[Tadao Hamada] works for Fujitsu Tokki, a subsidiary of the more famous Fujitsu. In 1956, Fujitsu decided to compete with IBM and built a relay-based computer, the FACOM128. The computer takes up 70 square meters and weighs about 3 tons. By 1959, they’d learned enough to make a FACOM128B model that was improved. [Hamada’s] job is to keep one of these beasts operational at Fujitsu’s Numazu plant. According to the Japanese Computer Museum, it may be the oldest working computer.

The relays in the computer were adapted from telephone switch networks. The computer had an index register and an interrupt. It used a biquinary (base 5) system and it could automatically detect errors and recompute by using redundancy (each decimal digit took 7 bits) and checking results against their complements, separately calculated.

There’s not a lot of information about the old machine on the Internet and what there is tends to be in Japanese. The computer was made for speed (relatively speaking) and uses asynchronous control so it doesn’t have to clock at a rate suitable for the slowest part of the machine.

A word was 69 bits, although that’s misleading because some of that was dedicated to the automated self-check function. There were 180 words of general-purpose memory. There were also specialized banks of memory for paper tape, the printer, and other items. One read only memory held common constants needed in calculations. The computer even has a floating point unit that can hold 8 significant digits and an exponent between -19 and +19.

The FACOM128 series was a relay computer although at the time tube-based computers were appearing. However, experience with earlier computers convinced Fujitsu that computers needed to be error free. At the time, a relay was much more reliable than a tube and combined with the self-checking and automatic re-execution, the FACOM128 provided a very reliable computing platform for its day. The 128B was faster, added some instructions such as extended-length addition and subtraction, provided user-defined constants, and also had better documentation.