In September 1984, NEC announced the D5126 5.25-inch hard disk unit for OEM.
This unit was a thin model, with a thickness of about 4cm, and track density was doubled to achieve a memory capacity of 25.6MB (unformatted) -twice that of the previous D5124 model which was shipped in December of 1983.
This unit ensured reliability at a high track density by using an embedded servo system for head positioning control. The embedded servo system of this unit was an index servo system in which a special pattern of about 100 bytes length, including a pattern for detecting the offtrack amount, was written, at the time the unit was manufactured, into the part immediately preceding the index signal of all cylinders, but this system had the disadvantage that access time was slow due to the rotational delay until detection of servo information, and an average access time of 85ms, the same as 5-inch disk units with the ordinary open loop system, was achieved by performing look-ahead control based on information during calibration seek operation inserted at a rate of 1 time every few minutes.
In August 1987, three years after the D5126 went on sale, NEC shipped the millionth unit.
In March 1985, NEC announced the D5146 which doubled memory capacity to 51.2MB in the same cabinet size by doubling the number of magnetic disks to 4.
In November 1985, NEC announced the D5126H and D5146H which reduced average seek time from 85msec to 40msec by, among other things, changing the magnetic head movement system from a step motor to a torque motor, and using a rotary encoder as the head position detection mechanism.
|Number of disks||2||4||2||4|
|Number of heads||4||8||4||8|
|Number of cylinders||615|
|Memory capacity (unformatted)(MB)||25.6||51.2||25.6||51.2|
|Average seek time(msec)||85||40|
|Average rotational delay(msec)||8.42||8.33|
|Data transfer speed(kB/sec)||625|
|Unit dimensions (mm)||149.3(W)×208(D)×42.9(H)|
|Date first shipped||September 1984||June 1985||July 1986||March 1986|