Smart Hopper

shell.openTab("shell")

function rotate180()
    turtle.turnLeft()
    turtle.turnLeft()
end

function suckAndDrop(slot)
    local MC_STACK_DEFAULT_SIZE = 64

    turtle.select(slot)
    if (turtle.getItemDetail(slot) and turtle.getItemDetail(slot).count < MC_STACK_DEFAULT_SIZE) or (turtle.getItemDetail(slot) == nil) then
        local itemsNeededToFillStack = MC_STACK_DEFAULT_SIZE - (turtle.getItemDetail(slot) and turtle.getItemDetail(slot).count or 0)
        turtle.suck(itemsNeededToFillStack)
    end
    turtle.dropDown()
end

while peripheral.find("NBT_Observer").read_state().facing ~= "south" do
    turtle.turnLeft()
end

while true do
    local COAL_SLOT = 1
    local IRON_SLOT = 2
    local SEC_SLEEP = 1

    suckAndDrop(COAL_SLOT)
    rotate180()
    suckAndDrop(IRON_SLOT)
    rotate180()
    os.sleep(SEC_SLEEP)
end