History (1974): DEC RS03 and RS04 HDDs

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RS0x disk drive

The RS03 and RS04 disk drives were MASSBUS fixed-platter fixed-head disk drives. Their basic format was 18-bit words (for use in PDP-10 and PDP-15 machines), but they could be used in PDP-11s, where they simply ignored two bits.

RS03 mechanical components
Click to enlarge

They attach to any MASSBUS controller, e.g. RH11 (UNIBUS), RH70, or RH10. Up to 8 devices total can be attached to any MASSBUS controller.

The RS04 doubled the capacity (and bit rate) of the RS03 by recording on both sides of the platter; alternate bits in each word came from alternate sides.

They could store up to 512KB of data in 128B blocks (RS03), or 1,024KB of data in 256B blocks (RS04). They used 64 fixed read/write heads per side for data (8 spares were provided), and 2 heads to read timing/address tracks (a main and a backup).

The second timing track did not provide reliability/redundancy; the two tracks were not recorded synchonously. (This seeming oversight may be explained from DEC’s apparent intent that this device be used for swapping, where being unable to retrieve data would not be an issue. Indeed, the RS0x diagnostics scribbled over the disk – which caused an issue the first time they were run at Bell Labs, where UNIX used one as a root drive!) Switch-over from one to another was manual, by inverting a connector.

The drive had the ability to write-protect tracks 0-N (‘N’ set in switches).