If you have got as far as DMA, you will probably know that the microprocessor pins for the DATA BUS can be driven HIGH or LOW to WRITE to the memory, OR, they can be configured as inputs to READ data from the memory. In the latter case the microprocessor is driving the DATA BUS pins neither HIGH nor LOW.
In normal use the microprocessor pins for the ADDRESS BUS are always being driven HIGH or LOW by the microprocessor because it only WRITES addresses to the memory.
When you have a DMA CONTROLLER connected, the memory can be WRITTEN to, or READ from, by either the microprocessor OR the DMA CONTROLLER. When the DMA CONTROLLER needs to access the memory it needs to inform the microprocessor. The microprocessor then puts all of its pins that are connected to the memory into a state where it is not driving them HIGH or LOW. (This is just like what happens on the DATA BUS when the microprocessor READS from memory - except now it happens on ALL the relevant pins.)
All these microprocessor pins can now be described as 'floating' - they are free to be driven HIGH or LOW by an external source, in this case the DMA CONTROLLER.
A more formal definition can be found at:
http://www.answers.com/topic/3-state?hl=7400&hl=series
The data sheet for a very early (and simple) Tri-State device can be found at:
http://www.alldatasheet.co.kr/datash...C/DM74125.html
