Loop Implementation Scheme for Computer Serial Communication

    For users who require the interface to have long-distance (>1km) high-speed (up to 19200bps) communication capabilities, as well as strong anti-interference and noise suppression capabilities in industrial sites, for ordinary RS232 interfaces, point-to-point single-line common ground connection (Singleended) is used. In the flat drive mode, the maximum communication distance is 15.25m, which obviously cannot meet the requirements. Therefore, consider the 20mA current loop driving method. Compared with the level mode, the current loop has one advantage:

    the level mode 20mA current loop mode

    Signal is easy to attenuate Signal is not easy to attenuate

    Susceptible to interference and strong anti-interference ability

    Long distance, high frequency suitable for long distance

    Low transmission performance and high-speed communication

    In this design, Motorola's optocoupler TIL117 is selected for signal conversion and photoelectric isolation. In this way, data information is transmitted through a closed current loop. The low-resistance transmission communication method of this current loop has a strong ability to suppress interference that enters the communication loop.

    The 20mA current loop interface communication method is shown in Figure 1.

    The circuit consists of the following two parts:

    1.DC-DC converter

    The DC-DC module is used to adjust the +5V DC voltage provided by the device to ±12V DC voltage to provide the driving voltage for the current source.

    2. Signal converter (current loop driver/receiver)

    The circuit of the signal converter is shown in Figure 2.

    The selection of communication protocol is carried out through SW1, see Table 1.

    The transmitter (TD, DTR) and receiver (RD) of this current loop interface board can work in active and passive modes. By selecting DIP switch SW2 on the interface board, you can switch between the two modes. When switching, it must be noted that for both parties in communication, the sending and receiving of the same signal must be active on one side, and only one side can be active.

Table 1 SW1 settings

Select DIP switch SW2, there are 4 communication methods, see Table 2. SW2 settings are shown in Table 3.

Table 2 SW2 settings

Table 3 SW2 settings

    "O" means ON, and a space means OFF.
    This product has been successfully developed and passed debugging at the user's site, and has now entered the mass production stage.