PPT Slide

7 – Sequential Logic Examples

© 1996 Gaetano Borriello

Open-collector gates and wired-AND

• 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

44

open-collector NAND gates

with ouputs wired together

using "wired-AND"to form (AB)'(CD)'

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