ESP8266 UDP to/from Raspberry Pi running LUA

(click here to see index of all ESP8266 posts)

This is just a different twist to my prior post

ESP8266 Reading/Writing GPIO and Transmitting/Receiving UDP Packets

In that post I send UDP packets to a Raspberry Pi running the echo service. In this post, I send the same packet in the same manner, but it is received by a LUA program being run on the Raspberry Pi.

To make this work, LUA and the lua-sockets package must be installed on the RPI. Please see here to make that happen:

Installing LUA on Raspberry Pi and Getting it Running

I used the following resource as an aid to writing the LAU program on the Raspberry Pi:

https://love2d.org/wiki/Tutorial:Networking_with_UDP-TheServer

The ESP8266 program used in the last ESP8266 post is almost right for this new experiment. The difference is I no longer want to use the echo service port, but will simply use UDP port 9999 instead. Here is the new code:

-- interrupt called when button is pressed
function btnINT(level)
    if inInt then                   -- don't allow interrupt in interrupt
        return
    else 
        inInt = true
    end
    tmr.delay(100000)               -- 100ms debounce
    cu:send('1234')
    gpio.write(4, gpio.LOW)
    inInt = false
    end -- btnINT

-- function called when UDP packet received
function rxPkt(cu,c)
    if c == '1234' then
        print('correct response')
        gpio.write(4, gpio.HIGH)
    else
        print('incorrect response')
        end
    end -- rxPkt

-- setup gpio pins
gpio.mode(3, gpio.INT, gpio.PULLUP)
gpio.trig(3, 'down', btnINT)
gpio.mode(4, gpio.OUTPUT)
gpio.write(4, gpio.LOW)

-- setup UDP port
cu=net.createConnection(net.UDP)
cu:on('receive',rxPkt)
cu:connect(9999,'192.8.50.106')

inInt = false

Now, I need a LUA program on the RPI that will accept a UDP packet, and simply echo it back (for this example, I’m not going to do anything any fancier than that, but it illustrates the necessary techniques I’ll need later down the road).

#!/usr/bin/lua

-- Setup UDP socket. Bind to localhost, port 9999.
local socket = require "socket"
local udp = socket.udp()
udp:settimeout(0) -- indicates not to wait. If no data, return immediately
udp:setsockname('*', 9999)

local data, msg_or_ip, port_or_nil

print "Beginning server loop."
while true do
    data, msg_or_ip, port_or_nil = udp:receivefrom() -- receive UDP packet
    if data then
        print('data ', data)
        print('msg ', msg_or_ip)
        print('port ', port_or_nil)
        udp:sendto(data, msg_or_ip,  port_or_nil) -- xmit same UDP packet back
    elseif msg_or_ip ~= 'timeout' then
        error("Unknown network error: "..tostring(msg))
    end
    socket.sleep(0.01) -- sleep .01 secs
end

My understanding is this will work exactly the same way on any linux or windows implementation of LUA. So if you don’t have a Raspberry Pi but have a PC, you can still use this code.