Service manual

STP 11-25R13-SM-TG
Q - 30
c. Clocked R-S Flip-Flop
. The clocked R-S FF is a special version of the R-S FF.
(1) Suppose you had two signal sources such as two video signals to switch between. You could
use an R-S FF to accomplish this. The only problem would be that when you command the FF to switch,
it will do exactly that. You are familiar with switching between video sources; it is desirable to switch
when the transition will not be seen. This means during the vertical blanking interval (VBI).
(2) Now if you had a 60-Hz clock in your body, you would be able to command a switch during the
vertical interval, but most of us are not that accurate to press a button at the exact time a switch should
be made. To overcome this problem, you could replace your R-S FF with one that is clocked by a vertical
interval pulse. A clocked R-S FF will not switch when the R or S inputs are changed but will wait until a
third input (clock) is present. So it can be easily seen that you can command the FF to switch and then it
will wait until the next clock pulse to do so. Figure Q-28 shows the symbol for the clocked R-S FF and its
gate equivalent. Table Q-18 illustrates the truth table for the clocked R-S FF.