*Digital Design,Principles and Practices*

*3e*by John F. Wakerly frustrated me on the characteristic equation of S-R latch.

In the chapter describing sequential circuit,John just list the characteristic equations in a table without even a word for explaination.It reads the equation of S-R latch is \[ Q* = S + R' \cdot Q\].

I thought it over and over,again and again with the conclusion that this equation is wrong.When both S and R are 1,Q*(the next state of Q) should be 0.However,according to this equation,\[Q* = 1 + 0 \cdot Q\],we could calculuate that \[Q* = 1\] which is obvious wrong.

After wikipediaed S-R latch,I found there is a condition \[S \cdot R = 0 \]for this equation of High-Active S-R latch in http://books.google.com/books?id=4sX9fTGRo7QC&pg=PA344&lpg=PA344&dq=sr+characteristic+latch+equation&source=web&ots=9WdHN5uzTF&sig=ewGWuNX8g_4KozJuL5VyAS2yErc#PPA343,M1

So,My final question comes.Why does we need this condition for the equation?Why don't we just use \[Q* = S \cdot R' + R' \cdot Q\] so that we could emit the condition \[S \cdot R = 0\]

Thanks in advance.

MichealY.