Massbus
| Company type | Technology / Computer bus |
|---|---|
| Industry | Computer hardware |
| Founded | 1970s |
| Founder | John Levy |
Key people | John Levy (lead developer), Gordon Bell (sponsor) |
| Products | High-speed computer input/output bus for connecting DEC computers to storage devices |
| Parent | Digital Equipment Corporation |
The Massbus represents a high-speed computer input/output bus engineered during the 1970s by Digital Equipment Corporation (DEC), with development led by John Levy and sponsored by Gordon Bell.
Designed to link DEC's premium computer systems with magnetic disk and magnetic tape storage devices, Massbus utilized a unified controller architecture capable of supporting various peripheral models. This approach enabled peripheral sharing among the PDP-10, PDP-11, and VAX series, minimizing the necessity for distinct drivers tailored to each device, operating system, or machine family. The design integrated magnetic technology components directly into storage units, allowing CPU I/O or memory bus adapters to remain consistent across evolving storage device generations.
From a commercial standpoint, Massbus was developed to provide storage subsystem solutions at considerably lower initial costs compared to IBM's expensive, device-specific controllers optimized for large-scale arrays.
The initial Massbus-compatible device was the RP04, derived from Sperry Univac ISS’s reproduction of the IBM 3330. Subsequent offerings included the RP05 and RP06, which were based on Memorex’s 3330 clone; these models incorporated DEC’s design innovations by relocating data separator electronics into the drive itself. This set a precedent for future disk technology enhancements such as double-density 3330s, CDC SMD drives, and "Winchester" drives. Drives were procured from ISS/Univac, Memorex, and Control Data. Various tape technologies also employed Massbus in master-slave configurations. DEC created the RS03/RS04 Massbus tape drives for high-performance swapping needs and ultimately developed the RM80 internally as they shifted focus toward proprietary large disk designs.
Logical implementation
The Massbus architecture is logically segmented into two primary parts:
- An asynchronous control bus responsible for accessing memory-mapped I/O registers within individual storage peripherals
- A high-speed synchronous data bus dedicated to transferring data between storage devices and the host bus adapter; this data bus features an 18-bit width plus parity
Disk
- RP04: 88 MB removable disk drive utilizing Sperry Univac pack technology
- RP05/RP06: 100 MB and 200 MB removable disk drives from Memorex; the RP04 and RP06 corresponded in capacity to IBM's 100 MB 3330 Model I and 200 MB Model 11 respectively. The RP05 was positioned as a higher-performance alternative to the RP04
- RP20: 1.2 GB fixed disk drive
- RM series: RM02, RM03, RM05, RM80 disk drives
- RS03/RS04: disks featuring a head-per-track design
Tape
- TU45: 9-track tape drive capable of 800 and 1600 BPI at a throughput of 120,000 bytes per second
- TU78/TA78: high-density GCR tape drives operating at 6250 BPI; TU78 interfaces via DEC's Massbus while TA78 functions with the HSC50 controller within a VMScluster
Massbus CPU interfaces
- RH11—Provides connection between Massbus and the PDP-11’s Unibus
- RH750—Serves as an interface linking Massbus to the VAX-11/75x bus