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====== An example of custom protocol for Modbus ASCII ======

  ***Type**: Serial port
  ***Address validation**: ^(C[0-9]+)$|^(DI[0-9]+)$|^(HR[0-9]+)$|^(IR[0-9]+)$
  ***Validation error message**: Invalid register address. Valid ModBus addresses are Cxxx, DIxxx, IRxxx, HRxxx.

Code:

<code lua>
-- MODBUS ASCII Demo Driver

function parseAddress ()
  -- do something here
end

function createDevices ()
  addDevice({name = "C",  shift = 0, base = 10, xtraFields = {1, 5}});
  addDevice({name = "DI", shift = 0, base = 10, xtraFields = {2, 0}});
  addDevice({name = "HR", shift = 0, base = 10, xtraFields = {3, 6, 16}});
  addDevice({name = "IR", shift = 0, base = 10, xtraFields = {4, 0}});
end

local errorCount = 0;

function intToPair(val, count) 
    local str = string.upper(string.format("%0" .. count .. "x", val));
    local res = {};
    for i = 1, count do
        res[i] = string.byte(string.sub(str, i, i));
    end
    return res;
end

function pairToInt(a, b)
    local str = string.char(a) .. string.char(b);
    local n = tonumber(str, 16);
    return n;
end

function LRC(table, start, count)
    local lrc = 0;
    local i = start;
    while (i < (start+count)) do
        local str = string.char(table[i]) .. string.char(table[i + 1]);
        local n = tonumber(str, 16);
        lrc = lrc + n;
        i = i + 2;
    end
    lrc = bit.band(lrc, 255);
    return 256 - lrc;
end
    

function readRegister (reg, device, unitId)
  local request = {};
  
  -- ASCII header
  request[1] = 58;
  
  -- slave address
  local addr = intToPair(unitId, 2);
  request[2] = addr[1];
  request[3] = addr[2];

  -- function code
  local fnCode = intToPair(device.xtraFields[1], 2);
  request[4] = fnCode[1];
  request[5] = fnCode[2];

  -- address of register
  local intAddr = intToPair(reg.internalAddr, 4);
  request[6] = intAddr[1];
  request[7] = intAddr[2];
  request[8] = intAddr[3];
  request[9] = intAddr[4];

  -- count of registers
  local count = 1;
  if (reg.dataType == 3) then -- double word
    count = 2;
  end
  count = intToPair(count, 4);
  request[10] = count[1];
  request[11] = count[2];
  request[12] = count[3];
  request[13] = count[4];

  -- LRC
  local lrc = LRC(request, 2, 12);
  lrc = intToPair(lrc, 2);

  request[14] = lrc[1];
  request[15] = lrc[2];
  
  -- CR LF
  request[16] = 13;
  request[17] = 10;

  local res = sendBytes(request);
  
  if (res == false) then
      DEBUG("Can't send bytes");
      return false;
  end


  local response = {};

  -- read Header
  response = readBytes(7);
  if (response == false) then
      DEBUG("Can't read response");
      return false;
  end
  res = #response;

  if (res ~= 7) then
      errorCount = errorCount + 1;
      if (errorCount > 3) then
          closeConnection();
          errorCount = 0;
      end
      DEBUG("Can't read header");
      return false;
  end


  if (response[1] ~= request[1]) then
      ERROR("Wrong protocol");
      return false;
  end

  if (response[2] ~= request[2] or response[3] ~= request[3]) then
      ERROR("Wrong UnitID in response");
      return false;
  end

  if (response[4] ~= request[4] or response[5] ~= request[5]) then
      ERROR("Wrong function code in response");
      return false;
  end

  local length = pairToInt(response[6], response[7]);
  if (length < 1) then
      ERROR("Wrong length in response");
      return false;
  end
  
  local response2 = {};

  response2 = readBytes(length * 2 + 4);
  if (response2 == false) then
      DEBUG("Can't read response2");
      return false;
  end
  res = #response2;

  local result = {};
  
  local resp = {};
  for i=1,#response do 
      resp[i] = response[i];
  end
  local c = #response;
  for i=1,#response2 do 
      resp[c+i] = response2[i];
  end
  
  local lrc_resp = LRC(resp, 2, 6 + length * 2);
  local lrc_actual = pairToInt(response2[length*2 + 1], response2[length*2 + 2]);
  
  if (lrc_resp ~= lrc_actual) then
      ERROR("Wrong LRC in response");
      return false;
  end

  if (length >= 1) then
      for i = 1, length do
          result[i] = pairToInt(response2[i*2 - 1], response2[i*2]);
      end
  end

  return result;
end



function writeRegister (reg, device, unitId, newValue)
    local request = {};
    
