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