1. RS485 interface is a serial communication protocol. It allows multi-address communication.
2. To communicate with multiple RS485 ports, first set the main communication port, then set different communication addresses for other ports and set the communication timing. Build 485 bus communication network can be.
How to achieve multiple RS485 communication
RS485 adopts differential signal negative logic, +2V～+6V means “0”, – 6V～- 2V means “1”, RS485 has two-wire and four-wire wiring, four-wire is a full-duplex communication mode, two-wire is a half-duplex communication mode. RS485 has two-wire and four-wire wiring.
RS485 and RS232 are based on the same serial communication interface, data sending and receiving operations are the same, so the use of the same WinCE underlying driver. But they are in the actual application of the communication mode has a big difference, RS232 interface for full-duplex data communication mode, while the RS485 interface for half-duplex data communication mode, data can not be sent and received at the same time, in order to ensure that the data sent and received without conflict, the hardware is through the direction of switching to achieve, and accordingly also requires the software must be sent and received in the process of strict separation.
RS485 interface composed of half-duplex network, generally two-wire system (formerly four-wire connection, can only achieve point-to-point communication, now rarely used), more shielded twisted-pair cable transmission. In the RS485 communication network is generally used in the master-slave communication mode, that is, a host with multiple slaves. In many cases, connecting the RS-485 communication link is simply a pair of twisted-pair cable will be connected to the various interfaces “A”, “B” end. RS485 interface connector using DB-9 9-core plug seat, and intelligent terminal RS485 interface using DB-9 (now rarely used), mostly shielded twisted-pair cable transmission. The RS485 interface connector adopts DB-9 9-core plug holder, and the RS485 interface with intelligent terminal adopts DB-9 (hole), and the RS485 interface with keyboard adopts DB-9 (pin).
Why RS485 to achieve multi-host control?
RS485 uses a single-duplex mode of operation, in order to prevent competition on the bus generally can only have a host, and the use of a host to ask, a slave to answer the way. But sometimes, we do need more than one host to monitor RS485 devices, such as the need for three computers can monitor the same few RS485 devices, and it is best to share the RS485 bus. Although this problem is often encountered in practice, but there is no good solution. The difficulty lies in the fact that the 2 hosts are completely independent, then they do not have a coordination mechanism, if the 2 hosts at the same time to ask the slave device, then it is bound to cause data conflicts on the RS485 bus.
RS485 Wiring Specification
1. RS485 signal line can not be routed with the power line. In the actual construction, due to the alignment are through the pipeline, the construction side sometimes for convenience, direct RS485 signal line and power line tied together, due to the strong power has a strong electromagnetic signal to the weak power interference, which leads to 485 signal instability, resulting in unstable communications.
2. RS485 signal line can use shielded wire as wiring or unshielded wire as wiring. As the RS485 signal is transmitted using differential mode, i.e. by the voltage difference between RS485+ and RS485- as the signal transmission.
Correct wiring for RS485 communication
Ideal for twisted-pair wires: half-duplex two wires are best used in a pair of twisted-pair wires, so that the two wires are twisted, plus the interference level on the two wires to achieve the anti-jamming effect of cancellation. Full-duplex when receiving two lines with a pair, send two lines with another pair of RS485 does not have power transmission requirements, so the wire diameter requirements are not high.
In the actual project, usually procured outdoor waterproof twisted pair wire to ensure the protection performance of the line. However, some engineers will use RVV cable, which is also possible, but the anti-interference to be worse. So that there are engineers using RVVP cable (with shielding), this is not good, because the increase in capacitance between the lines will affect the quality of transmission, the need to reduce the baud rate of transmission.
Baud rate settings and cable length (including the total length of the branch) is a certain correspondence, the longer the line, the lower the baud rate should be set to more stable.
No matter what kind of cable, as far as possible using the bus architecture, reduce the star connection, branch line as short as possible, as far as possible using the daisy chain connection, that is, the bus to the first node, and then jump to the next node.
Not connected to the device’s branch line is best removed from the bus, otherwise it is easy to form interference. The most unterminated end of the bus if the reception of the signal is poor, you can add 120 ohm line unresistor across the signal line at both ends. Intermediate devices do not add, otherwise it will increase line loss, reduce the number of devices and distance.
The RS485 chips of different devices are usually different, there are different load types of chips, which are usually not directly visible to engineers. So that is to say that the maximum number of devices connected to different devices on the bus is not certain, the same number of devices connected to see the device description requirements on the line.