Rs-485 adopts balanced transmission and differential reception to realize communication: the transmitting end converts the ttl level signal of the serial port into differential signal a, b two outputs, and reduces the differential signal to ttl at the receiving end after transmission through the cable. Flat signal. Since the transmission line usually uses twisted pair and differential transmission, it has strong anti-common mode interference capability, and the bus transceiver has high sensitivity and can detect voltages as low as 200mv. Therefore, the transmission signal can be recovered beyond the kilometer. The maximum communication distance of rs-485 is about 1219m, and the maximum transmission rate is 10mb/s. The transmission rate is inversely proportional to the transmission distance. At the transmission rate of 100kb/s, the maximum communication distance can be achieved. Distance, need to add 485 repeater. Rs-485 adopts half-duplex working mode and supports multi-point data communication. The rs-485 bus network topology generally adopts a terminal-matched bus type structure. That is to say, each bus is connected in series by a bus, and a ring or star network is not supported. If you need to use a star structure, you must use a 485 repeater or a 485 hub. The rs-485 bus generally supports up to 32 nodes. If a special 485 chip is used, it can reach 128 or 256 nodes, and the largest can support up to 400 nodes.
Rs485 wiring specificationBecause of its simple wiring, stable and reliable, the 485 bus is widely used in various fields such as video surveillance, access control intercom, building alarm, etc. However, there are many problems in the 485 bus wiring process due to many incomplete and accurate concepts. Now some clarifications are made about some of the wrong ideas.
The 1.485 signal line cannot be routed along with the power line. In the actual construction, since the traces are all taken through the pipeline, the construction party sometimes directly binds the 485 signal line and the power line for the convenience of the figure, because the strong electric power has a strong electromagnetic signal to interfere with the weak electricity, thereby The 485 signal is unstable, resulting in unstable communication.
2.485 signal lines can use shielded lines as wiring, or unshielded lines can be used as wiring. Since the 485 signal is transmitted using differential mode, the voltage difference between 485+ and 485- is used as the signal transmission. If there is an external interference source to interfere with it, when using twisted pair for 485 signal transmission, due to its twisted pair, the interference effect is the same for 485+, 485-, and the voltage difference is still unchanged. The interference to the 485 signal is reduced to a minimum. By the same token, if a shielded wire is used as a shielding function, the influence of external interference sources on it can be reduced as much as possible.
3. Choose to use the ordinary super five shielded twisted pair cable, that is, the network cable. Due to the rising prices of raw materials, the wire rods on the market are now mixed, and some bad businessmen use some kind of alloy to replace the copper wire to make the network cable, and copper plating on the outside to mix customers. The specific difference method: look at the cross section of the network cable. If it is copper, it is copper wire. If it is white, it is filled with alloy. Alloys are generally brittle, easy to break, and are far less conductive than copper wire, which can easily cause problems in engineering construction. Generally, it is recommended to select the standard 485 line, which is a shielded twisted pair. The transmission line is not a single strand of copper wire like a network cable, but a plurality of strands of copper wire are twisted together to form a line, so that even a small copper wire Broken, it will not affect the entire use.
4.485 wiring can be arbitrarily arranged into star wiring and tree wiring by means of 485 hub and 485 repeater. The 485 wiring specification is a wiring that must be hand-held. Once it is not directly connected to a star connection and a tree connection by means of a 485 hub and a 485 repeater, it is easy to cause signal reflection to cause bus instability. Many construction parties use star wiring and tree wiring in the 485 wiring process. Sometimes the whole system is very stable, but sometimes there are always problems, and it is difficult to find the cause, generally due to irregular wiring. Caused by it.
The 5.485 bus must be grounded. In many technical documents, it is mentioned that the 485 bus must be grounded, but there is no detailed description of how to ground. Strictly speaking, the 485 bus must be reliably grounded at a single point. The single point is that there can only be one point grounding on the entire 485 bus, and it is not possible to ground more points because it is grounded because the voltage on the ground wire (generally the shielded wire is used as the ground wire) is kept consistent to prevent common mode interference. If multiple points are grounded, it is counterproductive. When grounding reliably, the ground wire of the entire 485 line must have good contact, so as to ensure the voltage is consistent, because in actual construction, for the convenience of wiring, the wire is cut into multiple sections and then connected, but the shielded wire is not well connected. The ground wire is divided into multiple sections, and the voltage cannot be consistent, resulting in common mode interference.
The ideal line is twisted pair: the half-duplex two-wire is preferably a pair of twisted pairs, so that the two-wire twisted pair and the interference level added to the two lines cancel the anti-jamming effect. When two-duplex is used, two pairs are used to receive one pair, and two lines are used to send one pair. RS485 has no power transmission requirements, so the wire diameter is not high.
In actual engineering, the outdoor water-blocking twisted pair cable is usually purchased to ensure the protection performance of the line. However, some engineering companies will use RVV cables, which is also possible, but the anti-interference is worse. So there are engineers using RVVP cable (with shielding), this is not good, because the addition of line capacitance affects the transmission quality, you need to reduce the baud rate of transmission.
The baud rate setting has a certain correspondence with the cable length (including the total length of the branch). The longer the line, the lower the baud rate should be set.
No matter what kind of cable you choose, use the bus architecture as much as possible, reduce the star connection, and make the branch line as short as possible. Try to use the daisy chain connection method, that is, the bus receives the first node and then jumps to the next node. . The branch line of the unconnected device is preferably removed from the bus, otherwise interference is easily formed. If the receiving signal is not good at the far end of the bus, a line with 120 ohms can be added across the signal line. Do not add intermediate equipment, otherwise it will increase line loss and reduce the number and distance of equipment.
RS485 chips of different devices are usually different, and chips with different load types are not directly visible to the engineers. Therefore, the maximum number of connected devices of different devices on the bus is uncertain. For the same device connection number, refer to the device description requirements. The connection diagram of the daisy chain is given below.
Under the full-duplex four-wire system, the transmitting end line of the host is connected to the receiving end line of the slave, which is positively connected and negatively connected. The slave's transmit line corresponds to the host's receive line.
Under the half-duplex two-wire system, it is positively connected and negatively connected.
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