7,288 views
19 votes
19 votes

Choose the correct alternatives (More than one may be correct).

Two NAND gates having open collector outputs are tied together as shown in below figure.

The logic function $Y,$ implemented by the circuit is,

  1. $Y=ABC + DE$
  2. $Y=\overline{ABC + DE}$
  3. $Y=ABC.DE$
  4. $Y=\overline{ABC.DE}$

5 Answers

Best answer
13 votes
13 votes

From wikipedia (third paragraph),

By tying the output of several open collectors together, the common line becomes a "wired AND" (positive-true logic) or "wired OR" (negative-true logic) gate. A "wired AND" behaves like the boolean AND of the two (or more) gates in that it will be logic 1 whenever (all) are in the high impedance state, and 0 otherwise. A "wired OR" behaves like the Boolean OR for negative-true logic, where the output is LOW if any of its inputs are low.

So, after tying the open-collector NAND Gates, the common line becomes a wired AND.

So, $Y = \left(\overline{ABC}\right) \cdot \left(\overline{DE}\right)$

By D’ Morgan’s law, $Y = \overline{ABC + DE}$

Hence,

Correct Answer: Option (B)

selected by
9 votes
9 votes

A Wired Connection can create both an AND gate and an OR gate , however it cannot implement a not gate .

but to know whether this intersection acts as AND gate or OR gate , we should know whether the intersection is at a HIGH or a LOW.

i think here they have assumed it to be at a HIGH , hence the answer will be Y=(ABC)' . (DE)' .

https://en.wikipedia.org/wiki/Wired_logic_connection

2 votes
2 votes

Unfortunately options provided to the question are wrong...

refer this : https://questions.examside.com/past-years/year-wise/gate/gate-cse/gate-cse-1990/XD7yFvea5NWQTFJe/

Right options are :

Y= ABC + DE

Y= (ABC)' + (DE)'

Y= ABC . DE

Y= (ABC)' . (DE)'

*Referring to the "wired logic" connection, mentioned in this wiki page

A wired logic connection can create an AND or an OR gate. The limitations are the inability to create a NOT gate and the lack of level restoration.

*Then what we are left to wonder is what will be the logical choice between AND and OR in this scenario...(option B or D??), At this point I will agree with shaurya vardhan, here it's assumed to be AND by default as is hinted by this image in wikipedia :

Example wired AND.svg

 

Hence answer is option D) Y= (ABC)' . (DE)'

If anyone comes up with a reasonable explaination, please share...

edited by
Answer:

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