AN18 Lua access to RS-232 serial port | NETIO products: Smart power sockets controlled over LAN and WiFi

The AN18 Application Note contains a Lua script that controls the RS-232 serial port and the sockets. The result is the ability to switch individual electrical outputs on or off over RS-232 using a text-based protocol that is the same as the Telnet M2M protocol.   

AN18 demonstrates the control of the RS-232 serial port (with a request-response protocol) using a Lua script. The commands to control the power outputs can be modified (in the Lua script) to change the serial port protocol.



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Supported devices: NETIO 4C


The NETIO model 4C includes a (3-pin) RS-232 serial port.

In the device configuration, the serial port (serial console) can be connected to a specified TCP/IP port, or redirected to a Lua script (as used in the script below).


The Lua script reacts to the string received over the serial port, sends a response and switches an output.

In order to work with the serial port, several parameters need to be specified (baudrate, block delimiter, handshake). The script itself then uses several basic functions to read the message received over the serial port, parse it, perform the requested action and send back a response. Example of a command to switch on the output no. 1: port 1 1


The AN18 Lua script includes 2 user-defined messages. The user can define the format of these messages and the contained commands.


Serial port settings

In the web administration, go to M2M API Protocols, click Serial Console and select Use in Actions (Lua scripts).

  • The block delimiter is a character that terminates the message and starts the Lua script. It is possible to choose from pre-defined characters (CR+LF, CR, LF, NULL), or to define a custom delimiter.
  • For a custom delimiter, the requested character must be entered in hexadecimal form (0x followed by the character code) in the field on the right.
  • For the pre-defined characters, the field is filled automatically (the hexadecimal form of CR+LF is “0xd” and “0xa”).
  • Speed sets the baudrate.
  • Click Save Changes to confirm the settings.



  • After the serial port is activated, a dark green dot appears next to the tab and the Status changes to “Connected”.



Creating the rule

To create and run the Lua script, do the following:

1) In the Actions section of NETIO 4 web administration, click Create Rule to add a rule.


2) Fill in the following parameters:

  • Enable rule: checked
  • Name: Serial console to Lua (user-defined)
  • Description: Parse message from Serial console and respond (user-defined)
  • Trigger: Serial console has read line
  • Schedule: any (does not affect the function in this configuration)


3) Copy the following code and paste it into the field for the Lua script:

------------Section 1------------
local shortTimeMs = 1000 -- "short" time for short on/off [ms]
local definableQuestion1 = "^all%-off$" -- prepared customer string 1
local definableQuestion2 = "^all%-on$" -- prepared customer string 2
local endingString = "\r\n" -- symbol sent at the end of each answer (CR+LF in the default setting)
---------End of Section 1---------

------------Section 2------------
-- Serial port - Telnet commands definition
local portValue = "^port %d$"
local portSet = "^port %d %d$";
local portListValue = "^port list$"
local portListSet = "^port list %d%d%d%d$"
---------End of Section 2---------

local id = -- internal variable, do not change
local message = event.args.message -- internal variable, do not change
local function short(output,state)

local function isBetween(number,lowLimit,highLimit)
  if number >= lowLimit and number <= highLimit then
    return true
  return false

local function setOutput(output,action)
  if action == 0 then -- turn off
    devices.system.SetOut{output = output, value = false}
  elseif action == 1 then -- turn on
    devices.system.SetOut{output = output, value = true}
  elseif action == 2 then -- short off
    devices.system.SetOut{output = output, value = false}
    milliDelay(shortTimeMs,function() short(output,true) end)
  elseif action == 3 then -- short on
    devices.system.SetOut{output = output, value = true}
    milliDelay(shortTimeMs,function() short(output,false) end)   
  elseif action == 4 then -- toggle
    if devices.system["output" ..output.. "_state"] == 'on' then 
      devices.system.SetOut{output=output, value=true}
  elseif action == 5 then
    -- do nothing

