When repairing DEC PDP-11s there's always the need to see whats happening on the main system bus. On newer 11s this is the QBUS.

So after UniProbe, I also designed a "QProbe".

The card has these features:

• it acts optionally as bus terminator via changeable resistor packs.
• it visualizes the QBUS signals. 
• Two display modes:
  a) ADDR and DATA are demultiplexed, INTR and DMA pulses are elongated to be visible.
  b) "passthrough": LEDs just show state of bus lines.
• Pin headers provide access to signals via logic analyzer (LA) or multimeter.
• Patch field to connect LA probes to QBUS signals in a clean way. LA ports with changeable impedance-matching resistor packs.
• LED drivers and demultiplexer logic realized with two EPM240T100 CPLDs. These can be programmed with own logic via an onboard JTAG header and Intel QUARTUS tool.

Read on!