Open-collector gates and wired-AND
open-collector NAND gates
with ouputs wired together
using "wired-AND" to form (AB)'(CD)'
Open collector: another way to connect gate outputs to the same wire
- gate only has the ability to pull its output low
- it cannot actively drive the wire high (default – pulled high through resistor)
Wired-AND can be implemented with open collector logic
- if A and B are "1", output is actively pulled low
- if C and D are "1", output is actively pulled low
- if one gate output is low and the other high, then low wins
- if both gate outputs are "1", the wire value "floats", pulled high by resistor
- low to high transition usually slower than it would have been with a gate pulling high
- hence, the two NAND functions are ANDed together