if string.match(message,portValue) then -- return output state
  local output = tonumber(message:sub(6,6))
  if not isBetween(output,1,4) then
    devices.system.SerialWrite{id=id,message="501 INVALID PARAMETR" .. endingString}
  local answer = "250 0"
  if devices.system["output" ..output.. "_state"] == 'on' then 
    answer = "250 1"
elseif string.match(message,portSet) then -- set output
  local output = tonumber(message:sub(6,6))
  local action = tonumber(message:sub(8,8))
  if not isBetween(output,1,4) or not isBetween(action,0,5) then
    devices.system.SerialWrite{id=id,message="501 INVALID PARAMETR" .. endingString}
  devices.system.SerialWrite{id=id,message="250 OK" .. endingString}
elseif string.match(message,portListValue) then -- return output status
  local answer = "250 "
  for i=1,4 do
    if devices.system["output" ..i.. "_state"] == 'on' then 
      answer = answer .. "1"
      answer = answer .. "0"
  devices.system.SerialWrite{id=id,message=answer .. endingString}
elseif string.match(message,portListSet) then -- set outputs
  local actions = {};
  for i=1,4 do
    actions[i] = tonumber(message:sub(10+i,10+i))
    if not isBetween(actions[i],0,5) then
      devices.system.SerialWrite{id=id,message="501 INVALID PARAMETR" .. endingString}
  for i=1,4 do
  devices.system.SerialWrite{id=id,message="250 OK" .. endingString}
elseif string.match(message,definableQuestion1) then
  -- command 1 start
  for i=1,4 do
  devices.system.SerialWrite{id=id,message="250 OK" .. endingString}
  -- command 1 end
elseif string.match(message,definableQuestion2) then
  -- command 2 start
  for i=1,4 do
  devices.system.SerialWrite{id=id,message="250 OK" .. endingString}
  -- command 2 end
  devices.system.SerialWrite{id=id,message="502 UNKNOWN COMMAND" .. endingString}

4) To finish creating the rule, click Create Rule at the bottom of the screen.


Description of the code

  • The message received over the serial port is read to the message variable with this command: local message = event.args.message
  • The message contains everything sent over the serial line, except the message-terminating character.
  • The message format is compared to the predefined patterns, or the user-defined patterns if appropriate (see below) using the string.match function. In case of a match, the corresponding section is executed and the action performed.


  • The code contains two variables that can be set:
  • shortTimeMs
    •  Specifies (in milliseconds) for how long is the output switched on or off for actions 2 and 3 respectively (short off / short on).
    •  The minimum value is 100 ms.
    •  Example – to turn on for 2 seconds: shortTimeMs = 2000
  • endingString
    • Specifies the symbol to terminate all responses that are sent back over the serial line.
    • The default is the CR+LF symbols (in Lua expressed as \r\n).
    • In a robust protocol, this terminating string will be the same as the string that starts the Lua script.

List of basic commands:

  • The patterns for these commands are coded in Section2 (portValue, portSet, portListValue, portListSet variables).
    • port <output> <action>
      •  Controls one particular socket.
      •  <output> is the socket number
      •  <action> is one of the possible parameters
      •  If <action> is omitted, the command prints the socket state.
      •  Parameters:
        • 0 – turns the socket off
        • 1 – turns the socket on
        • 2 – briefly turns the socket off (if the socket was off, the command turns it on)
        • 3 – briefly turns the socket on (if the socket was on, the command turns it off)
        • 4 – toggles the socket state
        • 5 – leaves the socket state unchanged
          •  Example – turn on socket 1: port 1 1
          •  Example – turn off socket 3: port 3 0
    • port list [xxxx]
    •  Controls all sockets at once.
    •  Without parameters, lists the socket states.
    •  List of parameters:
      • 0 – turns the socket off
      • 1 – turns the socket on
      • 2 – briefly turns the socket off (if the socket was off, the command turns it on)
      • 3 – briefly turns the socket on (if the socket was on, the command turns it off)
      • 4 – toggles the socket state
      • 5 – leaves the socket state unchanged
        •  Example – turn all sockets on: port list 1111
        •  Example – turn on sockets 1 and 3, turn off socket 4 and toggle socket 2: port list 1410
        •  Example – turn on sockets 2 and 3 and leave sockets 1 and 4 unchanged: port list 5115
    •  Note: All “x” in the command need to be replaced with an actual parameter. If the state of a particular socket should not change, use “5” as the parameter. For example, port list 10x1 is not a valid command.


