The DEC RP04 Disk Drive


Disk Drive Capacity:     20.48 million 36-bit words (= 92MB) formatted.
Peak Transfer Rate:       5.6  microseconds/Word
Access time:
 Track-to-track:          7    milliseconds
 Average:                28    milliseconds
 Maximum:                50    milliseconds
                        128    Words/sector
                         20    Sectors/track
                         19    Tracks/cylinder
                        411    Cylinders/pack
Number of heads:         20
Recording surfaces:      19
Max disks/system:         8
Max drives/controller:    4

Date: 1975. The RP04 is equivalent to the IBM 3330 Mod I. Compare with RP06.


This is the RP04 on our own PDP-11/50. As you can see, the disk packs are removeable. The glass door slides open and the disk is extracted using the clear plastic round cover, then coupled with the case bottom for storage. Exposure of disk heads and surfaces to air particles made these disks much less reliable than today's sealed disks. Also the heads were relatively massive, and rapidly seeking heads caused considerable vibration, and sometimes even made the drives "walk" across the floor.

Photo: Ben Beecher.

Here's a photo of an RP04 that someone sent me, origin unknown, apropos of the well-known phrase "washing-machine size" applied to the disk drives of the 1970s and 80s. I'll be glad to give this photo proper credit if anybody can tell me where it's from.

Disk drive in laundry room

Frank da Cruz / / Columbia University Computing History / Jan 2001 / Updated: Dec 2014