My friend gave me a broken digital clock (it's losing a few minutes a day all of a sudden). Rather than try to fix it, I decided to use it for parts. I took the display off and decoded the segments. There appear to be three different ground wires and then all the other pins. To light a particular segment you have to use a particular ground and a particular pin (i.e. the pins are re-used for different segments depending on which ground is connected). I understand how to use a port on an 8051 to select one of the segment pins, but how can I also use the 8051 to select one of the ground pins on the display?

On previous projects where I've used an LED, I connected Vcc (and a resistor) to one side of the LED pins and then used the 8051 to turn on or off a particular port which would let current flow to ground. Can someone explain how this is done with a display board like this? Thanks.