Firefox Workstation Backplane

This document details the Firefox Workstation Backplane, Revision 3.0, developed by Digital Equipment Corporation in December 1987.

Key Features and Characteristics:

• Physical Design: An 11.9-inch x 16-inch, 12-slot, quad-height assembly using press-pin connector blocks. It is a four-layer printed circuit board (two signal, one +5V power plane, one ground plane) designed with a 64-ohm matched impedance to enhance signal integrity on the high-speed backplane. The backplane is fixed in size and cannot be expanded.

• Bus Architecture: It implements a new etch for M-bus and local subsystem interconnects for the Q-bus and Graphics VI-bus.

  • Slots 1-9 primarily use L-series connectors (188 pins per slot) supporting VI-bus and M-bus.
  • Slots 10-12 use Q-bus connectors (144 pins per slot) supporting the Q22-bus, which is active only if a Q-bus adapter module is in slot 9. Q-bus DMA grant and interrupt-acknowledge signals are daisy-chained, requiring specific slot prioritization for module insertion.

• Clock Circuit: An M-bus clock circuit is integrated directly onto the backplane module to minimize system clock skews, distributing a divided clock signal (derived from a TBD ns oscillator) to M-bus slots 3-9.

• Power Distribution: The backplane has two dual 28-pin card-edge power connectors:

  • Primary Power Supply Connector (J50): Near slot 1, distributes +5V, +12V, and fan power to slots 1-6.
  • Expansion Power Supply Connector (J51): Near slot 12, distributes +5V and +12V to slots 7-12.
  • Ground is common throughout. A -12V signal is present across slots but not powered by the Firefox power supply.

• Signal Integrity: Critical M-bus signals (MCMD, MSTATUS, MDAL, MPARITY) have a maximum stub length of 1.5 inches, while other M-bus signals are limited to 3.0 inches. Module IDs are assigned to M-bus slots using MID signals.

• External Connections: A fan connector (J52) provides power to fans, and an external connector (J53) sends signals to an RF distribution panel.