RS485 serial bus is widely used in the communication distance from tens of meters to thousands of meters. RS485 uses balanced transmission and differential reception, so it has the ability to reject common mode interference. Coupled with the high sensitivity of the bus transceiver, which can detect voltages as low as 200 mV, the transmitted signal can be recovered from kilometers away.
RS485 adopts half-duplex working mode, and only one point can be in the sending state at any time, so the sending circuit must be controlled by the enable signal. RS485 signal to Ethernet signal, that is, serial server equipment, in industry, security, remote meter reading and other scenes have its figure, using its signal conversion function, to achieve signal conversion and improve the network.
Working principle of RS485
RS485 to Ethernet provides the function of converting serial port to network, which can convert RS485 serial port into TCP/IP network interface, and realize bidirectional and transparent data transmission between RS485 serial port and TCP/IP network interface. The serial device can immediately have the function of TCP/IP network interface, connect the network for data communication, and expand the communication distance of the serial device.
RS485 is a differential signal negative logic, + 2V ~ + 6V represents "1", and -2V ~ -6V represents "0". RS485 communication interface is divided into two types, one is two-wire system, and the other is four-wire system. The two-wire system is a half-duplex communication mode, and the four-wire system is a full-duplex communication mode. RS485 communication interface, strong anti-interference ability, good anti-noise interference. In industrial control, RS485 bus is widely used because of its simple interface, convenient networking, long transmission distance and other characteristics.
The RS485 interface is a half-duplex data communication mode. The half-duplex network composed of the interface is generally a two-wire system in use, and the shielded twisted pair is often used for transmission. Such a wiring mode is a bus-type topology, and 32 nodes can be connected to the same bus at most. RS485 in the communication network uses a master with multiple slaves.In most cases, a simple pair of twisted pairs is used to connect the two ends of the "A" "B" "of each interface when connecting RS485 communication. The RS485 interface connector adopts the 9-core socket head of DB-9, the RS485 interface of the intelligent terminal adopts DB-9, and the keyboard interface RS4485 connected with the keyboard adopts DB-9.
The wiring is simple: only 2 wires are needed, and the shielded wire interface of the network cable is adopted;
Long communication distance: differential mode communication is adopted, and the communication distance is long;
Multi-machine networking: RS485 supports mounting multiple terminals on line, and realizes multi-machine communication by means of query-response.
The RS485 to Ethernet hardware converter is divided into two types: serial server and serial networking module.
The serial server realizes the conversion from RS485 interface to TCP/IP interface, and with the use of the virtual serial driver on the computer side, the TCP/IP connection is virtually converted into a serial port again.
The virtual serial port driver creates virtual serial port numbers such as COM5 and COM6 in the driver layer of the computer, and binds the remote serial server to the virtual serial port to realize communication.
After using the hardware and software system of the solution, the RS485 device is equivalent to the expansion of the network, and the original hardware and software programs do not need to be modified.
RS485 serial server
Basic functions: RS485 serial port to Ethernet, bidirectional transparent transmission, support Modbus forwarding;
Guide rail installation: exquisite volume, standard 35mm guide rail installation, easy layout and installation, convenient for later maintenance;
TCP Server: As a Server, the device can be set with two TCP clients to perform transparent data transmission with the device at the same time, so as to solve the problem of simultaneous communication between multiple clients and the device. It can also be used as a standby network channel for switching communication;
TCP Client: As a Client, the device can be set to face four server addresses at the same time, actively initiate connections to the server, and as a Client, support the setting of custom heartbeat packets and handshake packets;
UDP Server: as the Server end, the device supports multiple UDP clients to transmit data with the device;
UDP Client: as the Client end, the device actively communicates with the UDP Server and transparently transmits data with the RS232 serial port;
UDP multicast: as the Client end, the device can realize the mutual communication between the devices in the multicast;
Http server: as the HTTP server, the device supports HTTP communication in the local area network through HTTP (get).
What are the features of the RS485 repeater
RS485 is very convenient when used for multipoint interconnection, and many signal lines can be omitted. The application RS485 can be networked to form a distributed system that allows up to 32 drivers and 32 receivers to be connected in parallel. In view of the shortcomings of RS-232-C, the new standard RS-485 has the following characteristics:
(1) Electrical characteristics of RS485: logic "1" is represented by the voltage difference between two lines + 2 V ~ + 6 V, and logic "0" is represented by the voltage difference between two lines -6 V ~ -2 V. The interface signal level is lower than that of the RS232-C, so that the interface circuit chip is not easy to be damaged; and the level is compatible with the TTL level, so that the interface is convenient to be connected with a TTL circuit.
(2) Maximum data transmission rate: 10Mbps
(3) RS485 interface adopts the combination of balanced driver and differential receiver, which has strong anti-common-mode interference ability, that is, good anti-noise performance.
(4) The standard maximum transmission distance of RS485 interface is 4000 feet, which can actually reach 3000 meters.
(5) The RS232-C interface allows only one transceiver to be connected on the bus, that is, single-station capability; The RS-485 interface only allows up to 128 transceivers to be connected on the bus, that is, it has multi-station capability, so users can easily establish a network of devices using a single RS485 interface.
