【NEC】 N7751 and N7755 Hard Disk Units

In April 1976, NEC announced the N7751 high-capacity hard disk unit with 14-inch media. It was announced together with the ACOS Series 77 System 800 and 900, and the first shipment was in January 1978.

This unit was the first "Winchester" hard disk unit made by NEC. It was configured to house two magnetic disk modules with a memory capacity of 317MB in a single cabinet. Each module contained eight 14-inch media, and high-density and high-reliability were achieved using a completely sealed structure.
Access time was shortened by improving the servo system and providing two heads for each disk side in order to shorten head movement distance.
A model with fixed head and 1.144MB per module could be selected, and was effective for recording information with a short access time.

Later, in October 1979, NEC announced the N7755 hard disk unit which had a memory capacity of 1270MB, twice that of the N7751. It was announced together with the ACOS system 350, 450 and 550, and first shipped in July 1980.

With the increased density of magnetic disk media, this unit improved positioning of the magnetic head and ameliorated the various factors involved in track slippage, such as thermal offtracking. To ensure software compatibility, it inherited the specifications of the N7751, and memory capacity was increased by doubling track density and the number of cylinders.

Specifications of N7751 and N7755 Magnetic Disk Units (per module)
Model name N7751 N7755
Memory capacity(MB) (Movable head) 317.5 635
(Fixed head) 1.144
Average seek time(msec) 20
Average rotational delay(msec) 8.3
Data transfer speed(MB/sec) 1.198
Disk diameter (inch) 14
Number of disk sides(information recording part) 15
Number of heads(information recording part) 30
Number of cylinders (figure in parentheses is for alternates) 555+(5) 1110
Recording density(bpi) 6400
Track density(tpi) 480 960
Recording system MFM
Rotation speed(rpm) 3600
Total memory capacity(MB)(Entire unit) 635 1270
Unit dimensions(mm) (Entire unit) 1040(W)×800(D)×1100(H)

N7751 Fixed Disk UnitN7755 Fixed Disk Unit