1. Bus matching.

There are two methods for bus matching. One is to add matching resistance. 120 Ω matching resistance shall be connected between the differential ports VA and VB at both ends of the bus to reduce the reflection and absorption noise caused by mismatch and effectively suppress the noise interference. However, matching resistors consume large current, which is not suitable for systems with strict power consumption restrictions.

Another power-saving matching scheme is RC matching. Using a capacitor C to isolate the DC component can save most of the power, but the value of capacitor C is a difficulty, which needs to make a compromise between power consumption and matching quality.

In addition to the above two, there is also a matching scheme using diodes. Although this scheme does not achieve real matching, it uses the clamping effect of diodes to quickly weaken the reflected signal, so as to improve the signal quality, and the energy-saving effect is remarkable.

Analysis of design steps of RS-485 bus interface circuit

Di and RO pull-up terminal configuration.

Asynchronous communication data is transmitted in bytes. Before each byte is transmitted, handshake must be realized through a low-level start bit. In order to prevent the interference signal from triggering ro (receiver output) to generate negative jump and make the MCU at the receiving end enter the receiving state, it is recommended that ro be externally connected with 10K Ω pull-up resistor

3. Ensure that the RS-485 chip is in the receiving input state when the system is powered on.

For the transceiver control terminal TC, it is recommended to use MCU pin to control through inverter, rather than MCU pin to control directly, so as to prevent the interference to the bus when MCU is powered on.

4. Bus isolation.

RS-485 bus is a parallel two-wire interface. Once a chip fails, the bus may be “pulled dead”. Therefore, its two-wire ports VA and VB should be isolated from the bus. Usually, a 4 ~ 10 Ω PTC resistor is connected in series between VA, VB and the bus, and a 5V TVS diode is connected with the ground to eliminate the line surge interference. If there is no PTC resistor and TVS diode, ordinary resistor and voltage stabilizing tube can be used instead.

5. Reasonably select chips.

For example, for external equipment, in order to prevent strong electromagnetic (lightning) impact, it is recommended to select ti’s 75lbc184 and other lightning protection chips, and Sipex’s SP485R can be selected for those requiring more nodes.

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