Flip-flop

In 1918 William Eccles and F. W. Jordan described a circuit of two vacuum tubes wired so that each kept the other switched off. The output could sit in either of two states, both stable, and it stayed where it had last been pushed. It was the first memory built with no moving parts: no cam, no relay armature, just two conditions that held as long as the power did. They called it a trigger relay and had radio telegraphy in mind.

ENIAC's accumulators were ring counters made of flip-flops, ten stages per decimal digit, and a large share of the machine's 18,000 tubes sat right there. Colossus counted the same way, at thousands of characters a second. As long as a tube cost more than a week's wages, every bit was a budget decision.

The variants followed: SR, D, JK and T; level-sensitive latches that stay transparent while the clock is high, edge-triggered flip-flops that capture the value in the transition itself. A CPU register is flip-flops. An SRAM cell is six transistors in the same bistable pattern. A synchronous chip is fundamentally flip-flops with combinational logic between them — a century after Eccles and Jordan.