USB to RS485 serial port circuit design "suggestions collection"

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USB The serial port signal of the serial port chip is generally TTL/CMOS level , In half duplex RS485 External connection is required for serial port 485 Level conversion chip , Design A signal is required to control 485 Transmit and receive enable end of the transfer chip , It is recommended to choose to bring 485 Switching chip of control pin （ Such as CH340/CH342 Series of chips TNOW Pin ）, This pin defaults to low level , When the serial port is in the sending state, it will be automatically raised to be in the effective state , The transmission is completed and then the low level is restored . Empathy , It can be extended to other application scenarios , Such as MCU serial port to 485 Can be used in circuit design GPIO Mouth to control 485 Transmit and receive enable of the transfer chip .

With MAX485 For example ：

1. DE Enable... For sending ,RE Enable... For reception ,DI by TTL/CMOS Serial transmission ,RO by TTL/CMOS A serial port to receive .

2. TNOW Control signal pins are connected at the same time DE and RE; In the serial port sending state ,TNOW Output high level , open DE Receive enable ; When the serial port does not send ,TNOW Output low level , The default RE Receive enable ;

common problem Q&A

Q： The serial port often receives 0x00 Bit error value ？

A： In the receive enable state , commonly 485 Of A and B The signal terminal is in high resistance state , In the process of transmitting and receiving A and B The signal remains , Did not return to idle state . Lead to RO End sampling error . here , stay A Terminate a pull-up resistor ,B Terminate a pull-down resistor , The resistance value shall be calculated according to the manual and practical application .

Q：AB Whether the terminal needs parallel terminal matching resistance ？

A： According to the manual , At present, many chips are built-in 120 Ohm matching resistance , It's not a mandatory option .

Q： How to calculate the number of cascade devices ？

A： It shall be determined according to the received input impedance marked in the manual , Generally, the maximum number of transceivers allowed will also be directly marked .

Application circuit 1

The following figure shows the use of the built-in TNOW Pin 485 Control signal USB Switch to dual serial chip CH342F Designed two-way RS485 Serial port circuit ：

notes ：CH342F Of DTR When the pin is connected to the pull-down resistor, it will switch to TNOW Pin , Used in the above figure is 4.7K.

Application circuit 2

If the selected model does not 485 Control signals , Use only TXD and RXD Two signal lines 485 The need for circuit switching is also very common . The following is the schematic diagram ：

Realization principle ： take RE Keep setting 0, Enable receive enable （RE And DE Direct short connection is also possible ）, take TXD Reverse post connect send enable end DE.TXD The output is high , Connected 485 The chip send enable is invalid , The bus output is idle （ High level ）,TXD Output low level , Send enable is valid . Compared to the application 1, Because there is a triode reverse circuit , The signal output control will have a delay , It is not recommended to use in high baud rate communication .

The above conversion circuit and design principle are in 485 High frequency is used in the conversion circuit , Users can flexibly select according to their needs .

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