libraries, it can be easily adapted to an existing framework or integrated with an existing application. The uBACstac can be run on wide range of architectures, ranging from bare-metal micro-controllers, to real-time OS, to general purpose OS. It does not impose any restrictions on the platform: no threads are required, no synchronization primitives (mutexes, semaphores, etc), no memory allocation (malloc/new). These facilities can be used with uBACstac, but the portable uBACstac code does not use these.

3. uBACstac features a highly effective implementation of BACnet protocol.

  - Zero copy architecture all the way from ASN.1 encoders in application to sending data to UART, and back. The PDU payload is never copied or moved.

  - State of the art MS/TP implementation with optional fully backward-compatible extension to the BACnet standard allowing windows size >one on MS/TP network.

  - Low-level receive/transmit UART functions are fully decoupled from the portable MS/TP FSM implementation, allowing the most effective implementation for a particular platform (DMA/ISR, even a daughter micro-controller).

  - Implementation is compatible with lock-free implementation. The libraries do not explicitly use any synchronization primitives, so they are compatible with any synchronization implementation.

4. uBACstac features predictable latency, and is compatible with hard real time devices. The libraries do not 'sleep' or run a 'busy loop' or 'polling loop' internally.

   The application code is fully decoupled from the BACnet protocol transaction state machines, so application code is not blocked when the protocol state machine is "waiting" for a message or "sending" a message to the network.

5. uBACstac supports segmentation. This is a non-trivial task, and if the framework is not engineered for segmentation in the first place, then it is very hard to add segmentation support at a later stage.

6. UBACstac is configurable at compilation time, allowing an application developer to disable certain features, like segmentation or DCC service support, minimizing memory footprint and processing burden.

7. The software is well structured and interfaces are documented. The provided portable example is truly portable: it is compiled from the same source on all supported platforms.

The example is not a trivial "switch()/case()" all-properties-hardcoded program, but is a flexible code, maintaining a simple object/property database. The code is a good starting point for developing a real-world production-grade BACnet device.

8. The software is designed and implemented to be robust in a real world environment. The corner cases and possible protocol errors are treated thoroughly. For example, ASN.1 encoders check for buffer overflows and prevent memory corruption, if the encoded value does not fit in the allocated buffer. In addition, the uBACstac software transparently allocates buffers for segmented responses as needed; the application developer does not need to be concerned with this.




Svetlana Lyons
Svetlana Lyons

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