RS-485 bus smart home system architecture

The RS-485 bus is a circuit standard for multiple transmitters and multiple receivers. This standard specifies only the transmission level, rate, etc., and does not specify the transmission protocol. The communication protocol between the sender and the receiver needs to be designed by the developer himself. Its characteristics are low cost, mature technology, simple structure, high reliability and strong anti-interference ability.

1 appliance control module

The common feature of appliances used in modern homes is relay control. Connect the corresponding control line of the function relay to the module MCU on the control panel of the home appliance, and the system can be conveniently controlled through the RS-485 bus interface.

Use the 7 bit input/output Darlington MC1413 driver relay to control the related switching. For air conditioners, temperature control, wind direction control, etc., are key switches, all of which can be controlled by relays. Taking into account the need to embed home appliances, the control node to do as little as possible, and there is no complicated control object, so use AT89C2051 microcontroller as a controller. The RS-485 communication interface uses the MAX1487 chip, which allows 125 home appliances to be connected and uses half-duplex communication. The communication speed is 2.5 Mbit/s. Home appliance control communication is only the transmission of control commands and home appliance status information, and the amount of data is small. The chip communication rate can meet the requirements.

2. Security monitoring module

The security system is composed of a smoke sensing section, a gas sensing section, and a real-time monitoring section, which realizes functions such as fire prevention, anti-gas poisoning, and anti-theft monitoring. These components are composed of smoke sensors and gas sensors, etc., and combine AD conversion chips to transmit information.

The real-time monitoring part uses the OV7141 image acquisition chip produced by Ovinmin. OV7141 is a highly integrated camera chip that supports a variety of formats: The Serial Camera Control Bus (SCCB) interface is provided. It provides a simple control method and can modify all register values ​​inside the OV7141 chip. OV7141 contains 8bit data, synchronization signals, these signals need to be sent to the CPU to read the image data and ensure synchronization.

Since the data transmission speed is limited by the serial port speed, the serial port baud rate must be increased. According to the characteristics of the system, when the serial port transmission mode 3 is used, the baud rate is variable and can be changed in real time according to the transmission effect.

The system uses RS-485 bus communication. The "Polling System" is adopted in communications, and the repeater, as the master device, continuously sends a device request response frame downward, and the accessed slave device cannot actively send data. At any moment, the bus is only in one way, receiving data or sending data. Once the system initialization is completed, the bus is set to the receiving state. When the slave device receives the message sent from the master device, the bus is immediately set to the sending state through software. After the slave device sends a frame of data, it immediately changes to Receive status, waiting for the master device's request frame. If the device ID address and the self ID address match in the sent request frame, the receiving device checks whether the frame is correct.

The system uses an embedded control chip as a control unit and a general-purpose RS-485 bus as a transmission network to achieve low-cost, long-distance transmission, which can meet the needs of remote home appliances for general households, and has high economical efficiency and practicality.

The RS-485 wiring specification is as follows:

1) The 485 signal cable must not be routed with the power cable. In actual construction, because the cabling passes through the pipeline, the constructor sometimes directly binds the 485 signal line and the power cable together to save time. The strong electric signal has a strong electromagnetic signal to interfere with the weak electricity, which leads to instability of the 485 signal. , leading to communication instability.

2) The 485 signal line can be wired using shielded wires, or it can be wired using unshielded wires. Since the 485 signal is transmitted using the differential mode, the voltage difference between 485+ and 485- is used as the signal transmission. If there is an external interference source to interfere with it, when the twisted-pair cable is used for 485 signal transmission, the interference effect is the same for the 485+ and 485- interferences due to its twisted pair, and the voltage difference remains unchanged. The interference to the 485 signal is minimized. By the same token, if a shielded wire acts as a shield, then the interference from external sources of interference can also be minimized.

3) Select the use of ordinary super five shielded twisted pair cable that can be. Due to the increase in raw material prices, the wire rods on the market are now mixed. Some bad traders have used some kind of alloy instead of copper wires to make network cables, and copper plating on the outside to deceive customers. The specific difference method: Look at the cross section of the network cable. If it is copper, it is copper wire; if it is white, it is to use the alloy to shoddy. The alloy is generally brittle, easy to break, and the conductivity is far inferior to copper wire, which is very easy to cause problems in the construction. The wire is generally recommended to select the standard 485 line, which is a shielded twisted pair. The transmission line is not a single strand of copper wire like the cable, but a plurality of strands of copper wire twisted together to form a line, so even if a small copper wire is broken Loss will not affect the use of the entire line.

4) 485 wiring can be arbitrarily arranged into a star-shaped wiring and a tree-shaped wiring with a 485 hub and a 485 repeater. The 485 cabling specification is a cabling that must be "hands in hand." Once it is not routed directly into a star and tree connection with a 485 hub and 485 repeater, signal reflections can easily occur, resulting in bus instability. Many construction parties use star-type wiring and tree-type wiring in the 485 wiring process. Sometimes the entire system is very stable, but sometimes there are always problems and it is difficult to find the reasons. Generally, it is due to irregular wiring. caused.

5) The 485 bus must be grounded. In many technical documents, it is mentioned that the 485 bus must be grounded, but there is no detail how to ground it. Strictly speaking, 485 bus must be a single point of reliable grounding. The single point is that only one point on the entire 485 bus can be grounded, and it cannot be grounded at multiple points because it is grounded so that the voltage on the ground (usually the shielded ground) is consistent to prevent common mode interference. If you ground more points, it will be counterproductive. When reliable grounding, the ground line of the entire 485 line must have good contact, so as to ensure the voltage is consistent, because in the actual construction, in order to facilitate the connection, the line shear is reconnected in multiple segments, but the shield line is not well connected, The ground is divided into multiple segments, so that the voltage can not be consistent, leading to common-mode interference.

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