July 26, 2023
Basic knowledge of RS485 interface
Introduction to RS485:
To expand its scope, Electronic Industries Association of America (EIA) formulated RS-485 standard from RS-422 in 1983, adding multipoint and two-way communication capabilities, that is, allowing multiple transmitters to connect to same bus, control and anti-collision characteristics of transmitter are increased simultaneously, and common-mode bus signal range is expanded, later referred to as TIA/EIA-Standard 485-A.
The origin of RS485 goes back to RS232 communication interface. The disadvantages of RS232 communication interface are as follows
(1) The interface signal level is relatively high, which makes it easy to damage interface circuit chip, and since it is incompatible with TTL level, it is necessary to use a level conversion circuit to connect with TTL circuit.
(2) The transfer rate is slow. During asynchronous transmission, baud rate is 20kbps, so in "Old south tree 51cpld breadboard", total program baud rate can only be 19200 baud, which is also reason.
(3) The interface uses a signal line and a signal return line to form a common ground transmission waveform. This common ground transmission is subject to common mode interference, so anti-noise interference is weak.
(4) The transmission range is limited, maximum standard transmission range is 50 feet, in fact, it can only be used at a distance of about 50 meters.
Due to lack of an rs232 interface, new interface standards have constantly emerged, and rs-485 is one of them.
It has following characteristics:
1. RS-485 electrical characteristics: logical "1" is represented by voltage difference between two lines as +(2-6)v, logical "0" is expressed by voltage difference between two lines as -(2-6)v said. The interface signal level is lower than rs-232, so it is not easy to damage interface circuit chip, and level is compatible with TTL level, which can be easily connected to TTL circuit.
2. The maximum RS-485 data rate is 10 Mbps.
3. The rs-485 interface uses a combination of a balanced driver and a differential receiver, which improves ability to withstand common-mode dryness, that is, it has good noise immunity.
4. The standard value of maximum transmission range of rs-485 interface is 4000 feet, which can actually be up to 3000 m. In addition, rs-232 interface allows you to connect only one transceiver to bus, which makes it possible to work with one station. The RS-485 interface allows up to 128 transceivers to be connected to bus. That is, it supports multi-station operation, so that users can easily set up a network of devices with one RS-485 interface.
Using RS485 interface
This communication interface provides multipoint two-way communication over a simple pair of twisted pairs. Its interference suppression capability, data rate, cable length and reliability are unmatched by other standards.
Therefore, RS-485 is used as a communication channel in many different fields, such as automotive electronics, LAN telecommunications equipment, smart buildings, etc. Another reason why this standard has become widespread is its versatility. The RS-485 standard only defines electrical characteristics of interface and does not include connecting cables or protocols. On this basis, users can set their own high-level communication protocol.
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