When the value of counter 620 is zero, {HEAD ~ the output signal} of NOR gate 621 generates {TAIL ~ an active signal} on terminal R of SR flip flop 631 
generates
When the value of counter 620 is zero , { HEAD ~ the output signal } of NOR gate 621 generates { TAIL ~ an active signal } on terminal R of SR flip flop 631
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH