On the control equipment of public broadcasting and intercom broadcasting system, the single chip microcomputer is used to send and receive control signals through the RS-485 serial bus interface. RS-485 serial bus interface is adopted. In the half-duplex working mode, the bus interface only needs a pair of twisted pairs. MCS-51 series single-chip microcomputers all have serial transmit and receive data ports TXD and RXD. Coupled with the RS-485 serial bus interface chip MAX1487, a complete two-wire control data sending and receiving system can be formed. Among the previous public broadcast system control equipment, the remote control microphone is an important sound transmission, zone control selection equipment. With a partition, there is a partition control output. When it is necessary to control multiple partitions, it is necessary to have multiple control terminals, and correspondingly to control the receiving and executing equipment, many terminals are required. That is, a large number of cable cores are required. In this way, inconvenience to the project construction and increase in cable costs are caused. In this way, the disadvantages are particularly prominent in large-scale multi-zone public broadcasting systems. RS-485 serial bus interface is used to simplify the control interface and reduce the number of cores used in the cable. The single-chip microcomputer is used to simplify the control circuit of the public broadcast system control equipment and reduce the number of keys on the equipment panel.

 

    1. Composition of public broadcast system control equipment

 

    The control equipment of the public broadcasting system using the RS-485 serial bus interface includes: front-end control equipment remote microphone, zone control decoder, zone control expander, and layer decoder. The system application is shown in Figure 1.

 

    2. Principles of each device

 

    (1) Remote-control microphone:

 

    The remote control microphone is composed of a microphone, a microphone amplifier, a cascade control circuit, a MCS-51 series single-chip microcomputer, and an RS-485 serial bus interface chip MAX1487. The zone control input is completed by ten numeric keys from 0 to 9 and four function keys. The function keys are "full call", "on / off", "enter" and "speak". There is also a function switch. The keys have two colors, most of which are partition selection keys, and the other of which are function keys. The function keys include "full call" key, "talk" key, and "off" key.

 

    When in use, to call a zone by broadcasting, first press “On / Off”, enter the number of the zone to be broadcasted through the number keys, and then press the talk button; for full-call broadcasting, first press “On / Off”, and then directly press the full-call button. Broadcast. For fire emergency broadcasting, press "On / Off" first, and enter the zone number to be broadcast through the number keys, and then broadcast according to the relevant requirements of the fire alarm. Control signal RS-485 is issued.

 

    Function switch: When the function switch is set to "fire", this remote microphone can only perform emergency fire broadcasting. When the function switch is set to "normal", this remote microphone can only perform call broadcasting.

 

    (2) Partition control decoder

 

    It consists of RS-485 serial bus interface chip MAX1487, MCS-51 series single-chip microcomputer, and control latch output circuit. Receive control signals from RS-485 serial bus, automatically distinguish between fire emergency broadcast mode control signals and call broadcast mode control signals, and decode and output them (there are 16 output control terminals).

 

    (3) Partition control expander

 

    The circuit principle is the same as the partition control decoder. The difference is that there is an additional coded control switch for setting the control zone number range to be expanded. Each unit has 16 partitions.

 

(4) Layer decoder

 

    The layer decoder is composed of MCS-51 series single-chip microcomputer, coding switch and relay switching circuit. The layer decoder receives the control signal from the RS-485 serial bus and automatically recognizes the type of the control signal. When the received control signal is related to the decoder, the relay switching circuit operates to switch between background music broadcasting and emergency broadcasting.