HDL integration with KNX through LM2

local socket = require('socket')
local crc16 = require('encdec').crc16
local ifinfo = require('ifinfo')
 
HDL = {
  -- destination ip
  dstip = '192.168.0.2',
  -- packet constant data
  magic = 'HDLMIRACLE',
  lcode = string.char(0xAA, 0xAA),
  -- source device settings
  srcsubnet = 1,
  srcdevice = 254,
  devicetype = 0xFFFE,
  -- command types
  cmd = {
    chanreg = 0x0031, -- single channel regulate
    chanregreply = 0x0032, -- single channel regulate answerback
    chanstat = 0x0033, -- read status of single channel targets
    chanstatreply = 0x0034, -- single channel targets status answerback
  }
}
 
HDL.init = function()
  local info, chunk, chunks
 
  info = ifinfo()
 
  if not info or not info.eth0 then
    error('cannot get interface info')
  end
 
  -- ip header
  HDL.iphdr = ''
  -- broadcast address
  HDL.bcast = info.eth0.bcast
 
  -- split ip address into chunks
  chunks = info.eth0.inetaddr:split('.')
 
  -- add ip address chunks
  for i = 1, 4 do
    chunk = tonumber(chunks[ i ])
    HDL.iphdr = HDL.iphdr .. string.char(chunk)
  end
end
 
HDL.decode = function(packet)
  local len, data, src, crc
 
  -- primary header
  if packet:sub(5, 14) ~= HDL.magic then
    return nil, 'magic'
  end
 
  -- leading code
  if packet:sub(15, 16) ~= HDL.lcode then
    return nil, 'lcode'
  end
 
  -- get data length and check against
  len = packet:byte(17)
  if len and len + 16 ~= packet:len() then
    return nil, 'len'
  end
 
  -- get packet data and check crc
  data = packet:sub(17, len + 14)
  crc = packet:byte(len + 15) * 0x100 + packet:byte(len + 16)
  if crc16(data) ~= crc then
    return nil, 'crc'
  end
 
  -- return parsed packet
  return {
    srcip = string.format('%d.%d.%d.%d', packet:byte(1, 4)),
    srcsubnet = packet:byte(18),
    srcdevice = packet:byte(19),
    devicetype = (packet:byte(20) * 0x100 + packet:byte(21)),
    opcode = (packet:byte(22) * 0x100 + packet:byte(23)),
    dstsubnet = packet:byte(24),
    dstdevice = packet:byte(25),
    additional = packet:sub(26, len + 14)
  }
end
 
HDL.word = function(v)
  return string.char(bit.band(bit.rshift(v, 8), 0xFF), bit.band(v, 0xFF))
end
 
HDL.encode = function(cmd, dstsubnet, dstdevice, extra)
  local packet, len, crc, data
 
  -- perform init if required
  if not HDL.iphdr then
    HDL.init()
  end
 
  -- start packet: ip, magic and leading code
  packet = { HDL.iphdr, HDL.magic, HDL.lcode }
  -- base data
  data = string.char(HDL.srcsubnet, HDL.srcdevice) ..
    HDL.word(HDL.devicetype) ..
    HDL.word(cmd) ..
    string.char(dstsubnet, dstdevice)
 
  -- add extra data parameters
  if type(extra) == 'string' then
    data = data .. extra
  end
 
  -- calculate length and crc
  len = string.char(data:len() + 3)
  crc = crc16(len .. data)
 
  table.insert(packet, len)
  table.insert(packet, data)
  table.insert(packet, HDL.word(crc))
 
  return table.concat(packet)
end
 
HDL.send = function(packet)
  local client = socket.udp()
  client:sendto(packet, HDL.dstip, 6000)
end
 
HDL.chanreg = function(dstsubnet, dstdevice, chan, value, delay)
  local extra, packet
  delay = delay or 0
 
  if type(value) == 'boolean' then
    value = value and 100 or 0
  end
 
  extra = string.char(chan, value) .. HDL.word(delay)
  packet = HDL.encode(HDL.cmd.chanreg, dstsubnet, dstdevice, extra)
 
  HDL.send(packet)
end