Answers Database

94 DATA BOOK 2nd: Presettable Down Counter on page 8-72 is misdrawn


Record #230

Problem Title:
94 DATA BOOK 2nd: Presettable Down Counter on page 8-72 is misdrawn


Problem Description:
The figure "Presettable Down Counter" is misdrawn.  The AND gate below T5 in
the upper-right-hand corner of the drawing calculates whether bits 1-4 of the
counter are low.  This is passed on to the T6 AND gate, and this gate is
supposed to make T6 active (high) when bits 5-0 of the counter are all low,
and similarly for T7 and bits 6-0 of the counter.  However, the schematic as
drawn makes T6 high whenever bits 5 and 0 are low *and* at least one of bits
1-4 is high!  Similarly, T7 is high whenever bits 6, 5, and 0 are low *and*
at least one of bits 1-4 is high.  This also messes up the terminal count.


Solution 1:



Fixing this can be accomplished in one of two ways:

  1. Removing the second bubble down off the T6 and T7 AND gates, and the
     first bubble down on the AND gate feeding the terminal-count flip-flop.

  2. Placing a bubble on the output of the AND gate below T5.

The second solution was implemented in the 94 data book 3rd edition





End of Record #230