The RS-485 interface chip can play a role of level conversion, converting TTL signals, COMS signals, etc. into differential signals that can be transmitted on the 485 bus, and converting the received 485 differential signals into TTL or MCU-recognizable. COMS level has been widely used in many fields such as industrial control, instruments, meters, multimedia networks, mechatronics products and so on. However, in RS485 communication, there are often problems such as short communication distance and poor communication quality. In order to improve the communication quality of RS485, in addition to adopting the bus-type structure of terminal matching, the following problems are usually considered in the system design.
1. Fault protection
According to the RS-485 standard, the receiving sensitivity of the receiver is ± 200mV, which means that when the differential voltage at the receiving end is greater than or equal to + 200mV, the output of the receiver is high, and when it is less than or equal to -200mV, the output is low. Between ± 200mV, the receiver output is in an uncertain state. When the bus is idle (that is, all nodes on the transmission line are in the receiving state) and the transmission line is open or shorted, if no special measures are taken, the receiver may output high or low levels. Once the receiver of a certain node generates a low level, it will make the serial receiver (UART) unable to find the start bit, thereby causing communication anomalies.
To solve this problem, many RS485 interface chips have introduced fault protection. For example, the input sensitivity of UM3085 / UM3088 of Shanghai Yinglian Electronics is -50mV / -200mV, that is, when the input voltage of the differential receiver UA-B≥-50mV, the receiver outputs a logic high level, if UA-B≤-200mV Output logic low. When the bus at the receiver input is short-circuited or all transmitters on the bus are disabled, the differential input at the receiver is 0V to ensure that the receiver outputs a high level when the bus is idle and shorted.
2. Lightning protection
The RS-485 interface chip may be damaged by electrostatic shock during use, welding or transportation of the device. In the occasion where the transmission line is installed outdoors, the interface chip and the entire system may also be struck by lightning. The use of anti-static or anti-lightning chips can effectively avoid such losses. The UM3085 / UM3088 chip integrates an ESD protection circuit. The human body model ESD protection and the machine model ESD protection reach 15kV and 2kV, respectively. In addition, Yinglian Electronics has a complete set of ESD protection solutions (Figure 1), which makes the system more effective Reliable operation in harsh transient high-pressure shock environments.
Figure 1: RS485 chip lightning and surge voltage design
The power supply of the RS485 chip usually comes from the transformer. When relatively severe voltage changes occur, some transient surge high voltages may appear on the power supply. In the figure, UM07 can clamp these transient voltages well to protect the interface chip. At the same time, UESD712 is used to protect the two communication ports of the bus, which can play a very good lightning protection effect.
3. Optocoupler isolation
In some industrial control fields, due to the complexity of the scene, there is a high common-mode voltage between each node. Although the RS-485 interface uses differential transmission and has a certain ability to resist common mode interference, when the common mode voltage exceeds the limit receiving voltage of the RS-485 receiver, that is, greater than + 12V or less than -7V, the receiver will not work properly. Work, even in severe cases will burn chips and instruments and equipment.
The solution is to isolate the system power supply and the power supply of the RS-485 transceiver through a DC / DC converter (Figure 2), completely eliminate the effects of common-mode voltage, and filter out interference caused by ground loops and conducted noise.
4. Wide signal swing
In RS485 communication, in order to prevent signal reflection, usually 90 ~ 120 ohm resistor is connected to the terminal. When the RS485 interface chip carries such a resistive load, the output voltage swing is only 1.5 ~ 1.8V, so when carrying out long-distance communication, due to the attenuation of the signal, the receiver may not receive the signal correctly, resulting in errors. code. The UM3085 / UM3088 driver improves the output signal swing to greater than 2.2V, thereby providing higher reliability in long-distance communication. In addition, UM3085 / UM3088 has the characteristics of slope limitation, can reduce EMI, and can reduce reflection when using the transmission line with impedance mismatch, improve communication quality.
In summary, when selecting the RS485 interface chip, in addition to considering the parameters such as quiescent current and power supply voltage width range, it is also necessary to consider its fault protection function, antistatic performance, driving ability, as well as all aspects of the external environment and communication cables. Factors, so as to build a high-quality RS485 communication network.
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