GMCGUI RS485 software

[last updated: 2024-09-04]
RS485 home page
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Communication on RS485 bus has no built-in error correction. All data validation must be done in software.

This is my custom code approach to the problem.
Starting from lots of online resources, notably Nick Gammon's RS485_protocol library, here's what I'll do in this project:

• Data will be sent in packets. Each packet will be composed of:
  • a start byte ("\2")
    This is ctrl-B, "start-of-text", STX, decimal=2, hex=2
  • a one-byte address of destination node
  • a one-byte address of sender node
  • a one-byte byte-count of how many bytes in the message
    (just the message, ie. not counting start, destination, sender, byte-count, stop, or crc bytes)
  • some number of bytes (248 bytes max?) of actual message
  • one-byte crc (cyclic redundancy check) of just the message itself.
  • stop byte ("\3")
    This is ctrl-C, "end-of-text", ETX, decimal=3, hex=3
• Receiving nodes listen for start bytes followed by their node ID.
  • If they receive that, they will echo back their node ID.
  • Then as the message progresses, they will echo back every byte received
    • If at any time before the stop byte they happen to receive another start byte, they start over, ie. waiting for their node ID
  • When they receive the stop byte, they will collect one more byte (the crc) from the buffer
  • They will then validate the data byte count and crc.
    If it's correct (or not?), they will respond with [what message?]
  • They will then process the message and act on any instructions it's given them.
• Transmitting nodes (initially the Master node) will:
  • send the packet, one byte at a time.
    when it sends the start byte, it will not expect a reply.
    however for every byte sent after that, including the stop and crc bytes,
    it will expect to receive an echo/response of the byte it sent.
    If it does not receive the correct echo, it will start over sending the whole packet again.
  • Once the packet is sent, it will go to receive mode, and will listen for acknowledgement of successful receipt of the message
  • if it has asked the remote node to send it some sensor data, eg, it will wait to receive it...

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