Therefore, the deterministic behavior is mandatory to avoid collisions of data packets. When RS485 communication line is ready for operation at the physical level, it's time to think about the data transfer protocol - an agreement between the system's devices on the format of the data packets transmission.īy the nature of RS-485 interface, RS-485 devices cannot transmit and receive data at the same time, as it leads to a conflict of transmitters. For instance, a 20-meter cable allows a maximum data rate of 5 Mbits/s. A general guideline, however, is that the product of line length (in meters) and the data rate (in bits per second) should not be more than 108. All this allows sending data over long distances at relatively high speeds, which can reach 100 kbits/s at 4000 feet.Ĥ000 feet or about 1200 meters is the maximum cable length in RS-485 communications. In addition, the twisted pair may be shielded, which ensures the protection of transmitted data. This ensures high resistance to common mode interference. With that, one wire of the pair transmits the original signal and the other one transports its inverse copy.Īs a result of differential signal transmission there is always a potential difference between the wires. That means one signal is transported over two wires. The basic principle of RS-485 interface is differential (balanced) data transmission. Communication network built on the RS-485 interface consists of transceivers connected by a twisted pair (two twisted wires).
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