I have read the Wikipedia article Three-state logic, but it is not clear enough. What is a more simple explanation for what a three-state circuit is?
When and where do we use it? What has CMOS to do with three-state circuits?
Electronic – a three-state circuit
digital-logic
Related Solutions
The question seems rather confused in several respects...
1) It is unclear whether you want to implement 3-state logic or an open-drain interconnection.
In the former, the driving device DOES use a push-pull output, driving 0 or 1 onto the bus. The other devices, meanwhile, must abstain from driving until some separate system signals that it is their turn.
In the latter, the bus is always pulled to '1' by a resistor, and any device may pull it to '0'. In this case, there is no harm done if several devices drive the bus simultaneously, though any messages may be corrupted.
2) You say you want a "floating" bus to indicate that the bus is free. In neither case is this normally possible (there is no logic primitive that can detect that a bus is floating). This is why in tri-state logic there must be another system (bus arbitration logic) to keep track of who has the bus, and give each device a turn.
Now as to the specific question of transmission gates : either of these systems can be trivially implemented using transmission gates, though there are other and sometimes better ways.
Tri-state logic can be implemented using a normal push-pull (totem-pole) output, and a transmission gate between the push-pull output and the bus. The bus arbiter simply switches the transmission gate on or off. It is often more economical to turn off both transistors in the push-pull output instead.
Open drain logic can be implemented with a transmission gate by simply connecting one side of it to ground, and the other to the bus. Now simply turn it on to pull the bus low.
Your mistake comes in your logic circuit schema. You have written s instead of c and vice versa. It should be as shown below:
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Best Answer
Normal push-pull outputs drive the output either high or low by turning one of the output transistors on and the other off. Tri-state outputs can turn both transistors off, effectively cutting off the output completely. This allows another output on the same wire to drive it either high or low without creating a short circuit between chips.
Tri-state outputs can be implemented with either BJTs or MOSFETs, so there is no direct relation between them and CMOS.