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