Book Correction ? -- SR Latch

4 posts / 0 new
Last post
Last seen: 14 years 6 months ago
Joined: Mar 17 2005 - 19:55
Posts: 2
Book Correction ? -- SR Latch

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

Last seen: 6 years 7 months ago
Joined: Dec 20 2003 - 10:38
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.


Tom Owad's picture
Last seen: 2 days 19 hours ago
Joined: Dec 16 2003 - 15:14
Posts: 2964
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.

Last seen: 14 years 6 months ago
Joined: Mar 17 2005 - 19:55
Posts: 2

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.

Log in or register to post comments