M7264 Troubleshooting

[Brent Hilpert]

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.