    -- ASCII header
    request[1] = 58;
    
    -- slave address
    local addr = intToPair(unitId, 2);
    request[2] = addr[1];
    request[3] = addr[2];
    
  
    if (reg.dataType == 3) then -- double word should be handled in a special way

        if (device.xtraFields[3] == 0) then
            ERROR("Can't write these type of registers (" .. device.name .. ")");
            return false;
        end
        
        -- function code
        local fnCode = intToPair(device.xtraFields[3], 2);
        request[4] = fnCode[1];
        request[5] = fnCode[2];
    
        -- address of register
        local intAddr = intToPair(reg.internalAddr, 4);
        request[6] = intAddr[1];
        request[7] = intAddr[2];
        request[8] = intAddr[3];
        request[9] = intAddr[4];

        local count = intToPair(2, 4);
        request[10] = count[1];
        request[11] = count[2];
        request[12] = count[3];
        request[13] = count[4];

        -- message length
        local dataLen = intToPair(4, 2);
        request[14] = dataLen[1];
        request[15] = dataLen[2];
        
        local valpairs = intToPair(newValue, 8);
        for i=1,8 do 
            request[15+i] = valpairs[i];
        end
        
        -- LRC
        local lrc = LRC(request, 2, 22);
        lrc = intToPair(lrc, 2);
        request[24] = lrc[1];
        request[25] = lrc[2];
    
        -- CR LF
        request[26] = 13;
        request[27] = 10;
        
        local res = sendBytes(request);
        if (res == false) then
            DEBUG("Can't send request");
            return false;
        end


        local response = {};

        response = readBytes(17);
        if (response == false) then
            DEBUG("Can't read response");
            return false;
        end
        res = #response;
        
        if (res ~= 17) then
          DEBUG("Wrong response length");
          return false;
        end
        

        if (response[1] ~= request[1]) then
          ERROR("Wrong protocol");
          return false;
        end
        
        local lrc_resp = LRC(response, 2, 12);
        local lrc_actual = pairToInt(response[14], response[15]);
        if (lrc_resp ~= lrc_actual) then
          ERROR("Wrong LRC in response");
          return false;
        end
        
        if (response[2] ~= request[2] or response[3] ~= request[3]) then
          ERROR("Wrong UnitID in response");
          return false;
        end
        
        if (response[4] ~= request[4] or response[5] ~= request[5]) then
          ERROR("Wrong function in response");
          return false;
        end
        
        if (response[6] ~= request[6] or response[7] ~= request[7] or response[8] ~= request[8] or response[9] ~= request[9]) then
          ERROR("Wrong register address in response");
          return false;
        end

        if (response[10] ~= request[10] or response[11] ~= request[11] or response[12] ~= request[12] or response[13] ~= request[13]) then
          ERROR("Wrong register count");
          return false;
        end
        
        if (response[16] ~= request[16] or response[17] ~= request[17]) then
          ERROR("Wrong footer in response");
          return false;
        end

    else

        if (device.xtraFields[2] == 0) then
            ERROR("Can't write these type of registers (" .. device.name .. ")");
            return false;
        end
        
        -- function code
        local fnCode = intToPair(device.xtraFields[2], 2);
        request[4] = fnCode[1];
        request[5] = fnCode[2];
    
        -- address of register
        local intAddr = intToPair(reg.internalAddr, 4);
        request[6] = intAddr[1];
        request[7] = intAddr[2];
        request[8] = intAddr[3];
        request[9] = intAddr[4];

        -- data
        local val = newValue;
        if (reg.dataType == 0) then
            if (val > 0) then
                val = 255*256;
            else
                val = 0;
            end
        end
        local valpairs = intToPair(val, 4);
        request[10] = valpairs[1];
        request[11] = valpairs[2];
        request[12] = valpairs[3];
        request[13] = valpairs[4];
    
        -- LRC
        local lrc = LRC(request, 2, 12);
        lrc = intToPair(lrc, 2);
        request[14] = lrc[1];
        request[15] = lrc[2];
    
        -- CR LF
        request[16] = 13;
        request[17] = 10;
  
  
  
        local res = sendBytes(request);
  
        if (res == false) then
            DEBUG("Can't send bytes");
            return false;
        end
        
        local response = {};
        local requestLen = #request;
        
        response = readBytes(requestLen);
        if (response == false) then
            DEBUG("Can't read response");
            return false;
        end
        res = #response;

        if (res ~= requestLen) then
          DEBUG("Wrong response length");
          return false;
        end
        
        for i = 1,res do
            if (response[i] ~= request[i]) then
                DEBUG("Wrong response");
                return false;
            end
        end
    end
    return true;
end
</code>