Let's first get to know what RS-485 is and what characteristics RS-485 has.
RS-485, in which the full term RS is called Recommended Standard, which means recommended standard, or a standard level signal. In 1983, the Electronics Industries Association (EIA) approved RS-485 as a standard communication interface. After 40 years of development, RS-485 has become one of the most commonly used buses in industrial sites.
The reason why RS-485 is so popular is mainly due to its transmission rate of up to 10Mbps, transmission distance of 1200 meters and its multi-point topology connection characteristics. RS-485 uses differential form for data transmission, which allows it to receive correct transmission data even when it is interfered by the environment.
Ⅰ. The charm of RS-485 communication protocol
RS-485, with its unique half-duplex and multi-point communication capabilities, can connect multiple devices on a bus at the same time (theoretically up to 32), and the maximum transmission distance can be extended to 1200 meters, which makes it show unparalleled cost-effectiveness in the face of large-scale deployment. However, with the diversification of application requirements, simple point-to-point connections can no longer meet the communication requirements in complex scenarios. At this time, the introduction of RS-485 hubs is particularly important.
Ⅱ. The current limitations of RS-485 hubs
Due to the characteristics of differential transmission, the RS485 hub makes its bus transmission mode half-duplex (supports bidirectional transmission and reception, and does not support bidirectional simultaneous transmission and reception). Therefore, the protocol running on the 485 bus is usually a question-and-answer protocol (the host actively sends, and the slave sends after receiving the host request), such as the ModBus protocol. This means that there can only be one host in a system, otherwise the data sent between each other will interfere, resulting in no normal data being received.
Ⅲ. The "magic" of 485 hub
- Built-in arbitration mechanism to resolve bus conflicts
The RS-485 hub has multiple host ports. After two or more serial ports send control commands at the same time, the commands are sent in order, and data will not be sent at the same time. After the multi-host gateway is turned on, after any host sends data, the data of other hosts will be blocked until the slave command is received or the host timeout period is reached. In addition, the data returned by the slave will only be restored to the corresponding host, and will not be replied to other hosts, so there will be no data conflict.
- Break the transmission distance limit and realize long-distance communication
Although the transmission distance of RS-485 bus can reach more than 1000 meters, this is the theoretical value of RS-485 transmission distance. In fact, it will be affected by many factors such as environment and wire lighting. For application scenarios with long-distance coverage, relying on only one set of RS-485 cables cannot meet the long-distance transmission requirements. The built-in signal enhancement and relay technology of RS-485 hub can receive the attenuated signal, and resend the signal after amplification, effectively extending the communication distance of RS485 bus.
- Improve system stability and isolate interference
The electromagnetic environment at industrial sites is complex and changeable, and interference is everywhere. The RS-485 hub can effectively block external interference through electrical isolation design, prevent the influence of ground loops between different devices, and ensure the stability and security of communication. At the same time, when the RS-485 bus detects strong interference, it can prevent damage to the device at the other end of the bus.