summarize
RS-485 (which should now be called EIA/TIA-485, except that everyone is more accustomed to RS-485) is theStandard interfaces for the physical layer of communicationRS-485 is a signaling method that is the first level of OSI (Open Systems Interconnection) mode.RS232Likewise a serial interface, but younger and faster than RS232, it was created to extend the physical capabilities of the RS-232 interface, which is known for its ability to be used effectively over long distances and in electrically noisy environments. Because of this, and the ability to transmit data over long distances, RS485 is commonly used as a protocol for POS, industry and telecommunications, and is also common in technological applications for computers, PLCs, microcontrollers and smart sensors.
RS stands for Recommendation Standard; 485 is the identification serial number; EIA is the Electronic Industries Association (EIA).lectronics Industries Association EIA).
Copper wire diameter is usually measured in AWG (American Wire Gauge) The value in front of the AWG (e.g. 24AWG, 26AWG) indicates the number of holes the wire passes through to form its final diameter; the larger the value, the more holes the wire passes through and the smaller the diameter of the wire.The
Abbreviation: AWG (American Wire Gauge)
The EIA-485 connection is accomplished using a two- or three-wire cable: the data wire, the wire with inverted data, and usually the zero wire (ground, 0 V). In this way, the transmitter and receiver exchange data over a twisted-pair cable consisting of 22 AWG (0.643 mm diameter) or 24 AWG (0.511 mm diameter) solid wire. The key point is that two wires are used to transmit a signal, and while one wire transmits the signal, the other wire transmits the reverse signal, which improves high reliability against common mode interference.
RS485 communication interface
In RS485 communication protocol, commands are sent by the node defined as the master node. All other nodes connected to the master node receive data through the RS485 port. Based on the information sent, zero or more nodes on the line respond to the master node
Due to the nature of the RS-485 interface, an RS-485 device cannot send and receive data at the same time as it would cause a transmitter conflict. Therefore, deterministic behavior is mandatory to avoid packet conflicts.
A communication network built on an RS-485 interface consists of transceivers connected via a twisted pair (two twisted pairs of wires).The basic principle of the RS-485 interface is differential (balanced) data transmission, which means that a signal is transmitted over two wires. Thus, one wire of the pair transmits the original signal and the other transmits its reverse copy. The large voltage difference between Data A and Data B determines the signal, with +2V to +6V indicating a logical 1 and -2V to -6V indicating a logical 0 signal.
Due to differential signal transmission, there is always a potential difference between the wires. This ensures high resistance to common mode interference. In addition, the twisted pair can be shielded, which ensures protection of the transmitted data. All of this allows data to be sent over long distances at relatively high speeds, up to 100 kbits / s at 1,200 m. 4,000 ft. or about 1,200 m is the maximum cable length in RS-485 communications. However, the general guideline is that the product of the line length (in meters) and the data rate (in bits / s) should not exceed 10 to the eighth power. For example, the maximum data rate allowed for a 20-meter cable is 5 Mbits / s.
RS-485 hardware interface
DB9 RS485 Definition: RS485 commonly used half-duplex two-wire interface is defined as 1-DATA-, 2-DATA+, 5-GND. because of RS485's DATA+ and DATA+ correspond to each other, and DATA- and DATA- correspond to each other, there is no signal inconsistency between male and female RS485 connectors.
There are several different naming methods for RS485 in the wire system: British marking is TDA(-) TDB(+) RDA(-) RDB(+) GND; American marking is Y Z A B GND.
RS-485 Communication Features
Bidirectional half-duplex data transmission
Serial data streams can be transmitted in one direction, and transmitting data to the other side requires the use of a transceiver. A transceiver (often referred to as a "driver") is a device or circuit that creates a physical signal on the transmitter side.
symmetric communications channel
Receiving or transmitting data requires two equivalent signal wires. The wires are used to exchange data (or) in two directions. With the help of twisted pair cables, the symmetrical channel significantly improves signal stability and suppresses electromagnetic radiation generated by useful signals. The configuration uses 4 wires, also known as a full-duplex configuration, which uses two differential pairs: labeled TR (transmit) and RX (receive), and a ground reference
multipoint
RS-485 communication lines can be connected to multiple receivers and transceivers. At the same time, one transmitter and multiple receivers can be connected to one communication line at a time. All other transmitters that need to be connected should wait until the communication line is free for data transmission.
RS485 Modbus protocol
RS485 is a physical level interface standard, while Modbus is the protocol. Similar to our video conference, through the ZOOM or Tencent is part of the interface standard, and the language used for communication is the protocol, some people use English, some people use Chinese for. Therefore, RS485 can use Modbus protocol for transmission, you can also use the transmission Profibus protocol.
While RS-232 devices are connected via text (ASCII) protocol, most RS485 devices use Modbus.
ModbusIt is a serial communication protocol which is widely used by industrial electronic devices. In Modbus, the connection is established between a master (host) and a slave (COM based device).Modbus helps to access the configuration of the device and read the measures.
Data exchange is initiated by the host. The master can switch its RS-485 drives to transmit mode itself, while the other RS485 drives (slaves) operate in receive mode. In order for the slave to answer the master over the communication line, the "master" sends a special command to it, which entitles the target device to switch its drive to transmission mode for a certain period of time.
World Power related RS-485 products
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