1. What is RS485?
RS485 communication adopts differential signal negative logic control mode, that is, 2 6V means "0", and -6 -2V means "1". RS485 communication has two wiring methods: two-wire system and four-wire system. The four-wire system can only achieve point-to-point communication, and is rarely used nowadays. The two-wire wiring mode is currently used more of the bus topological structure, which belongs to the half-duplex transmission mode, that is to say, the transmission and reception are asynchronous.
Bus port guarding
In harsh environments, RS485 communication ports usually do additional protection such as electrostatic protection, lightning surge protection, and even need to do a good job of preventing 80V mains access solutions to avoid damage to smart meters and industrial control hosts. The following figure shows three common RS485 bus port protection schemes.
The first is to connect the TVS device to the protection ground in parallel between the AB ports, connect the TVS device in parallel between the AB ports, connect the thermistor in series with the AB port respectively, and connect the gas discharge tube to the protection ground to form a first-class protection scheme
The second is the best protection scheme for AB parallel TVS to ground, series thermistors, and parallel varistors between ABs
The third is to connect the pull-up resistor to the power supply and the ground respectively, connect the TVS between the AB, and connect the thermistor to a certain port of A or B.
2. RS485 networking
1. Bus networking
A typical network application circuit is shown in the diagram. These devices can also be used as linear devices for cable lengths longer than 4,000 feet, and to reduce reflections, they should be terminated at both ends of the transmission line with their characteristic impedance, and the branch length outside the trunk line should be as short as possible.
There are two types of signals that are reflected during communication: impedance discontinuity and impedance mismatch. Impedance discontinuity signal at the end of the transmission line suddenly encounters the cable impedance is very small or even no signal in this place will cause reflection, the way to eliminate this reflection is to cross the end of the cable with a terminal resistance of the same size as the characteristic impedance of the cable to make the impedance of the cable continuous. Since the transmission of the signal on the cable is bidirectional, a terminal resistor of the same size can be crossed at the other end of the communication cable, and the terminal resistor method is used for general terminal matching. The termination resistance is 120 in the RS485 network, which is equivalent to the resistance of the characteristic impedance of the cable, because the characteristic impedance of most twisted pair cables is about 100 120. This matching method is simple and effective, but there is a disadvantage, the matching resistor consumes a lot of power, which is not suitable for systems with strict power consumption constraints.
RS485 communication can be connected in general occasions by using ordinary twisted pair cables. In environments with high requirements, coaxial cables with shielding are used. When using the RS485 interface, for a specific transmission line, the RS485 interface is used to .
load, the maximum cable length allowed for the transmission of its data signal is inversely proportional to the frequency of the wave transmitted by the signal. This length data is mainly affected by signal distortion and noise. In the transmission process, the signal can be amplified by increasing the relay method, and generally up to 9 relays can be added, that is to say, the maximum transmission distance of RS485 can reach 96km。When long-distance transmission is required, optical fiber can be used as the propagation medium, and a photoelectric converter can be added at each end of the transmission and reception. The transmission distance of multimode fiber can reach 5 10km, and if single-mode fiber transmission is used, the transmission distance can reach 50km.
2. Hand-in-hand networking
Also known as daisy chain topology, it is the standard and specification of RS485 bus wiring, and is the RS485 bus topology recommended by TIA and other organizations. The wiring mode is that the master control device and a plurality of slave control devices form a hand-in-hand connection, as shown in the figure, leaving no branches is the hand-in-hand way. This wiring method has the advantages of small signal reflection and high communication success rate. At present, many applications basically use this networking method.