rednetLatch.lua

local sSide = "back" -- change this if you want to emit redstone signal on another side
local sRednetHost = os.getComputerLabel()
local sProtocol = "rn_redstone"
local sStateFilePath = "latch_state"

local fOldPullEvent = os.pullEvent
os.pullEvent = os.pullEventRaw

local function detectModem()
    local tPeripherals = {}
    for _,p in pairs(peripheral.getNames()) do
        local sType = peripheral.getType(p)
        if sType == "modem" then
            tPeripherals[#tPeripherals+1] = p
        end
    end

    if #tPeripherals ~= 1 then
        return
    else
        return tPeripherals[1]
    end
end

local function listener()
    repeat
        rednet.receive(sProtocol)
        print("Change output")
        redstone.setOutput(sSide, not redstone.getOutput(sSide))

        local tStateFile = fs.open(sStateFilePath, "w")
        local bState = redstone.getOutput(sSide)
        if bState then
            tStateFile.write("t")
        else
            tStateFile.write("f")
        end
        tStateFile.close()
    until false
end

local function initialState(sStateFilePath)
    local tStateFile
    if not fs.exists(sStateFilePath) then
        tStateFile = fs.open(sStateFilePath, "w")
        tStateFile.write("f")
        tStateFile.close()
    end

    tStateFile = fs.open(sStateFilePath, "r")
    local sState = tStateFile.readAll()
    tStateFile.close()

    return sState == "t"
end

local sModemSide = detectModem()
if not sModemSide then
    print("Many modems detected, please place only one modem on the computer.")
    return
end

local modem = peripheral.wrap(sModemSide)
if not modem.getNameLocal() then
    print("Configuration error, the modem should be connected.")
end

-- set initial state
local bInitialState = initialState(sStateFilePath)
redstone.setOutput(sSide, bInitialState)

-- start rednet listener
rednet.open(sModemSide)
rednet.host(sProtocol, sRednetHost)
term.clear()
term.setCursorPos(1, 1)
print("Emitting redstone on the "..sSide)
print("Hosting "..sProtocol.." for host "..sRednetHost)
print("Now listening to redstone messages...")

parallel.waitForAny(
    listener,
    function()
        local event = os.pullEventRaw("terminate")
        os.pullEvent = fOldPullEvent
        rednet.unhost(sProtocol)
        rednet.close(sModemSide)
    end
)