A Massbus Mystery, or, Why Primary Sources Matter, Even In Computer History

This document, "A Massbus Mystery, or, Why Primary Sources Matter, Even In Computer History" by Bob Supnik, details an error found in the documentation and implementation of DEC's Massbus, a high-speed interconnect for mass storage devices. While preparing a simulator for the VAX-11/780, the author discovered that printed documentation for RP04/RP05/RP06 controllers was incorrect, and this error was propagated into the VMS operating system.

The Massbus, designed in the early 1970s, connected CPUs to storage devices. The document highlights discrepancies in register offsets between RP and RM drive families, particularly concerning the SN and ER2 registers. Initially, the author hypothesized that the RH70/RH11 controller for PDP-11 systems might have handled these mappings differently. However, evidence from developers indicated that mixed configurations of RP and RM drives were supported, contradicting this hypothesis.

The investigation led the author to consult primary sources: the schematics for the RP04/05/06. These schematics revealed that the register decoding for SN and ER2 was correct in the hardware, but the maintenance manuals and, consequently, the VMS drivers had incorrect definitions for these registers. The error logger, however, had the correct definitions, indicating the error was likely a documentation oversight that persisted for over 25 years. The author concludes that even maintenance manuals cannot be trusted and that schematics are the ultimate primary source for resolving such historical technical discrepancies.