M7264 Troubleshooting

[Mister PDP]

Alright, I hooked the oscilloscope up to SRUN off of E68, and found that it
oscillates low at 58.68KHz. This oscillation is very short lived, with it
bouncing back up to high nearly instantly. Hitting the Run/Halt switch does
not have any effect on the period or amplitude of the oscillation.

On Mon, May 20, 2019 at 3:05 AM Brent Hilpert via cctalk <
cctalk at classiccmp.org> wrote:

> On 2019-May-20, at 12:11 AM, Glen Slick via cctalk wrote:
> > On Sun, May 19, 2019 at 11:48 PM Brent Hilpert via cctalk
> > <cctalk at classiccmp.org> wrote:
> >>
> >> Except the SRUN (K8 SRUN L) action is not latched, so it probably
> appears as an
> >> active-low heartbeat pulse with some periodicity when the processor is
> in run mode.
> >> A low-going pulse during PH 3 every instruction cycle perhaps.
> >
> > http://www.bitsavers.org/pdf/dec/pdp11/1103/1103_Schematics.pdf
> >
> > Page 171 of the PDF, Lights & Switches Board schematic
> >
> > The SRUN L signal (routed from the backplane through the power supply)
> > is fed through a 74123  retriggerable monostable multivibrator
> > one-shot before driving the RUN LED. If the SRUN signal is a pulsing,
> > the LED might effectively be driven constantly on.
> >
> > I don't have the H11 schematic. It's front panel might have similar
> > logic driving the RUN LED.
>
>
> Good, that makes sense together, as the duty cycle could be expected to be
> too low to light the LED driven directly.
>
> TTL monostables weren't the most reliable components.
>
>