【Fujitsu】 FACOM 856 Magnetic Disk Unit

The FACOM 856 was Fujitsu's first magnetic disk, completed in 1963.

With increasing computer processing capability, there was a need for high-capacity memory. Thus foreign manufacturers outside Japan began to develop magnetic disk units with random access capability and higher memory capacity than magnetic drum. These magnetic disk units came in three types: models with movable disk access heads, models with stationary disk access heads to increase speed, and models in which the disk plate could be replaced as in a magnetic tape unit.

The FACOM 856 magnetic tape unit was the type with movable disk access head, and was developed as an auxiliary memory device for the FACOM 241D computer. This unit was comprised of a magnetic disk controller and a maximum of 4 connectable magnetic disk mechanisms, and memory capacity was 4.8–19.2 million words (40bit/word). The magnetic disk was comprised of 16 disk plates for information memory, 1 plate for clock, and 1 plate for baffle. There were positioners for each of the 16 information memory disk plates, and 8 access heads were attached to each positioner arm. Average access time was 195ms (at 50Hz). This equipment was connected to main memory of the FACOM 241D via the data channel of the FACOM 241D, just like a magnetic tape unit.

Specifications of FACOM 856 and 855 Magnetic Disk Units
  FACOM 856 Magnetic Disk Unit
Completion date 1963
Disk Memory capacity 192,000,000bit
Recording length Fixed length sector system
Number of disks 18 disk plates(for information memory:16, for clock:1, for baffle:1)
Disk rotation speed 1,200rpm(60Hz)/1,000rpm(50Hz)
Average rotational delay 25ms(60Hz)/30ms(50Hz)
Positioning time Average190 ms(60Hz)/195 ms(50Hz)
Read/write speed Inside:396Kbit/s
Unit Number of installed disks Max. 4
Memory capacity 4.8–19.2 million words(40bit/word)
Connected system FACOM 241D

FACOM 856 Magnetic Disk Unit