【Fujitsu】 FACOM 6421 Magnetic Disk Unit

Completed in October 1981, the FACOM 6421 was a magnetic disk unit from Fujitsu which used small diameter media with a diameter of 10.5 inches first in the world.

This magnetic disk unit integrated technologies developed by Fujitsu over many years. It miniaturized everything -- the media, the disk access head, head positioning mechanism and spindle drive mechanism -- and used a revolutionary integrated DE (Disk Enclosure). This structure revolutionized design concepts for magnetic disks, and had a worldwide impact. The unit had the following features:

1) It achieved greatly increased surface recording density (2.8 times Fujitsu's previous value) and a high memory capacity of 1.78Gbyte per the unit (due to being equipped with 4 DEs, each with 446Mbyte).
2) It achieved high-speed data transfer (1.859Mbyte/s) and high-speed access time (average positioning time: 18ms, average rotational delay: 7.6ms)
3) It achieved higher reliability by using a compact DE (Disk Enclosure) structure which sealed the head/media section more perfectly than previous equipment.

The FACOM 6421 came in 3 types: model A equipped with one adapter for connecting with a controller, model C with two adapters, and model B with no adapter.

For these three types, there were also models A4F, B4F and C4F with a fixed disk access head. With the fixed head, the head positioning time is zero, and data can be accessed with the rotational delay only. This made it possible to use part of the disk volume like a high-speed magnetic drum.

This unit also had two operation modes: a native mode, and a FACOM 493 compatible mode for easing transition from the previous FACOM 493 magnetic disk unit.

Specifications of FACOM 6421A4/B4/C4,A4F/B4F/C4F Magnetic Disk Units
  FACOM 6421A4/B4/C4,A4F/B4F/C4F
Completion date October 1981
Operation modes Native modes FACOM 493compatible modes
Disk Memory capacity 446Mbyte 317.5Mbyte
Track capacity 26,793byte 19,069byte
Number of cylinders 833 + 7(Alternate) + 1(CE) 555 + 5(Alternate) + 1(CE)
Number of heads 20 30
Fixed head capacity 1.607Mbyte(FACOM 6421A4F/B4F/C4F) 1.144Mbyte(FACOM 6421A4F/B4F/C4F)
Positioning time Average18ms(Max. 35 ms,Min. 5ms)
Average rotational delay 7.6ms
Data transfer speed 1.859Mbyte/s
Unit Number of installed DEs 4
Memory capacity 1.784Gbyte 1.27Gbyte
Connected system FACOM M-100/300 Series

FACOM 6421 Magnetic Disk UnitFrom the left, the photo shows the FACOM 1774A Magnetic Disk Controller, FACOM 6421C4 and FACOM 6421B4Disk Enclosure for the FACOM 6421The structure of the revolutionary first DE (Disk Enclosure)