9-bit RS485

  • Nobody/Anonymous

    A request please:
    Many RS485 networks use the 9th bit to signal that the following byte is a "node address", ie to distinguish between address and data. I can't see a way to send and receive a 9-bit data sequence in order to communicate on an RS485 bus.

    Any help would be gratefully received.

    • cyberman_13

      cyberman_13 - 2009-07-28

      This is a scheme hatched by Intel in the mid 80's. Enough of history though.
      As far as I can tell you would have to modify realterm to support this.
      You should also realize this expands the number of bits sent from 10 to 11 (if you are using 8bits no parity 1 stop 1 start),  You are using a 8 bit word and the pairty bit is sent. 

      I might point out that 'many' is actually a few do this.  This is an old hack (it is a hack because it's not a standard for UART communications).  Common protocols use modbus fieldbus or IDEC's now obsolete Micro3 protocol.

      The only way to support this is direct support for that particular protocol used.  You have to change the MARK/SPACE parity bit on the fly to support this type of protocol. EX leading (address) byte is sent MARK then data is sent SPACE for this scheme. It comes from the venerable 8051 serial port (Intel -sigh- ) having this 'feature' easily programmable.



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