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QBUS grant continuity plug — KiCad board for a PDP-11 QBUS daisy-chain grant continuity slot filler
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qbus-grant-continuity-plug-cache.lib empty double size qbus board 2026-06-26 07:54:34 +02:00
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sym-lib-table empty double size qbus board 2026-06-26 07:54:34 +02:00

README.md

QBUS Grant Continuity Plug

A small PCB that fills an empty slot in a PDP-11 QBUS (LSI-11 bus) backplane to keep the daisy-chained grant signals flowing. When a slot is left empty, the two grant chains are broken and every device electrically downstream of that slot stops receiving DMA grants and interrupt acknowledges. This plug bridges each chain's input to its output so the bus keeps working with the slot unpopulated.

It implements the same function as DEC's M9047 grant continuity card.

Connections

The grant signals are on the A connector, solder side (row 2). The board carries two jumpers:

In Pin → Out Pin Chain
BIAKI L (Interrupt Acknowledge In) AM2 → BIAKO L (Interrupt Acknowledge Out) AN2 interrupt grant
BDMGI L (DMA Grant In) AR2 → BDMGO L (DMA Grant Out) AS2 DMA grant

Nothing else is connected — no power, ground, or other bus lines are needed for grant continuity. The B connector carries no grant-chain signals and is left unconnected.

Form factor

Double-height DEC FLIP CHIP module, shortened to 50 mm from the top edge to the connector finger tips. The reduced height keeps the plug from being removable while neighbouring cards are seated, which suits its role as a permanent slot filler.

Manufacturing

Recommended fabrication options for DEC-compatible edge connectors:

  • ENIG or hard gold surface finish (hard gold preferred for insertion durability)
  • Beveled connector edges
  • Board thickness 1.4 mm – 1.6 mm (1.6 mm works in practice)

Generated Gerbers, drill files, BOM and placement data are in production/.

Building from source

The project is a KiCad design (qbus-grant-continuity-plug.kicad_pro). The DEC edge-connector footprints live in DEC-FLIP-CHIP.pretty/ and are registered via the project fp-lib-table.

Credits and license

The board is derived from the dec-flip-chip-templates KiCad templates by Jay Logue (DEC FLIP CHIP double-height, extended-length), which are licensed under CC BY 4.0. This project is released under the same license.