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+===== Modbus RTU in custom protocol version =====
+An example of custom protocol for Modbus RTU.
+  *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 RTU Demo Driver
+function createDevices ()
+                                                     -- read FC write FC
+  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}})
+  addDevice({name = "IR", shift = 0, base = 10, xtraFields = {4, 0}})
+end
+local errorCount = 0
+-- template
+local request = {1, 2,      -- slaveId FC
+,  4,  -- addr high lo
+,  6, -- count hi lo
+,  0      -- crc high lo
+                }
+EXCEPTIONS = {"Illegal Function",     "Illegal Data Address",
+               "Illegal Data Value",  "Slave Device Failure",
+               "Acknowledge",         "Slave Device Busy",
+               "Negative Acknowledge", "Memory Parity Error",
+               "Gateway Path Unavailable", "Gateway Target Device Failed to Respond"
+              }
+function readRegister (reg, device, unitId)
+                             --- FORMING REQUEST -----------
+      -- slave address
+  request[1] = unitId;
+  -- function code
+  request[2] = device.xtraFields[1]
+  -- address of register
+  request[3] = GetHiByte(reg.internalAddr)
+  request[4] = GetLoByte(reg.internalAddr)
+  -- count of registers
+  local count = 1
+      if (reg.dataType == 3) then -- double word
+        count = 2
+      end
+  request[5] = GetHiByte(count)
+  request[6] = GetLoByte(count)
+   -- CRC
+  local crc = GetCRC(request, 2)
+  local crcLo,crcHi = 0,0 -- will be used below too
+  crcLo = GetLoByte(crc) ; request[7] = crcLo
+  crcHi = GetHiByte(crc) ; request[8] = crcHi
+                           -- SENDING REQUEST
+  if not (sendBytes(request)) then
+      DEBUG("Can't send bytes")
+      return false
+  end
+                           -- RECEIVING REPLY ---
+  local respHead, respData, respCRC = {}, {}, {}
+    -- read Header with length
+  respHead = readBytes(3)
+  if (respHead == false) then
+      DEBUG("Can't read response")
+      return false
+  end
+  if (respHead[1] ~= request[1]) then
+      ERROR("Wrong slaveID in response!")
+      return false
+  end
+  if (respHead[2] ~= request[2]) then
+      if (respHead[2] >= 0x81) then
+          ERROR("EXCEPTION: "..EXCEPTIONS[bit.band(respHead[2], 0x0F)])
+          readBytes(2) -- read till the end
+      else
+          ERROR("Wrong Func Code in response")
+      end
+   return false;
+  end
+  local resp_Lentgh = respHead[3];
+        respData = readBytes(resp_Lentgh)
+  if (respData == false) then
+      DEBUG("Can't read response");
+      return false;
+  end
+  -- check CRC in reply
+  local tmpResponseTab = {}
+    for i,v in ipairs(respHead) do
+        table.insert(tmpResponseTab, v)
+    end
+    for i,v in ipairs(respData) do
+        table.insert(tmpResponseTab, v)
+    end
+  crc = GetCRC(tmpResponseTab, 0)
+  crcLo = GetLoByte(crc)
+  crcHi = GetHiByte(crc)
+  respCRC = readBytes(2)
+  if (respCRC == false) then
+      DEBUG("Can't read response");
+      return false;
+  end
+  if (respCRC[1] ~= crcLo) or (respCRC[2] ~= crcHi) then
+      DEBUG("Wrong CRC in reply! "..string.format("%X", crcLo).." "..string.format("%X", crcHi));
+      return false;
+  end
+  if (device.name == "DI") or (device.name == "C")  then
+      if (resp_Lentgh ~= count) then
+          ERROR("Wrong length in response");
+          return false;
+      end
+  else
+      if (resp_Lentgh ~= count*2) then
+          ERROR("Wrong length in response");
+          return false;
+      end
+  end
+                                  -- RETURN DATA --
+    return GetHexFromTable(respData)
+end -- readRegister
+function writeRegister (reg, device, unitId, newValue)
+    -- for read-only don't write
+                                    DEBUG("Going to write this value "..newValue)
+    if device.name == "IR" or device.name == "DI" then
+         ERROR("Can't write these type of registers (" .. device.name .. ")")
