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<title>fritzm.github.io - PDP-11/45: CPU debug VI - RK11 NPRs and first disk boot!</title>
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<h1>PDP-11/45: CPU debug VI - RK11 NPRs and first disk boot!</h1>
Sun 14 May 2017
by <a class="url fn" href="https://fritzm.github.io/author/fritz-mueller.html">Fritz Mueller</a>
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<div><p>Took a little of time to sort through BC11 cables to find a good one for drive interfacing, but in the end I
found one that worked okay and got the seek tester code from the previous post working reliably. At this
point I mounted one of the recently cleaned RK05 packs, but found that the M9301 bootstrap would hang the bus
on the first read operation.</p>
<p>A little scoping around on the RK11-C showed that it was asserting NPR to the processor, but never receiving
NPG. A quick check of NPG continuity on the backplane showed a missing jumper on one slot of my DD11-D.
Wrapped this on, verified NPG continuity all the way out to the RK11, but still no joy. Turns out the CPU
is not asserting NPG at all.</p>
<p>Threw the CPU UBC card out on extenders and had a go at chasing though the NPG logic with a logic probe.
Turns out to be a failed 8881 bus driver for NPG at the end of the line (E55 on KB11-A drawing UBCD). Pulled,
socketed, replaced.</p>
<p>After this, the CPU was asserting NPG, but the signaling still looked a little squirrelly. Turns out that
there are jumpers (W1-W5) on the M9301 bootstrap terminator that need to be installed to provide grant
pull-ups when they are not otherwise provided internally by the processor (and the 11/45 is one such
case). After installing the jumpers, NPG signaling looked solid.</p>
<p>Tried mounting a booting a few packs. Packs marked as having RT-11 would run for a short while and then hang.
But an RKDP pack <em>successfully booted!</em> Wow, that feels pretty good after about two years of working
seriously on this restoration. :-)</p>
<p>Going to stop here on a high note, and pick up trying to get a good RT-11 boot next time.</p>
<p><img src='/images/pdp11/rkdp-boot_thumbnail_tall.jpg' title='First boot off disk -- RKDP diagnostics pack!' onclick='pswipe("pdp11",57);'/></p></div>
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