RS485 communication problem, can receive but not send, confirmation process of 485 chip failure

1. Problem Analysis

1. Start the serial port debugging assistant

1. question

The 485 to SSI device supports the 485 interface and MOUDBUS RTU protocol. Under normal circumstances, the host (PLC, computer) and the slave (485 to SSI device) work in master-slave mode, that is, the host sends commands and the slave responds and gives feedback. The problems encountered are:

After the 485-to-SSI device works normally for a period of time, it suddenly stops working, that is, the host cannot receive feedback from the slave.

Build a test environment and use it for debugging:

1. Laptop + serial port debugging assistant + USB to 485 interface

2. 485 to SSI equipment

3. MCU Debug Tools

Build a test environment

2. Problem Analysis

1. Start the serial port debugging assistant

Click "Send", the data receiving area does not receive data

The problem is that after pressing "Send", the data receiving area does not receive any data.

Normally, click "Send" and the data receiving area will display the feedback data.

Click "Send" and the data receiving area receives the data.

2. Use the MCU debugging tool to check whether the serial port can receive data

(1) Use a multimeter to measure the 485 chip transceiver control pin signal

485 interface protection circuit

The measured DE=0 is in the receiving state.

(2) Start Keil uVision5 and enter debugging

Keil debugging interface

It is found that 485 receives data normally, but the serial port debugging assistant cannot receive the sent data.

(3) 485 sends data, but the serial port debugging assistant cannot receive it

Finally it was determined that the SP485EE chip was broken, and communication was normal after replacing it.

Cyclic sending, the data receiving area continuously displays data

Conclusion

The problem I encountered this time is a bit strange. I made two units of this device. The SP485EE chips were from the same batch. One unit worked normally, but the other unit failed to send messages. The first time, I solved the problem by replacing the SP485 chip after repeated tests. However, the problem did not last long before it reappeared. I tested the circuit again and found that the SP485EE was still broken. To be honest, my 485 circuit application is very mature. This is the first time I have encountered the continuous failure of the SP485 chip. Is it because the SP485 is made in low-cost domestic products?