+        return true
+    end
+    local wrRequest = {};
+    -- slave address
+    wrRequest[1] = unitId;
+    -- function code
+    wrRequest[2] = device.xtraFields[2]
+    -- address of register
+    wrRequest[3] = GetHiByte(reg.internalAddr)
+    wrRequest[4] = GetLoByte(reg.internalAddr)
+    local dataTable = GetDataAsTable(newValue, device.dataType)
+        local coilsTmp = 0
+        if (device.name == "C") then
+            coilsTmp = dataTable[2] * 0xFF
+            dataTable[2] = dataTable[1]
+            dataTable[1] = coilsTmp
+        end
+    DEBUG("#dataTable now "..#dataTable)
+    wrRequest[5] = dataTable[1]
+    wrRequest[6] = dataTable[2]
+    -- CRC
+    local crc, crcLo, crcHi = 0,0,0
+    crc = GetCRC(wrRequest, 0)
+    crcLo = GetLoByte(crc)
+    crcHi = GetHiByte(crc)
+    wrRequest[7] = crcLo
+    wrRequest[8] = crcHi
+    DEBUG("Going to send this packet "..table.concat(wrRequest))
+    local res = sendBytes(wrRequest);
+    if (res == false) then
+        DEBUG("Can't send request");
+        return false;
+    end
+    -- читаем ответ
+    res = readBytes(8)
+    if (res == false) then
+        DEBUG("Can't receive reply")
+        return false
+    end
+    if (table.concat(res) ~= table.concat(wrRequest)) then
+        DEBUG("Response does not match!")
+        return false
+    end
+    if (#dataTable == 4) then
+        -- DWORD
+        -- repeat steps for 2nd Word
+        wrRequest[3] = GetHiByte(reg.internalAddr + 1)
+        wrRequest[4] = GetLoByte(reg.internalAddr + 1)
+        wrRequest[5] = dataTable[3]
+        wrRequest[6] = dataTable[4]
+        crc, crcLo, crcHi = GetCRC(wrRequest, 0), GetLoByte(crc) , GetHiByte(crc)
+        wrRequest[7] = crcLo
+        wrRequest[8] = crcHi
+        res = sendBytes(wrRequest);
+        if (res == false) then
+            DEBUG("Can't send request");
+            return false;
+        end
+    end
+    return true
+end
+-- Calculating CRC for RTU
+function GetCRC(req, offset)
+  local crc = 0xffff
+  local mask = 0
+  -- iterate bytes
+for i=1,#req-offset do
+      crc = bit.bxor(crc,req[i])
+      -- iterate bits in byte
+      for j=1,8 do
+          mask = bit.band(crc,0x0001)
+          if mask == 0x0001 then
+              crc = bit.rshift(crc,1)
+              crc = bit.bxor(crc,0xA001)
+          else
+              crc = bit.rshift(crc,1)
+          end
+      end
+end
+     return crc
+end
+function GetHiByte(c)
+                                   DEBUG("Going to get high byte from "..c)
+   return bit.rshift(c,8)
+end
+function GetLoByte(input_val)
+                                    DEBUG("Going to get low byte from "..input_val)
+    return bit.band(input_val,0xFF)
+end
+function GetHexFromTable(inputTab)
+    -- get hex and concat it to number via string operatoin
+                                    DEBUG("entered GetHexFromTable with table - "..table.concat(inputTab))
+    local numberAs_String = ""
+    local tmpStr = ""
+        for i,v in pairs(inputTab) do
+            tmpStr = string.format("%X",v)
+            if (#tmpStr == 1) then
+                tmpStr = "0"..tmpStr
+            end
+            numberAs_String = numberAs_String..tmpStr
+        end
+                                DEBUG("number is "..numberAs_String.." decimal = "..numberAs_String)
+    return tonumber(numberAs_String, 16)
+end
+function GetDataAsTable(value, datatype)
+    local highWord, lowWord, tmpTable = 0, 0, {}
+    if (datatype ~= 3) then
+        DEBUG("getdataastable - going to process value  "..value)
+        tmpTable[1] = GetHiByte(value) ; DEBUG(tmpTable[1])
+        tmpTable[2] = GetLoByte(value) ; DEBUG(tmpTable[2])
+    else
+        highWord = bit.rshift(value,16)
+        lowWord = bit.band(c,0xFFFF)
+        tmpTable[1] = GetHiByte(highWord)
+        tmpTable[2] = GetLoByte(highWord)
+        tmpTable[3] = GetHiByte(lowWord)
+        tmpTable[4] = GetLoByte(lowWord)
+    end
+    return tmpTable
+end
+</code>