I am trying to master "AT89C51 datasheet" by understanding it completely. But my limited knowledge is hindering me. help me to overcome all obstacles. It says that Port 0 is an open-drain port. Does it mean that it can only sink current and can't source current? On the next line it says that "as an output port, each pin can sink eight TTL inputs." what does 'eight TTL input' mean here? Next it says that "When 1s re written to port 0 pins, the pins can be used as high impedance inputs." Does it mean that when the port 0 pins are set high, they won't sink any more current. They won't source any current either. all the current from the pull-up resistor will drive my circuit?