A counter which can be made to count in either the forward or reverse direction is called an up-down, a reversible or forward-backward counter.
A binary counter with a reverse count is called a binary down counter. In a down counter, the binary counter is decremented by 1 with every input count pulse. The count of a 4-bit down counter starts from binary 15 and continues to binary counts 14, 13, 12… 0 and then back to 15. In a binary down counter, outputs are taken from the complement terminals Q’ of all flip flops.
For a down counter, when Q goes from 0 to 1, Q’ will go from 1 to 0 and complement the next flip flop.
A binary counter with a normal count is called a binary up counter. In a up counter, the binary counter is incremented by 1 with every input clock pulse. Outputs are taken drom the normal output terminal Q of all flip flops. For a up counter when Q goes from 1 to 0, it complements the next flip flop.
In above diagram, mode control line is also called up down counter line.
When mode control line is 1, all gates labeled as 1 will be enabled and all gates labeled as 2 will be disabled. The counter works like a Up Counter.
When mode control line is 0, all gates labeled as 1 will be disabled and all gates labeles as 1 will be enabled. The counter works like a Down Counter.