Why are RS485 frames corrupted between ESP32 and ESP8266?

Projects Networking, Protocols, and Devices
10

Think it goes wrong in readBytes().

What does this print? (I expect a value less than packet size = 4)

Analysis

If there are 3 bytes available of the 4 to receive, these will be copied in buffer by readBytes and in the next Loop these 3 bytes are ignored. So good bytes are dropped because the packet is not complete.

Better is to wait until at least 4 bytes are in the buffer (still not fool proof, just a bit better)

void loop() {
  if (rs485.available() >= 4) {
    uint8_t buffer[128];
    size_t len = rs485.readBytes(buffer, 4);

    Serial.print("SLAVE <- Otrzymano: ");
    printHex(buffer, len);

A robust solution should do something like

  • read one byte at the time
  • check if this is a PACKET_START byte (define a start of the packet byte)
  • if so collect the next N bytes as payload and CRC (one byte at the time)
  • check CRC
  • process data
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