Adrianne's Digital Basement: Plexus P20 Part 5

Introduction

• Rare Plexus P20: dual processor multi-user Unix workstation (circa 1985).
• Part five in the series documenting efforts to restore it to working condition.
• Recap of parts 1-4:
  • Part 1: Disassembly, motherboard examination, hard drive imaged.
  • Part 2: Power supplies serviced, initial power on, serial port pinout figured out.
  • Part 3: Original hard drive reinstalled, system booted into Unix but then failed.
  • Part 4: Diagnostic journey, Discord community assistance, identified and replaced a faulty flip-flop chip.

Current Problems and Goals

• Issues to resolve:
  • Diagnostic ROM error.
  • Clock circuit failure (likely clock chip).
  • Multibus error (related to expansion chassis).
• Goal: resolve errors and achieve stable Unix boot.

Diagnostics and Investigations

Multibus & Power Supply Configuration

• Multibus board: Not required for basic boot, supplies power to other cards if multiple power supplies are used.
• Power Supply: Connected to backplane or directly to motherboard.

Initial Diagnostic Check

• Powered up system: serial output shows system does not boot due to multibus error and clock calendar failure.
  • Ta (test all): Multibus passes but clock/calendar fails.
• Clock chip address: D000000 (memory mapped IO in 68k system).
• Clock chip pins: data not as expected, indicated possible chip or connecting logic issues.

Examining Clock Circuit

• Probed and verified signals on clock chip using oscilloscope.
• Identified power supply connections and chip select logic as possible failure points.
• Simulation of circuit: Verified clock chip enable logic.
• Contributors: Discord user SuperSVGA provided simulated logic for enabling clock chip.
• Attempted direct grounding of chip enable line, observed expected results.

Replacing Components

• Replaced clock chip: Confirmed faulty, socketed replacement worked with test runs.
  • Issues persisted with diagnostic calls.
  • Confirmed functionality by running timeframe tests on new chip.
• Formatted new Dallas clock chip with external 2032 battery for persistence.
  • Added resistor to initiate Motorola mode for compatibility.

System Boot and Configuration

Boot Sequence and Error Resolution

• Powered up system: verified retaining configuration.
  • Multiple iterations of setup confirming SC7 settings for tape drive resolved diagnostic errors.
• Unix boot successful, confirming operational state.

Future Plans

• Next Steps: Further Testing and Adjustments:
  • Continued validation: RJ45 check, system longevity stress testing.
  • Floppy drive repair.
  • BlueSCSI: potential for virtual storage, easier data transfer.
  • Hard drive image sanitization and sharing for broader access.
  • File transfer solutions: investigating viable methods suitable for Unix System V.

Reflections and Acknowledgments

• Acknowledged Discord community for support and collaboration.
• Thanks to contributors for data recovery and logic simulations.
• Intention for further documentation to aid restoration and usage of vintage systems.

Conclusion: System operational and future-focused testing ongoing for stable and full utilization of the Plexus P20.