RS485 repeater, as RS485/422 data relay communication product, features and functions:
High efficiency: The RS485 repeater adopts the professional I/O circuit and uses the automatic control technology of data flow direction to automatically distinguish and control the direction of data transmission. It can easily switch between the RS485 bus and the RS422 bus without changing the software and hardware. Zero delay design is adopted, and the transmission speed is fast. Automatic detection of serial port signal rate, no need to set the serial port baud rate.
Anti-interference: RS485 bus requires 485 equipment to be as close as possible to the bus backbone. Using 485 bus repeater, the bus topology can be changed into "T" topology, which is convenient for field construction. If the RS485 transmission line reaches a certain distance and is in a complex external environment, it is vulnerable to external interference such as electromagnetic induction.
Stability: The lightning protection tube in the RS485 repeater can effectively suppress lightning and ESD, and provide lightning surge protection power of 600 W per line. Absorbs external interference such as electromagnetic induction of external environment, thereby protecting the stability of the RS485 bus.
Expansibility: Due to the extension of RS485 bus transmission distance or the increase of devices in RS485 bus, the RS485 signal will be continuously attenuated, which may lead to the instability or even unavailability of RS485 bus. The RS485 repeater can enhance and amplify the existing RS485 signal. Increase the number of RS485 devices in the RS422/RS485 bus network.
Isolation: In the field construction, due to the long distance of the RS485 bus, the potential difference is formed, resulting in common mode interference, resulting in reduced stability. Photoelectric isolator can provide isolation voltage, effectively isolate and prevent common-ground interference. The DC/DC module can completely isolate the power supply at both ends.
Difference between RS485 and RS232
The interface standard of RS232. The RS232 interface conforms to the interface standard of serial data communication formulated by the Electronic Industry Alliance (EIA) of the United States. It is widely used in the connection of computer serial interface peripherals. For example, some old PCs are equipped with RS232 interface. RS232 is one of the mainstream serial communication interfaces. RS is the abbreviation of English "Recommended Standard", 232 is the identification number, and C represents the number of revisions.
The RS232 bus standard has 25 signal lines, including a main channel and an auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines are needed, such as a sending line, a receiving line and a ground line. Another reason for the short transmission distance is that RS232 is a single-ended signal transmission, which has problems such as common-ground noise and inability to suppress common-mode interference, so it is generally used for communication within 20 m.
Due to the early appearance of RS232 interface standard, it is inevitable that there are shortcomings. The RS232-C standard specifies data transfer rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second.In a device network composed of RS232 or RS485 devices, if there are more than two devices, RS485 must be used as the communication medium. The devices in the RS485 network can only communicate with each other through the transfer of a "Master" device, which is usually a PC, and only one Master device is allowed in such a device network. The rest are all Slave devices.Fieldbus technology is based on ISO/OSI model and has a complete software support system, which can solve the problems of bus control, conflict detection, link maintenance and so on.
Application field of RS485 serial serv
RS485 is often used in long-distance transmission with high data transmission rate, and the commonly used equipment includes point-of-sale terminals (POS), measuring instruments and large-scale special automation machines. The commonly used RS232 is now generally used in short-distance transmission with low data transmission rate, and can also work effectively in noisy environments, such as factories, public sites, etc. Its commonly used equipment includes low-speed modems, industrial control equipment, and programmable logic controllers (PLC)., computer numerically controlled (CNC) machine tools, robotics, embedded control computers, medical instruments and equipment, and embedded controller development systems.
However, in normal applications, we often need to convert between two different interface types. At this time, we can use the popular USB interface, which can help us convert to RS232 interface and RS485 interface.
To sum up, RS232 is suitable for short-distance and low-rate transmission, while RS485 is suitable for long-distance communication transmission. RS485 is a new interface standard for the shortage of RS232 interface. Because of its good anti-noise interference ability and multi-station ability, it has become the preferred standard.
RS485 bus frequent fault
RS485 repeater, RS485 converter and other equipment in the conventional RS485 bus communication will encounter many problems in the application process, such as: unable to communicate, abnormal data upload and download, communication interference, abnormal flickering of communication indicator light, communication instructions on and off, etc., which affect the normal use. The usual detection methods are as follows:
1. Common ground method: connect the grounding points of all RS485 equipment with one wire or shielded wire to avoid potential difference affecting communication between all equipment.
2. Terminal resistance method: Connect a 120 ohm terminal resistor in parallel on RS485 + and RS485- of the last RS485 device to improve the communication quality.
3. Middle section disconnection method: check whether the equipment is overloaded, the communication distance is too long, and the damage of a certain equipment has an impact on the entire communication line by disconnecting from the middle;
4. Separate wire pulling method: simply and temporarily pull a wire to the equipment to rule out whether the communication failure is caused by wiring.
5. Converter replacement method: Replace several converters to rule out whether the converter quality problem affects the communication quality.
6. Notebook debugging method: First ensure that the notebook computer is a device with normal communication, and then replace it with another computer for communication. If it is possible, it indicates that the serial port of the computer may be damaged or injured.