M7255

This document is an Engineering Change Order Log for the M7255 Disk Control unit (RK11-D, RK11-E), used in PDP-11 and PDP-15 systems. It details a series of identified problems and their corresponding corrections, spanning from October 1972 to February 1974.

The key issues addressed include:

1. Manufacturing & Design Flaws: Correcting errors in etch runs, updating artwork, and redesigning the board layout to increase density, add spare IC locations, shorten etch runs, and improve Hardware Poll timing.
2. Component & Timing Instability: Addressing loose tolerances on one-shot circuit components and standardizing clock pulses throughout the control unit.
3. Logic & Functionality Errors: Fixing an interrupt handling issue that caused incorrect disk drive status reporting, and resolving a problem where a one-shot circuit caused false "SCH CMP" errors when not polling.
4. Reliability & Power Integrity: Preventing data damage during power failures by modifying gate control logic, and mitigating susceptibility to noise (causing data, checksum, or write errors) by improving power and ground distribution through the addition of jumper wires.
5. Documentation & Implementation Clarifications: Correcting erroneous or unreasonable statements regarding the installation and effectivity of previous engineering change orders, including a typographical error in rework instructions.

In essence, the log records crucial updates to the M7255 Disk Control unit to enhance its manufacturing quality, functional correctness, stability, and overall reliability.