startup

updateInterval = 0.2
update = os.startTimer(updateInterval)

while true do
    local e, p1,p2,p3,p4,p5,p6 = os.pullEvent()
    if e == "timer" then
    if p1 == update then
      if redstone.getInput("back") then
        if turtle.detect() then
          for i=1,16 do
            turtle.select(i)
            turtle.dropUp()
          end
        else
          turtle.select(1)
          turtle.place()
        end
      else
        if turtle.detect() then
          for i=1,16 do
            turtle.select(i)
            turtle.dropUp()
          end
          turtle.select(1)
          turtle.dropUp()
          turtle.dig()
        end
      end
      update = os.startTimer(updateInterval)
    end
  end
end