TCP Socket响应头两位十六进制格式问题：发送后设备返回未知错误

Let's walk through the likely issues causing the unknown error from your device—your header calculation might be misaligned with what the spec actually expects:

• Check if your hex header is sent as a string vs. raw byte
  The spec says "two-digit hexadecimal format for the data length"—this almost always means sending the hex value as a human-readable string, not the raw byte. For example, instead of sending the byte 0x19 (which is an unprintable control character in ASCII), you should send the two characters "19" (two separate bytes: 0x31 and 0x39). If you sent the raw byte, the device probably can't parse it as a valid length header.

• Verify your payload length count is accurate
  You said your response data is 25 characters, but let's recount: "202000000000000TEST0_OK " breaks down to:

  • "202000000000000" (15 characters)
  • "TEST0_OK" (8 characters)
  • Trailing space (1 character)
    That adds up to 24 characters, not 25. If you're counting bytes (assuming ASCII encoding, each character is 1 byte), that's 24 bytes. Double-check if there's an extra hidden character (like a newline or null byte) in your payload that you're not seeing, or if you miscounted. Even an off-by-one error here would throw off the device's parsing.

• Confirm if the header counts payload length or total message length
  Some specs define the header as the length of the entire message (header + payload), not just the payload. For example, if your header is two characters (2 bytes) and your payload is 24 bytes, the total length is 26 bytes (0x1A in hex). If the device expects total length but you sent payload length, it will misinterpret the data and throw an error.

Quick Debug Steps #

1. Use a packet sniffer like Wireshark to capture exactly what you're sending over the socket—this will let you verify the header format and payload length at the byte level.
2. Re-read the spec carefully to confirm:
   • Whether the length refers to payload or total message
   • Whether the hex header is a string or raw byte value
3. Test with both possible header formats (string "19" vs. raw byte 0x19) to see which one the device accepts.

内容的提问来源于stack exchange，提问作者Joseph