Controller

modem = peripheral.find("modem")

if not modem.isOpen(1298) then
    modem.open(1298)
end

function Sender()
    while true do
        Send(io.read(), 1297)
    end
end

function Send(msg, channel)
    modem.transmit(channel, 1298, string.char(12, 97) .. msg)
end

function Listen()
    while true do
        local _, _, _, replyChan, m, _ = os.pullEvent("modem_message")

        if string.find(m, string.char(12, 97)) then
            local formattedMessage = string.sub(m, 3)
            print(m .. " turned into " .. formattedMessage)
        else
            Reply("This is not the modem you are looking for!", replyChan)
        end
    end
end

function Reply(msg, replyChannel)
    print("Reply sent to " .. replyChannel .. " with message \"" .. msg .. "\"")
    modem.transmit(replyChannel, 1297, msg)
end

parallel.waitForAll(Sender, Listen)