As shown in Fig. (6), the core function of the software is to achieve ZigBee messages and BACnet Ethernet messages bidirectional parsing, encapsulating, addressing, sending and receiving process. The complete message transmission process is as follows: The local device encapsulates BACnet protocol messages NPDU into ZigBee frame; Get its ZigBee network address based on its MAC address of the remote device, then send ZigBee frame to the specified remote device using Z-Stack protocol AF_DataRequest function; When the remote device receives the ZigBee frame from the network, it calls afIncoming Data function to parse the data frame and extract NPDU messages; If the MAC address of the message is identical with its own address, it will save the ZigBee network address of the device initiating the request for message processing. Then the generated response messages are encapsulated into ZigBee frame and are sent to the ZigBee network address which is just saved.

Fig. (6) The basic framework.

In order to realize the above messages’ transmission process, the software assigns two separate tasks, i.e., BACnet Ethernet communication protocol processing and ZigBee communication protocol processing. At the same time, synchronous communication mechanism, synchronous transmission device address and data information need to be established between the different tasks. In other words, in addition to complete their separate protocol processing tasks, BACnet communication protocol and ZigBee communication protocol processing tasks are also need to handle collaborative tasks. In order to achieve this function, the “message” mechanism is used in the software design, namely, a “message” label is established for BACnet communication protocol and ZigBee communication protocol processing tasks, respectively,and it is designed to synchronize the relevant information of NPCI segment in the messages of BACnet and ZigBee message address field, and APDU segment of BACnet messages and ZigBee message data segment. In order to achieve multi-tasking capabilities, the software system architecture uses a free open-source embedded operating system ucOSII which has compact, scalable and real-time characteristics [7J. Nie, and C.H. Zhang, "Research on ZigBee and BACnet interconnection mechanism", Low-voltage Apparatus, vol. 22, pp. 1-3, 2008.]. The embedded operating system ucOSII is ported to Lm3s6911+ microcontroller and has its kernel configuration according to the need of protocol converter to achieve the corresponding functions.

Temperature data acquisition driver includes A/D driving function, digital zeroing function, mean filtering function. Since NTC is a non-linear sensing element, piecewise interpolation is used to have function approximation split into a number of broken lines or curve approximation based on their changes. After obtaining a sample value R, first determine in which broken line segment the size of R is located, and then the scale conversion value can be calculated by the linearized equation of the corresponding segment. Similarly, the more the number of segments, the higher the accuracy of linearization, and software overhead is also increased.

According to the BACnet standard, as BACnet standard device, temperature sensor must have a device object instance and an analog input object instance. The definition of object model of the smart temperature sensor is listed in Table 1 below [14B.B. Shao, Embedded Real-time Operating System ucOSII.. Beijing: University of Aeronautics and Astronautics Press, 2006.]:

Table 1
Definition of the object instance model of the sensor.

BACnet standard services can be divided into two types of services - confirmed services and non-confirmed services [6"Building automation and control systems-Part 5", Data Communication Protocol, 2010.ISO/TC205, ISO16484-5-2010, ]. The confirmed service is one-to-one communication, with one party initiating the service request, the other party to make the right response to receipt of the request. State machine TSM is used to describe the state of sender and receiver in the process of communication. This design uses a non-segmented confirm service, so as the state machine can be simplified into two states namely the idle state and waiting for a response state, as shown in Fig. (7).

Fig. (7) Service state machine.

In non-segmented confirm service, once the client device makes a request message, the timer starts counting. Upon receipt of the error message, rejected messages, aborted or simply confirmed messages, it will stop the clock and notify the client application. If the time value of the timer exceeds the APDU timeout attribute value of the client device object, then it will resend the confirm request message, with re-timing of the timer. If the number of retransmissions exceeds the required number of the property value of number of APDU retries of the customer terminal equipment and fails to receive proper acknowledgment message, it will discard the resend message and notify client to call request service so that the client application will deal with the messages that cannot be sent.

TSM's program implementation must first define an array of structures, which contains various parameters associated with the service status including invoke ID, retransmission count, request time, destination address, APDU messages etc. Then, define TSM management and state transition function. In TSM program, it manages the entire protocol stack invoke ID's distribution and use. Invoke ID is unique at the service request end, which is used to determine and identify whether a response message is corresponding to the request. In response end it may encounter the same invoke ID from requests of different devices, so the uniqueness of the state machine will be jointly determined by invoke ID and request source address.

BACnet ethernet communication protocol message consists of six parts: Application Protocol Data Unit (APDU), Network Protocol Data Unit (NPDU), Link Protocol Data Unit (LPDU),Logic Link Control (LLC), X82X82X03 for service access point information and physical layer message responsible for specific hardware transmitting/receiving message byte content. According to the definition of BACnet standard, a BACnet mark must be added before each encapsulated service data.

In data encapsulation process, the data block containing service information should be processed first. Then have APDU user data marked and encoded according to BACnet standard BACnet ASN.1 definition and service parameter structure. Then it has three parts - stationary application layer control information, network layer control information, and data link layer control information encoded. Parsing is the inverse process of the encapsulation. After receipt of message, the physical layer calls the decoding function of each layer to read the relevant parameters, and make judgments based on parameters.