RS485 is convenient for multipoint interconnects, eliminating many signal lines. Using RS485, a distributed system can be networked, allowing up to 32 drivers and 32 receivers to be connected in parallel. In view of the shortcomings of RS232-C, the new standard RS485 has the following characteristics:
(1) Electrical characteristics of RS485: logic "1" is represented by the voltage difference between two wires + 2V ~ + 6 V, and logic "0" is represented by the voltage difference between two wires -6 V ~ -2 V. The interface signal level is lower than RS232-C, so it is not easy to damage the interface circuit chip, and the level is compatible with the TTL level, so it is convenient to connect with the 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, but in fact it can reach 3000 meters.
(5) The RS232-C interface allows only one transceiver to be connected on the bus, that is, single-station capability. The RS485 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. The switching between RS485 bus and RS422 bus can be easily realized 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 RS485 equipment to be as close as possible to the bus backbone. Using RS485 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 of the external environment.
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. Absorb external interference such as electromagnetic induction of the external environment, there by 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, RS485 signals will be continuously attenuated, which may lead to RS485 bus instability or even unavailability. 485 bus repeater can enhance and amplify the existing RS485 signals. Increase the number of RS485 devices in an RS422/RS485 bus network.
Isolation: In the field construction, due to the long distance of the 485 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.
RS232 interface standard. RS232 interface conforms to the serial data communication interface standard 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. RS232 is the abbreviation of English "Recommended Standard", RS232 is the identification number, and C represents the number of revisions.
The RS232-C 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 transmit line, a receive 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 transmission 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 a communication medium, and 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.