Design of intelligent frequency acquisition node based on LONWORKS

    1 Introduction

    In recent years, LONWORKS measurement and control network technology has emerged among many field buses and has been widely used in data acquisition and supervisory control systems (SCADA), industrial control, building automation, intelligent transportation and other fields. The key to realizing LONWORKS measurement and control network is the development of LONWORKS intelligent nodes. This paper proposes two design and implementation methods of LONWORKS frequency acquisition nodes, and makes a comprehensive comparison of their advantages and disadvantages.

    2. LONWORKS Smart Node

    The core processor neuron chip of LONWORKS intelligent node is NEURON 3120 or NEURON3150. Neuron chip has a unique internal structure: three CPUs are integrated inside the chip, CPU-1 is the media access controller, CPU-2 is the network processor, the two together complete the network communication function of the node, CPU-3 is the application processor, which is mainly responsible for executing the code written by the user and the operating system commands called by the user code. There are two types of typical LONWORKS intelligent node structures. One is a node with NEURON chip as the core, which processes both user applications and network communications, as shown in Figure 1(a); the other is a node with MIP structure, in which NEURON chip only acts as a communication processor, and the node application is executed by the main processor, as shown in Figure 1(b).

    ECHELON has integrated the dual-wire transceiver FTT-10A with the original neuron chip and recently launched the dual-wire intelligent transceiver FT3150 and FT3120 chips. The new FT3150 and FT3120 chips are compatible with the original 3150 chip in the design of peripheral circuits, thus saving users time spent on peripheral circuit design, not only reducing the difficulty and cost of node development, but also improving the anti-interference ability of the node. In addition, the FT3150 and FT3120 chips also support higher input clocks, with the highest input clock reaching 40M Hz, which greatly improves the processing speed of the entire chip. Therefore, the design uses the FT3150 chip to replace the NEURON chip and transceiver in Figure 1, and the node structure is simpler.

    3. Frequency acquisition node design based on neuron chip

    LONWORKS is a control network dedicated to the field of industrial measurement and control. In order to facilitate the use of the 11 I/O ports of the NEURON chip, they can be defined as 34 application objects through programming. Among them, the period input object, pulse counting object and total input object can measure the field frequency signal. The following are simple applications of these three types of IO objects:

    The period input object can measure the time interval between two rising edges or two falling edges of the input signal. The simple application is as follows:

    When FT3150 uses a 10M crystal oscillator , the resolution of the period input object is 25.6us. The pulse count input object can measure the average frequency of the input signal by using the input edge within 0.8388608s. The simple application is as follows:

    The total input object records the total number of rising or falling edge transitions of the input signal through a timing counter. When FT3150 uses a 10M crystal oscillator, the maximum frequency of the input signal is 2.5M Hz.

    The simple application is as follows:

    A simple frequency measurement implementation method is to use the above three IO objects. The design block diagram is shown in Figure 2.

    The 8-to-1 multiplex analog switch CD4051 is connected to FT3150 through IO_0~IO_4. FT3150 can define the four adjacent IO ports IO_0~IO_7 as nibble IO objects for channel selection. We use IO_0~IO_3 to implement this function. The definition of the nibble IO object is as follows: IO_0 output nibbleio_select; IO_4 is the measurement signal input pin.

    This type of node can collect 8-channel frequency signals and has the characteristics of simplicity and small size. Its disadvantage is that it is only suitable for occasions with low input signal frequency due to the limitations of the characteristics of the NEURON chip itself.

    4. Frequency acquisition node design based on multi-processor mode

    For occasions with higher measurement frequencies, the node design can adopt multi-processor mode, using AT89C52 single-chip microcomputer for frequency measurement, and FT3150 is responsible for processing LONWORKS network communication. The input frequency signal enters the programmable device CPLD for frequency division after passing through signal conditioning circuits such as limiting or amplification. After the single-chip microcomputer collects the divided signal, it is sent to FT3150 through the asynchronous serial port, and forwarded to the LONWORKS network.

    The node hardware design block diagram is shown in Figure 3:

    In the design, CPLD uses the programmable device ISP1016E to realize the frequency division of input signal and channel switching.

    ISP1016E has 2000 PLD gates, 32 input and output (IO) pins, 4 dedicated input pins, and the pins are compatible with TTL level. ISP1016E adopts high-performance E2CMOS technology, with a maximum operating frequency of 125MHz, and is electrically erasable and programmable. The design uses 16 IOs of ISP1016E as the input end of the signal; 3 IO ports (A, B, C) realize 8-to-1 selection; one IO port (CS) controls whether to divide the frequency. When CS=0, the input signal is not divided, and 8 low-frequency signals are input. When CS=1, the input signal is divided; then it is output to the microcontroller acquisition port through IO.

    FT3150 can send an interrupt signal to AT89C52 according to the control message on the network, start data collection, and receive the collected data from the microcontroller through the asynchronous serial port after a delay.

    I0_0 output bit starts; //Interrupt signal is sent to I/O port. Starts is a custom object name, and output bit means that IO is defined as a bit output object. Sending the start acquisition signal can be achieved through the following statement.

    After the microcontroller detects the level change from high to low on the IO_0 pin, it interrupts and collects data. After collecting the data of 8 channels, it sends the collected data through the asynchronous serial port. The implementation program of FT3150 receiving data is as follows:

    IO_8 input serial baud(4800)data_in;

    IO_8 is defined as a serial input port with a baud rate of 4800b/s, io_in(data_in, in_buffer, 17);

    Receive collected data and store it in the input buffer.

    5 Conclusion

    The experimental results show that the frequency acquisition node based on the multi-processor mode is significantly superior to the node based on the neuron chip in terms of acquisition speed, acquisition accuracy and measurement range. It not only saves the IO port of the neuron chip, but also makes it easy to expand functions due to the use of multiple processors.