Design And Research Of Nonpolar RS485 Communication Interface Chip

RS485 protocol,as an international universal serial port communication standard,has the characteristics of long transmission distance,strong anti-interference ability,simple connection line and low price,etc.,so it has a wide range of applications in industrial fields,power communication,intelligent instruments,and data collection.The existing electricity meters generally adopt RS485 network communication and data collection.In this paper,an RS485 interface chip with non polarity function is designed.The chip meets the standard protocol of RS485 and adds the non-polarity function so that the A and B ports of the chip can adapt to the polarity on the bus to avoid communication failure caused by the reverse connection of the port and the bus.Based on the RS485 standard protocol,In this paper,HHgrace.18?m BCDMOS process is selected and Cadence simulation platform is used to design a non-polar RS485 chip for half-duplex communication.The chip is divided into four parts:driver,receiver,non-polar function module,and reliability module.The driver converts TTL single-ended signals into differential signals,and the receiver converts differential signals into TTL single-ended signals.This differential transmission method of driver and receiver can effectively suppress the common mode signal,greatly reducing the impact of noise on the transmitted data.The non-polar function part realizes the polarity on the A and B ports adaptive bus.Reliability design on the one hand is to strengthen the driver and receiver performance,For the driver,the over-voltage protection,over-current protection,over-temperature protection and other modules are added to make the performance of the driver more reliable.For the receiver,the fault protection structure is added to make the chip have the function of open circuit and short circuit protection,On the other hand,it is to design a specific structure to optimize the performance of the chip,including low power consumption when the chip is static,power-on reset,and hot swap protection.In this paper The above four parts separately is designed and simulated,and the results are in accordance with the RS485 standard protocol and preset parameters.In addition to the basic functions,the non-polar RS485 chip designed in this paper has the following characteristics:The first driver reduces the ringing of the output signal by adjusting the voltage swing rate,and Dual protection is used on the drive output port to prevent the reverse conduction of the bus and the VCC or GND inside the chip from causing the chip security risks.The design range of the second receiver differential threshold voltage is-200mv——50mv,which realizes the function of short circuit or open circuit protection.The third chip adds the non polarity module,which can modulate the polarity signal on the bus to the non polarity signal for transmission,In this design,the polarity adjustment time is set about 18ms.The fourth design of the low-power module allows the chip to minimize the power consumption of the circuit when it is not working,The simulation results show that the quiescent operating current is about 2.84uA.The fifth semiconductor device is very sensitive to temperature,and the over-temperature protection circuit is designed to limit the chip's operating temperature range to-40?-85?,which is conducive to protecting the chip and enhancing transmission The reliability of the data.Finally,the chip has the power on reset function,and the power on reset time is about 10us.