RModBus - free implementation of protocol ModBus.

• Ruby 1.8, Ruby 1.9, JRuby (without serial ModBus RTU)
• TCP, RTU, RTU over TCP protocols
• Client(master) and server(slave)
• 16, 32 -bit and float registers

• Read Coils (0x01)
• Read Discrete Inputs (0x02)
• Read Holding Registers (0x03)
• Read Input Registers (0x04)
• Write Single Coil (0x05)
• Write Single Register (0x06)
• Write Multiple Coils (0x0F)
• Write Multiple registers (0x10)
• Mask Write register (0x16)

Download and install RModBus with the following

$ gem install rmodbus

  require 'rmodbus'

  ModBus::TCPClient.new('127.0.0.1', 8502) do |cl|
    cl.with_slave(1) do |slave|
      # Read a single holding register at address 16
      slave.holding_registers[16]

      # Write a single holding register at address 16
      slave.holding_registers[16] = 123

      # Read holding registers 16 through 20
      slave.holding_registers[16..20]

      # Write holding registers 16 through 20 with some values
      slave.holding_registers[16..20] = [1, 2, 3, 4, 5]
    end
  end

You can checkout source code from GitHub repositry

$ git clone git://github.com/flipback/RModBus.git

Home page: http://rmodbus.flipback.net

RModBud on GitHub: http://github.com/flipback/RModBus

ModBus community: http://www.modbus-ida.org