【Fujitsu】 FACOM 631 Magnetic Disk Unit

The FACOM 631 was a high-capacity magnetic disk unit from Fujitsu, and the first model, called the FACOM 631A, was completed in 1965. At the time, magnetic disk units from Fujitsu were the only Japanese made disk units, and there were two models, the FACOM 631A and FACOM 631B. They were primarily used with the FACOM 230-20 and FACOM 230-30 mid-size general-purpose computers.

The disk module of the FACOM 631A had 41 disks (each 66cm in diameter) arranged on a horizontal shaft, and two sets of access arms driven by a hydraulic actuator. One unit was comprised of two disk modules. Each access arm set was made up of 20 arms (of which 4 were spare). The structure was such that all moved simultaneously, so that it gave a large cylinder capacity (capacity enabling reading and writing without changing position of access arms). Each arm accessed two disk sides (surfaces), and two heads were provided for one disk side. Each disk side was divided into zones -- an inside zone and outside zone - and each zone was comprised of 128 tracks. Each track was comprised of several sectors (record length 256 bytes) and the sector was the unit of access. It was possible to provide an arbitrary length of data, from 1 byte up-to maximum in a same cylinder. This equipment came in 2 types, model A and model B, and both used the same magnetic disk mechanism.

The FACOM 631A and 631B track format was fixed length, but in 1967 the variable length type FACOM 631K was developed for the FACOM 230-60 large computer. Except for the track format, The FACOM 631K was the same as the FACOM 631B.