The bistable flip-flop 60 years on
|flip-flop from 'Electronic Computers'
|- T.E. Ivall
The bistable flip-flop is a very simple circuit. It can store a single binary digit (bit) of information. The flip-flop was not new in 1960. The original patent was granted to the inventors (Eccles and Jordan) in 1918. That was long before the transistor was invented: the first flop-flops used valves (US: vacuum tubes).
Valves were still used in computers in 1960, but transistors were beginning to replace them. Transistors were expensive but I managed to buy a few from Proops*, a famous electronic components store in Tottenham Court Road, and I soldered two transistors together with a few other components to make a flip-flop.
* Proops still exists, albeit in a rather different format. They now support hobbyists, jewellers, craft makers and modelling enthusiasts, and run a mail order business from rural Leicestershire.
A bistable flip-flop is a symmetrical circuit. At any given time one of the transistors is 'on' and the other 'off'. In an S-R flip-flop (the one I made back in 1960) a pair of buttons allowed you to put the flip-flop into either of its stable states.
I had to check the output using a multimeter; the transistors I used couldn't drive a light bulb, and LEDs were not yet invented.
A few days ago I decided to recreate the circuit using modern components. I started with a breadboarded version but since then I've built a more permanent flip-flop using stripboard. The layout reflects the symmetry of the circuit.
Modern memory is based on a more complex version of the circuit, and modern computers contain a lot. You'd need 8 billion flip-flops to store a Gigabyte of data!
In last Wednesday's #MakersHour session on twitter, someone suggested that we should be capturing knowledge of original computer technology while olders like me are still around to recall it. Tweet to me (@rareblog) if you'd like to see more about early computer technology!