DECsystem20 Power System System Description

This document, EK-PWR20-SD-001, published in 1976, provides a modularly-structured technical description of the DECsystem-20 (2040) Power System, focusing on its operation theory for KL-based processors. It is organized into three levels of detail:

1. System Level Description:

   • Physical Configuration: The KL10-C Processor, typical for 2040 configurations, consists of three adjoining cabinets: Console Processor (housing PDP-11/40, peripherals, power controls, floppy disk), I/O (I/O logic), and CPU (Central Processing Unit). Each cabinet has self-contained power supplies. The DEC Power Control Bus interconnects the main power control system with peripherals.
   • Major Components: The system utilizes an 863 Power Control (main system control), an 861 Power Control (for console processor equipment), various H7420 Power Supplies, H760 Power Supplies (ferroresonant), H761 Series Pass Assemblies (for regulated negative outputs to the CPU ECL logic), and several voltage regulators (H744, H745, H754, H770) for different voltage outputs (+5V, -15V, +20V, -5V, +15V). A Capacitor Assembly provides power hold-up. A Switch Panel offers system control.
   • Electrical Requirements: The KL10-C Processor operates on 120 or 240 Vac, 60/50 Hz, 3-phase, 5-wire power, requiring approximately 21.6 KVA. Line voltage and frequency have specific tolerances.

2. Interface Level Description:

   • Power Control Flow: The 863 Power Control manages AC power for the entire system. The 861 Power Control further manages power for the PDP-11/40, DECwriter, Floppy Disk, and CPU cabinet fans. The H760 Raw Power Supply, Capacitor Assembly, and H761 Regulated Series Pass Assembly provide DC power for the CPU's ECL logic, including voltage regulation, sequencing, overvoltage protection, and current limiting.
   • Operation & Sequencing: Power-up is initiated via the Switch Panel's "POWER ON" switch. A critical 5-second delay ensures cabinet fans are up to speed before full power application, and a 13-second minimum delay is enforced between power-down and restart. The system uses "POWER WARNING" and "CROBAR" signals (from 863 to CPU) for coordinated power state changes.
   • Fault Detection: The system features an orange "FAULT" indicator on the Switch Panel for 8 fault conditions, including insufficient air flow, over-temperature, tripped circuit breakers, and open cooling assembly doors. Faults trigger an automatic 13-second power-down sequence.
   • Power-Failure Protection: In case of AC power drops, the 863 asserts a "POWER WARNING" signal, triggering an orderly shutdown sequence in both the KL10 CPU (to save data and clear memory) and the PDP-11/40 (to save volatile information within 2ms).
   • Automatic Restart: Only AC-related faults are automatically restartable, subject to the 13-second delay. Other faults require manual intervention.

3. Unit Level Description:

   • This section details the internal operation of each major assembly. It covers components like the 863 Power Control's Auxiliary Power Supply (W7000), AC Low Voltage Detector (W516), Power Control Logic (G8015), and Indicator Driver (G8017).
   • It describes the functions of the H7420 Power Supply (step-down transformer, line clock, AC LO/DC LO signals for power fail warning), the H760 Power Supply (ferroresonant design, fan power distribution), and the Capacitor Assembly (providing 20ms energy hold-up for critical data retention).
   • Each regulator (H744, H745, H754, H770) is described with its precision voltage regulation, overcurrent sensing, and overvoltage "crobar" protection circuits.
   • The H761 Series Pass Assembly's role in converting unregulated input to highly regulated negative outputs for the CPU, along with its control circuits and heat sinks, is detailed.
   • Air flow sensors are solid-state vane switches designed to detect reduced cooling and trigger a fault.

In essence, the document provides a comprehensive operational overview of the DECsystem-20's power infrastructure, emphasizing its modular design, hierarchical control, robust fault detection and protection mechanisms, and controlled power-up/down sequencing.