User-defined RS-232 commands

Format of requests (received over RS-232)

  • The definableQuestion1 and definableQuestion2 variables are prepared in the code for creating custom request patterns.
  • The custom pattern must be enclosed in double quotes and the ^ and $ characters. This convention ensures that the message received must exactly match the pattern.
    • Example – for the all-on request: definableQuestion = "^all%-on$"
  • If the request includes a symbol that serves as a variable, the pattern can represent it with a wildcard (. for any character, %d for a decimal digit).
    • Example – the predefined command to check the status of a socket uses “port %d”: portValue = "^port %d$"
    • The actual value of the number cannot be checked in the request. If you want to react to a specific number only, include it literally in the string (definableQuestion1 = "^getState 1$"). If the request pattern contains a space, the message must contain it, too.
  • When creating custom requests, care must be taken so that the patterns do not overlap. For example, ^port %d$and ^port 1$. In this case, a port 1 message would execute the command that is defined earlier in the code.
  • The following characters are called “magic” in Lua. If they need to be present in the message, they must be escaped with the % sign:
    • ( ) . % + - * ? [ ^ $
    • Example – for the all-off request: definableQuestion = "^all%-off$"
  • The process of composing a response is described below.

Creating a response (to be sent after receiving a request)

  • For both variables, a code section to perform the requested response is prepared. It is located near the end of the code (see the picture).


  • Replace the sample code (between the comments) with your code to perform the desired action.
    • To make things easier, the following helper functions are implemented:
      • setOutput(output,action)
        • Changes the state of an output according to the specified action.
        • The action parameter can have one of the 5 values (see the description of the port command).
        • Example – to toggle output 3: setOutput(3,4)
      • isBetween(number,lowLimit,highLimit)
        • Returns true if the value of the first parameter (number) is between the values of the second and third parameter (lowLimit and highLimit).
        • Examples:
          • isBetween(1,1,4) returns true
          • isBetween(0,1,4) returns false
        • This function is used to check the input parameters.
    • devices.system.SerialWrite{id=id,message="Your message" .. endingString} can be used to send a message back over the serial line.
      • Replace "Your message" with a message string (in double quotes) or a variable (without quotes).
      • The .. endingString adds the terminating string at the end of the message. If you do not wish to terminate the message with that string, delete this part (including the two dots).
    • Even if you do not want to send any message back, you must send at least the terminating string:                               devices.system.SerialWrite{id=id,message=endingString}



1)  Can I have more custom requests (commands)?

Yes. To add another pattern, create a new variable (e.g. definableMessage3) with the pattern. Then, it is necessary to add another condition and specify the corresponding action in the code. Insert the following condition just above the last else statement: elseif string.match(message,definableMessage3) then

Code your action immediately after this condition. If you chose a different variable name, remember to change it in the condition, too. Your code should now look like this:



2) I don’t want to use the basic commands. Is it possible to disable them?

Simply delete the corresponding section in the code below the setOutput function. The code should now look like this:


3) Why does the example use “\r\n” as the terminating string?

This is the default CR+LF string (see Serial port settings). It is necessary that the terminating string is the same as the string that activates the Lua script; in Lua, the CR+LF string is represented as “\r\n”.


4) Is it possible to get the consumption measurement data?

NETIO 4C cannot measure consumption, so it is not possible to get such data.


5) Is it possible to change the serial port parameters from the Lua script?

No. This is possible only in the serial port settings.


Supported FW versions:

3.1.0 and higher (Firmware archiv)



This Application Note is compatible with:

Smart power socket NETIO



NETIO 4C is a small 110/230V PDU (Power Distribution Unit) with four controlled IEC-320 C13 (max 8A) power outlets. It includes an Ethernet switch and two LAN ports for network connections. Each of the four power outputs can be individually switched on/off over the web, from a mobile app, or using various M2M APIs (SNMP, MQTT, XML, Modbus/TCP, ..). Custom Lua scripts can run directly in the NETIO 4C device. As a distinctive feature, NETIO 4C provides a RS-232 serial port (3-pin) that can be controlled with a Lua script or used as a remote serial port.

More about NETIO 4C


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