However you make a digital computer, you need something to represent a binary digit. Usually this is some form of switch: a relay, a tube, or a transistor, although there have been other ways to represent state. In 1954, [Eiichi Goto] of Japan invented the parametron, a resonant circuit using a ferrite core and a capacitor that could be moved between two phases.

According to [Goto’s] 1959 paper, the device is effectively a parametric oscillator, similar to some used at UHF frequencies by hams. The idea is that a tuned circuit is set to some frequency and driven with twice that frequency.

A What?

A parametron is essentially a resonant circuit made with inductors or capacitors whose reactance is varied at twice its resonant frequency. That “pumping” causes the circuit to oscillate at half the pump frequency. The neat trick is that there are two equally stable oscillation states at half the pump frequency, separated from each other by 180° of phase. Those two phases become binary 0 and 1. Depending on the incoming signals, one phase will win over the others.

We normally think of logic levels as voltages, although sometimes we use current direction. The parametron does neither. The phase of the AC waveform determines if the state is a one or a zero.

To make a logic gate, several preceding parametrons are weakly coupled into the next one. When that next stage is excited, the incoming signals provide a tiny initial oscillation. Because the parametron behaves rather like a regenerative amplifier, that initial signal rapidly grows into a full-amplitude oscillation. With three inputs, signals having one phase reinforce each other while opposite-phase signals cancel, so the new parametron naturally settles into the phase held by the majority of its inputs. Thus a three-input parametron is inherently a majority gate.

Ordinary Logic

Tie one input permanently to 0 or 1, and the same circuit becomes AND or OR; reverse the polarity of a coupling transformer and you get inversion. You can see this in the graphic from the original paper. In practical machines, groups of parametrons were driven by staggered clock phases so the result of one stage was established before the next stage was allowed to oscillate.

A chain of three parametrons can form a basic SR flip flop. [Goto’s] paper shows more complex forms and circuits like a parity checker. The catch, of course, is speed. Even a high-speed parametron topped out at about 6 MHz, and the clock frequency of a computer built with such devices would be down in the 140 kHz range. On the other hand, each parametron oscillates, and thus each gate regenerates a new output from its input signals, so a single output could easily drive 10 or 20 inputs.

In Practice

Still, the components to make a parametron were cheap and reliable. In the late 1950s, the PC-1 was operating at 15 kHz and could add fixed-point numbers in 270 microseconds. The PC2 (built in 1960 and commercialized as the FACOM 202) was set to operate at 100 kHz and had 9600 parametrons compared to the PC1’s 4200. The FACOM 212 used 8000 units. Then again, the FACOM used 5kW of power, the PC-1, 3kW, and the PC-2, 10 kW, so there is that. There were several other Japanese computers built on this technology around the same time, including HIPAC-1, MUSASINO-1, NEAC-1101, and SENAC-1.

The largest of these computers had 1,024 words of core memory. What’s more, parametrons could drive magnetic cores differently than conventional core memory. One advantage is that, unlike conventional core memory systems, reading these cores did not erase them.

Aftermath

Of course, active devices had the speed edge, and parametrons faded into history. There were some other uses, including numerical control of machining devices, a Morse code to Teletype converter, and even a prototype system that used parametrons to sense cars passing over a road.

While it is just a footnote now in the history of computing, a significant number of Japanese computers used the technology. The Engineering and Technology History Wiki has a great collection of details about this technology and the machines that used them.

What was it like to program the PC-1? You can get an idea by reading some of the old papers about it. We’ve noted before that you can make a logic gate out of almost anything. While switch logic is obvious, sometimes other methods make more sense.

Featured image: Fujitsu FACOM 201 Parametron Computer by [Daderot].