UART Modbus Read response

[stijnb1234]

I'm trying to send a 0x03 Read holding registers request as a Modbus master to the listening slaves. What is the best way to send a request (the read request), wait for the response, and then read it? I'm stuck at waiting for the response. Could you please help me further?

I tried a fixed delay, but that's not the best solution. I also tried working with events, but I can't find a stable way.

Example code:

Code: [Select all] [Expand/Collapse]

1. uint8_t fByte = (uint8_t)((startReg & 0xFF00) >> 8);
2. uint8_t sByte = (uint8_t)(startReg & 0x00FF);
3.  
4. ESP_LOGD(TAG, "Reading from register %d", startReg);
5. uint8_t data[8] = {slave_id, MODBUS_READ_HOLDING_REG, fByte, sByte, 0x00,
6.                     length};
7. uint16_t crc = calculateCRC(data, 6);
8. data[6] = (uint8_t)(crc & 0x00FF);
9. data[7] = (uint8_t)((crc & 0xFF00) >> 8);
10. int writeLen = Serial::write(data, 8);
11. ESP_LOGV(TAG, "Wrote %d bytes", writeLen);
12.  
13. //TODO Wait for response here
14.  
15. size_t dataLength = 0;
16. Serial::getBufferedDataLength(&dataLength);
17. if (dataLength < 6) {
18.   return false;
19. }
20.  
21. //Read response

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[ESP_alisitsyn]

Hi @stijnb1234,

It is hard to recognize what is the issue on your side. It will be better if you print the sent request and add some additional code.
If you provide your full request in HEX it will be more clear what goes wrong on your side.

For example the read holding registers request and response from slave:
Rx:006574-01 03 00 20 00 04 45 C3
Tx:006575-01 03 08 00 E4 40 46 00 E4 40 46 46 D3

Also, please make sure that your slave is configured correctly to be able to respond to your request.

I think the more simple way for you to read data from your slave is to use esp-modbus component https://github.com/espressif/esp-modbus
Documentation:
https://docs.espressif.com/projects/esp ... est/esp32/