Overview of RS232 Communication and RS485 Communication
The data transmission between computer and computer or computer and terminal can adopt two ways of serial communication and parallel communication. Because the serial communication method has few lines and low cost, especially in remote transmission, it avoids the inconsistency of the characteristics of multiple lines and is widely used. Both the RS-232-C interface and the RS-485 interface are the most commonly used serial communication interfaces.
The RS-232 standard stipulates that a 25-pin DB25 connector is used, and the commonly used 9-lead signal only needs three interface lines, namely "send data", "receive data" and "signal ground". The transmission cable length should be 50 feet, and the actual application is about 50 meters. Because the RS-232-C interface standard appeared earlier, it is inevitable that there are shortcomings. The main signal level value of the interface is high, which is not compatible with TTL circuits; the transmission rate is low, and the baud rate is 20Kbps during asynchronous transmission. ; Common mode interference is generated in the form of common ground transmission, and the anti-noise interference is weak; only one transceiver is allowed to be connected on the bus.
In view of the shortcomings of RS-232-C, a new RS-485 interface standard appeared, which has the following characteristics: the interface signal level is reduced and compatible with TTL level; the maximum data transmission rate is 10Mbps; the ability to resist common mode interference is enhanced , that is, good anti-noise interference; the maximum transmission distance standard value is 4000 feet, which can actually reach 3000 meters; up to 128 transceivers are allowed to be connected on the bus.
RS485 network wiring method
When using multiple devices, it is necessary to use 485 network to network, and the bus uses a set of twisted pairs of shielded wires of more than 5 categories. According to the 485 wiring method, the devices are connected to the bus. Note: The equipment in the network cannot have duplicate numbers, otherwise the communication will not be possible.
RS-485 network requirements
1. It is recommended to use Category 5 or super Category 5 twisted pair with shielding layer for the network cable. The shielding layer is used as the system ground wire, which can reduce electromagnetic interference and common mode interference and improve the reliability of the system.
2. Because the transceiver of 485 stipulates that the common mode voltage can work normally between -7V and +12V, exceeding this range will affect the communication, and will seriously damage the communication interface. Therefore, it must be connected to the system ground. The shielding layer in the network cable or another set of twisted pair wires can be used as the system ground wire to connect the equipments in the network such as tools and computers together, and reliably connect to the ground from one point.
3. The communication line should be kept away from high-voltage wires as far as possible, and should not be parallel with the power line, and should not be bundled together.
4. The length of the branch line from the bus to each terminal should be as short as possible, generally not exceeding 5 meters. If the branch line is not connected to the terminal, there will be a reflected signal, which will cause strong interference to the communication, so it should be removed.
5. In the same network system, use the same cable to minimize the number of contacts in the line. Make sure that the joints are well welded and tightly wrapped to avoid loosening and oxidation. A single, continuous signal path is guaranteed as a bus.
6. The effective length of RS485 is up to 1200 meters, and the effective length of RS422 is up to 15 meters.
