Riley had it right. Each row of LED's have common cathode, each column of LED's have common anodes. To light an LED up, you set the row LOW and the column HIGH. The only drawback to this method is you can only control one LED at a time. Even though there may be 42 LED's lit to make the clock read 12:58, it's persistence of vision and fast switching that gives a steady, flicker free display. I estimate the refresh rate of the clock display is around 300-400 fps.