Book Correction ? -- SR Latch

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Joined: Mar 18 2005
Posts: 2

Just working on the basic digital logic chapter and am having trouble with the SR latch setup on pg 66 figure 3.36

it says grab a 7402 chip but the diagram shows a 7432 chip and the symbols are for an OR (7432) gate but in the passage below the diagram it refers to NOR gates.

Should I use a 7432 (OR) chip or a 7402 (NOR) chip.

I have tried both but can't get the latch to work as expected.

Are S and R either connected to 5v or ground, what is the connection status at power up ?

Thanks for helping out a newbie and his 5th grader

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Joined: Dec 20 2003
Posts: 249
The answer to your question is below the fig 3.36

First, there maybe a misprint in the book. The correct circuit should be the one on the previouse page fig 3.35 except that is showing the same NOR. The pin #'s for the 2nd NOR (#2) should be 5 and 6 instead of 2 and 3, and then pin 4 instead of 1.

So, use a 7402 wire pin 1 to pin 5, and pin 4 to pin 3.

Pin 2 is 'S'
pin 6 is 'R'
pin 4 is 'Q'

run pin 4 to your resistor and your LED (don't forget polarity)

Pin 14 is 5V and 7 is GND

On page 66 it tells you to start with R to high (5V) and S to low (GND). Read that section to follow the rest of the experiment. In my opinion, Figure 3.36 is incorrect.

LOL, I actually had to do my math on that one to find that the truth table wouldn't work with OR's, it has to be NOR's. Hope this helps.

Vince

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Tom Owad's picture
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Joined: Dec 16 2003
Posts: 2460
Vince's explanation is correc

Vince's explanation is correct: it should be the 7402, not the 7432. The 7402 uses a different layout, which is why your circuit didn't work. The 7402 is layed out like this:

The pins Vince mentioned above correspond to this image.

I apologize for the error.

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Joined: Mar 18 2005
Posts: 2
Thanks

Thanks for your speedy reply. All I can say is that both my daughter and I are so impressed to have the two big names associated with our project respond to our questions. You guys really made a positive and powerful impression upon my child.