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Invalid Response or CRC

My Modbus master reports a CRC/checksum error or invalid response even though the WattNode® meter’s Comm LED flashes green.

  • Is your Modbus master using an RS-232 port to interface with the meter? If so, this can appear to work in one direction (the master’s command reaches the meter) but it won’t work in both directions (the meter’s response never reaches the master or appears garbled) because the RS-485 interface on the WattNode meter is not compatible with the RS-232 interface on your Modbus master. To fix this, you need an RS-232 to RS-485 adapter. The easiest type of adapter to use is one that converts to 2-wire (half-duplex) RS-485 and has an “automatic-send” feature (also called “auto-enable” or “auto turn-around”). Examples: CommFront ; U.S. Converters ; or Integrity Instruments.
  • This problem can also be caused by enabling the “echo” feature of the USB/RS-485 adapter. The “echo” feature should be disabled (e.g. for the B&B USOPTL4 adapter, switch all 4 DIP switches to the ON position).

Keywords: RS485, EIA485, EIA-485