How much do you know about RS-422 and RS-485?
RS-422 and RS-485 transient protection
The signal grounding measures mentioned above only protect against low-frequency common-mode interference, and are incapable of high-frequency transient interference. Since the transmission line is equivalent to an inductance for high-frequency signals, the ground line is actually equivalent to an open circuit for high-frequency transient interference. Although such transient disturbances are short-lived, they may have hundreds of kilovolts.
In the actual application environment, there is still the possibility of high-frequency transient interference. Generally, high-intensity transient interference will be generated during the switching of high-power inductive loads such as motors, transformers, relays, etc. or lightning, and RS-422 or RS-485 communication interfaces will be damaged if not properly protected. For such transient interference, isolation or bypass methods can be used to protect.
1. Isolation protection method. This scheme actually transfers the transient high voltage to the electrical isolation layer in the isolation interface. Due to the high insulation resistance of the isolation layer, no damaging surge current will be generated, which plays a role in protecting the interface. Usually high-frequency transformers, optocouplers and other components are used to achieve electrical isolation of the interface. Existing device manufacturers integrate all these components in one IC, which is very simple to use. The advantage of this scheme is that it can withstand high voltage and transient interference with a long duration, and it is relatively easy to implement. The disadvantage is that the cost is higher.
2. Bypass protection method. This scheme uses transient suppression components (such as TVS, MOV, gas discharge tube, etc.) to bypass hazardous transient energy to the ground. The advantage is lower cost, the disadvantage is that the protection ability is limited, and only protection within a certain energy Transient interference cannot last for a long time, and it needs a good channel to connect to the earth, which is difficult to achieve. In actual application, the above two schemes are combined and used flexibly, as shown in Figure 1. In this method, the isolation interface isolates large-scale transient interference, and the bypass element protects the isolation interface from excessive high transient voltage breakdown.
3. When using RS485 interface, how to consider the length of the transmission cable?
When using the RS485 interface, for a specific transmission line, the maximum cable length allowed for data signal transmission from the generator to the load is a function of the data signal rate. This length of data is mainly limited by the effects of signal distortion and noise. The relationship curve between the maximum cable length and the signal rate is obtained by using 24AWG copper-core twisted-pair telephone cable (wire diameter is 0.51mm), the bypass capacitance between the lines is 52.5PF/M, and the terminal load resistance is 100 ohms. When the data signal rate drops below 90Kbit/S, assuming the maximum allowable signal loss is 6dBV, the cable length is limited to 1200M. In practice, it is possible to obtain a larger cable length than it. When using cables with different wire diameters. The maximum cable length obtained is different.
Nine, how to realize RS-485/422 multipoint communication
Only one transmitter can send on the RS-485 bus at any time. In half-duplex mode, master and slave can only send one. In full-duplex mode, the master station can always send, and the slave station can only send one.
4. When using RS-485/RS422 interface communication, under what conditions should terminal matching be used? How to determine the resistance value? How to configure terminal matching resistance?
During long-line signal transmission, in order to avoid signal reflection and echo, it is necessary to connect a terminal matching resistor at the receiving end. The terminal matching resistance value depends on the impedance characteristics of the cable, regardless of the length of the cable.
RS-485/RS-422 is generally connected by twisted pair (shielded or unshielded), and the terminal resistance is generally between 100 and 140Ω, with a typical value of 120Ω. In the actual configuration, a terminal resistor is connected to the two terminal nodes of the cable, that is, the nearest end and the farthest end, and the node in the middle part cannot be connected to the terminal resistance, otherwise it will cause communication errors.
If you want to know more, our website has product specifications for RS-422 and RS-485, you can go to ALLICDATA ELECTRONICS LIMITED to get more information