With an access time of no more than 167 msec (including 100 msec for positioning), a new magnetic disk file can store from more than 30 to almost 620 million bits. The file can include from one to 20 storage disks, each 39 in. in diameter.
Manufactured by Bryant Computer Products, a division of Ex-Cell-O Corp., Walled Lake, Mich., the series 4000 disk file offers high-speed random access, very large storage, and low cost. Cost per bit ranges from 1/10 cent for the smallest file (with a single disk) down to 1/40 cent for the largest (with 20 disks). The 20-disk file allows one to read or write 40 bits simultaneously. Bryant guarantees zero dropouts and a minimum signal-to-noise ratio of 32 db.
Each disk offers a useful recording surface between radii of 6.75 and 18.75 in. Each recording area is divided into six 2-in. wide frequency zones with 128 tracks per zone. Bit densities vary downward in each zone from a maximum of 273 bits per in. Recording frequency ranges from 174 kc in the innermost zone to 431 kc in the outermost.
Aerodynamic read/write heads, 240 of them, serve the 240 frequency zones on the 40 disk surfaces. Each head floats within 1/2 mil of the microfinished surface of a rotating disk. Forty head-mounting bars, each holding six heads, are driven in unison by a single hydraulic positioner mounted on a base plate. The individual heads are selected electronically.
The head positioner is a hydraulic system of a digital, open-loop type. It is addressed by a 7-bit binary signal from a control unit. The positioner can repeat each of the 128 discrete data-track positions in each frequency zone to within 1 mil without adjustment for long intervals.
With its associated positioner and power supply, the file occupies a single cabinet, about 50-in. long, 40-in. wide, and 60-in. high. The complete assembly weighs about 1700 lb. Delivery on the series 4000 disk files is about six to nine months. Prices depend on quantity and configuration. For example, the model 4010, with a single disk, costs $41,000 for one and $32,000 in quantities of 10 or more. The model 4200, with 20 disks, costs $140,000 as a single unit and $110,000 in quantities of 10 or more. (Electronic Design, March 1, 1961, p. 82)
About eight megabytes—that's not bad. But those 20 disks have 39-in. diameters and the whole unit weighs almost a ton. Still, this is an impressive engineering achievement, just in getting such a massive system under control, and a necessary step in the evolution of disk drives.