QProbes QBUS interface consists of

  • labled FlipChip fingers
  • optional termination resistor packs
  • optional separate voltage for resistor packs for QBUS margin testing.
  • pinheaders to access every QBUS signal via Multimeter, oscilloscope or logic analyzer

qprobe qbus interface

Bus terminators

Resistor packs can be plugged onto QBUS signal wires to terminate the QBUS with 120 or 240 Ohm. Terminators force idle ("negated"= logic 0) signal voltage to 3.4V.

On typical single-backplane systems, termination is not required at all: QBUS CPUs (LSI11, 11/23, 11/73) have terminators onboard, additional some backplanes too have resistor packs. So the problem on QBUS may be to have TOO MUCH terminators.

When using the bus terminators, plug QProbe behind the last occupied QBUS slot.

The DEC part for sole bus termination is a mostly unpopulated variant of M9400, REV11-E.

Dive into DECs alchemic rules for termination resistor values yourself!


Terminator power

Optionally the resistor packs can be powered by a different voltage than standard +5V, open jumper "TERMPWR" then and plug your voltage supply there.

Separate terminator power can be used for "margin testing": defective bus drivers may fail early if driven with lower voltage. Normally you don't use this.


LSI11/03 M7264 and M7220

The old 16 bit LSI11/03 CPUs make special use of QBUS signals pins BC1, BD1, BE1 and BF1. On later CPUs these pins are used (or reserved) for 22 bit address extension via BDAL<21:18>.

On LSI11/03s these wires expose CPU internal state signals at TTL level, so terminator resistors here may disturb CPU operation.

Only multi-backplane setups need the bus-end termination, normally you can remove the terminators on QProbe. Or you disable termination of BDAL<21:18> by bending out pins 4,5,6,7 of resistor networks RN36 and RN38.