Active low/high

Hello,

I always seem to get confused with this active low and active high stuff Could someone please help me and explain a good way of uderstanding the concept of active high/low. What does it mean when INPUT is Active low and what does it affect? What does it mean when the OUTPUT is Active low and what it does?

Many Thanks,
vsg21

Active low or high only refers to the logic level a control signal is active at. For example, the set and presets of a flip flop might be active low, meaning a low applied to the preset pin will "set" the flip flop and active low applied to the reset resets the flip flop, while a high will do nothing. Or say a chip enable of a memory will be active high, meaning the memory is accessed with a high level is applied to the signal, and applying a low disables the memory.