DS1621 interface with PIC18F26J50
From page 15 of the datasheet the LOW period of the clock signal must be 1.3uS minimum in fast mode and 4.7 uS in standard mode. Without a delay after your 

CLOCK = LOW; 


The clock line goes high again after a few cycles of your sys clock. You can see this in your blue trace. Put a delay longer than 10 us in after this instruction, also you might want to shorten the delay after the 

CLOCK = HIGH; 


command or you're going to be waiting a second for 10 bits of data...