/ModbusControl

Primary LanguagePythonMIT LicenseMIT

ModbusControl

ModbusControl is a high-level protocol based on Modbus RTU, which facilitates remote controlling clients via serial interfaces like USB or RS485.

General principle: - All registers are 16-bit (Modbus standard) - Only functions READ_INPUT_REGISTERS, WRITE_MULTIPLE_HOLDING_REGISTERS, and READ_HOLDING_REGISTERS are used - Any other Modbus function codes return error code ILLEGAL_FUNCTION

  1. Read static values or time-critical reads with low communication overhead:

    • Use Modbus command READ_INPUT_REGISTERS
    • Direct read of input registers via target address
    • No parameters other than address are passed
    • Any valid address within [0,(MODBUS_NUM_INPUT_REG-1)] is allowed
    • Address 0 is reserved for ModbusControl protocol version MODBUSCONTROL_PROTOCOL
    • Measured execution time for read command is ~4.0ms (Debian Linux 18.04 via USB2.0)

  2. Trigger command execution on client with optional parameters:

    • Use combination of Modbus commands WRITE_MULTIPLE_HOLDING_REGISTERS and READ_HOLDING_REGISTERS
    • Write to and read from holding register holdReg[] must start at address 0
    • Command code is written to holdReg[0], parameters to holdReg[1..(N-1)]
    • A pending command is indicated via holdReg[0] bit 15 is set by master
    • After command completion, holdReg[0] bit 15 is cleared by slave
    • After successful command execution, return values are stored in holdReg[1..(N-1)]
    • An error on client side is indicated by setting holdReg[0] bit 14. In this case the error code is stored in holdReg[1]
    • Measured execution time for complete command (=write+read) is ~8.1ms (Debian Linux 18.04 and USB2.0)

This project contains:

  • Python example for ModbusControl master:
    • Tested for Windows and Linux. Should also work for MacOSX etc.
    • Requires Python installation (2.x or 3.x) and libraries serial and modbus_tk
    • Used serial port (e.g. USB) must shows up as device or VCP
  • Arduino example for ModbusControl slave:
    • Tested for Arduino Mega and Due. Should be compatible with all AVR, SAM and SAMD Arduinos with HW-Serial
    • Requires installed Modbus RTU slave library

For bug reports or feature requests please send me a note.

Have fun! Georg