IBM 350 Disk Storage Unit

In 1956, IBM introduced the 305 RAMAC system, generally acknowledged as the first (commercially successful) computer to feature what we would now call a “disk drive” or “hard drive” (i.e., data stored on a magnetic disk and accessed via a moving head). What is pictured above is the IBM 350 disk storage unit utilized by the IBM 305 RAMAC.

Introduced in 1956, the IBM 305 RAMAC (Random Access Memory Accounting System) was an electronic general purpose data processing machine that maintained business records on a real-time basis. The 305 RAMAC was one of the last vacuum tube systems designed by IBM, and more than 1,000 of them were built before production ended in 1961.”

The 305 was a flexible, electronic, general purpose data processing machine that enabled businesses to record transactions as they occurred and concurrently reflect each entry in affected accounts. It maintained records on a real-time basis, provided random access to any record, eliminated peak loads, and could simultaneously produce output by either print or punched cards.

The 305 system consisted of the IBM 305 Processing Unit (containing the magnetic process drum, magnetic core register and electronic logical and arithmetic circuits), the IBM 370 Printer (an 80-position serial-output printer with tape control carriage), the IBM 323 Card Punch (similar to the IBM 523 Gang Summary Punch, providing for 80 columns of output punching), the IBM 380 Console (containing the card feed, typewriter, keyboard and indicator lights and control keys), the IBM 340 Power Supply (supplying power for all components except the motors in the 350 disk storage unit), a utility table adjacent to the console, and the IBM 350 Disk Storage Unit.

The 350 Disk Storage Unit consisted of the magnetic disk memory unit with its access mechanism, the electronic and pneumatic controls for the access mechanism, and a small air compressor. Assembled with covers, the 350 was 60 inches long, 68 inches high and 29 inches deep. It was configured with 50 magnetic disks containing 50,000 sectors, each of which held 100 alphanumeric characters, for a capacity of 5 million characters.

Disks rotated at 1,200 rpm, tracks (20 to the inch) were recorded at up to 100 bits per inch, and typical head-to-disk spacing was 800 microinches. The execution of a “seek” instruction positioned a read-write head to the track that contained the desired sector and selected the sector for a later read or write operation. Seek time averaged about 600 milliseconds.

In 1958, the 305 system was enhanced to permit an optional additional 350 Disk Storage Unit, thereby doubling storage capacity; and an additional access arm for each 350.