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<title>fritzm.github.io - PDP-11/45: RK05 II - Head Load and Servo Calibration</title>
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<h1>PDP-11/45: RK05 II - Head Load and Servo Calibration</h1>
Sun 09 April 2017
by <a class="url fn" href="https://fritzm.github.io/author/fritz-mueller.html">Fritz Mueller</a>
</div>
<div><p>Okay, disassembled and cleaned a few RK05 cartidges, following advice from the vcfed forum and cctalk
mailing list (cleanroom gloves and wipes, 99% anyhydrous isoprop). Was surprised to find foam inside
the hub on the disks (see pic below) but folks on vcfed advise that it is high-density polyeurethane
and not subject to decay to the same extent as the other DEC foams, so I left it be.</p>
<p>Mounted one of the cleaned packs, and let it spin in the drive for a few hours with head load disabled
in order to get a good flush on the air filtration system, let the various bearings on the drive loosen up,
and make sure the replacement head retract batteries got a good charge. Drive ran quiet and balanced.</p>
<p>After that, took a deep breath and let the heads load -- no crash! Proceeded to work through the dynamic
off-line calibration procedure for the head positioning servo system. This involves jumpering the control
electronics on the drive to strobe simulated cylinder addresses from the sector counter. That provides
a convenient source of oscillating seeks that can be used to calibrate the servos. Video here shows head
load, a four cylinder oscillating seek, and a scope trace of the resulting sine position output of the electro-optical carriage position sensor:</p>
<p><span class="videobox">
<iframe width="640" height="390"
src='https://www.youtube.com/embed/b6p2ri-uM9c?rel=0'
frameborder='0' webkitAllowFullScreen
mozallowfullscreen allowFullScreen>
</iframe>
</span></p>
<p>Surprisingly, after about thirty years of non-operation, all of the servo calibration was within specified
error bars, so no adjustments were necessary! At this point I decided to go for broke, cabled the drive
to the RK11-C controller and attempted a boot. Some cncouraging front panel indicator activity, but soon
halted with a seek error flagged in RKER. Not too surprising.</p>
<p>Okay, on to debugging the drive online with the controller, then. Worked up the following test code, inspired
by something in one of the RK05 SPI workbooks. This reads two cylinder addresses from the high and low
bytes of the front panel switches, and instructs the controller to instruct the drive to seek alternately
between them:</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23</pre></div></td><td class="code"><div class="highlight"><pre><span></span> <span class="nt">177570</span> <span class="nt">SW</span><span class="o">=</span><span class="nt">177570</span>
<span class="nt">177400</span> <span class="nt">RKDS</span><span class="o">=</span><span class="nt">177400</span>
<span class="nt">177404</span> <span class="nt">RKCS</span><span class="o">=</span><span class="nt">177404</span>
<span class="nt">177412</span> <span class="nt">RKDA</span><span class="o">=</span><span class="nt">177412</span>
<span class="nt">000000</span> <span class="p">.</span><span class="nc">ASECT</span>
<span class="nt">001000</span> <span class="o">.=</span><span class="nt">1000</span>
<span class="nt">001000</span> <span class="nt">012706</span> <span class="nt">000700</span> <span class="nt">START</span><span class="o">:</span> <span class="nt">MOV</span> <span class="p">#</span><span class="nn">700</span><span class="o">,</span><span class="nt">SP</span> <span class="o">;</span><span class="nt">INIT</span> <span class="nt">STACK</span> <span class="nt">POINTER</span>
<span class="nt">001004</span> <span class="nt">013700</span> <span class="nt">177570</span> <span class="nt">L0</span><span class="o">:</span> <span class="nt">MOV</span> <span class="o">@</span><span class="p">#</span><span class="nn">SW</span><span class="o">,</span><span class="nt">R0</span> <span class="o">;</span><span class="nt">RETRIEVE</span> <span class="nt">SWITCHES</span>
<span class="nt">001010</span> <span class="nt">000300</span> <span class="nt">SWAB</span> <span class="nt">R0</span> <span class="o">;</span><span class="nt">LOWER</span> <span class="nt">SWITCHES</span> <span class="nt">TO</span> <span class="nt">UPPER</span>
<span class="nt">001012</span> <span class="nt">004767</span> <span class="nt">000012</span> <span class="nt">JSR</span> <span class="nt">PC</span><span class="o">,</span><span class="nt">SEEK</span> <span class="o">;</span><span class="nt">DO</span> <span class="nt">THE</span> <span class="nt">SEEK</span>
<span class="nt">001016</span> <span class="nt">013700</span> <span class="nt">177570</span> <span class="nt">MOV</span> <span class="o">@</span><span class="p">#</span><span class="nn">SW</span><span class="o">,</span><span class="nt">R0</span> <span class="o">;</span><span class="nt">RETRIEVE</span> <span class="nt">SWITCHES</span>
<span class="nt">001022</span> <span class="nt">004767</span> <span class="nt">000002</span> <span class="nt">JSR</span> <span class="nt">PC</span><span class="o">,</span><span class="nt">SEEK</span> <span class="o">;</span><span class="nt">DO</span> <span class="nt">THE</span> <span class="nt">SEEK</span>
<span class="nt">001026</span> <span class="nt">000766</span> <span class="nt">BR</span> <span class="nt">L0</span> <span class="o">;</span><span class="nt">START</span> <span class="nt">OVER</span>
<span class="nt">001030</span> <span class="nt">042700</span> <span class="nt">000377</span> <span class="nt">SEEK</span><span class="o">:</span> <span class="nt">BIC</span> <span class="p">#</span><span class="nn">377</span><span class="o">,</span><span class="nt">R0</span> <span class="o">;</span><span class="nt">MASK</span> <span class="nt">OFF</span> <span class="nt">LOWER</span> <span class="nt">BYTE</span>
<span class="nt">001034</span> <span class="nt">072027</span> <span class="nt">177775</span> <span class="nt">ASH</span> <span class="p">#</span><span class="nn">-3</span><span class="o">,</span><span class="nt">R0</span> <span class="o">;</span><span class="nt">SHIFT</span> <span class="nt">OVER</span> <span class="nt">TO</span> <span class="nt">CYL</span> <span class="nt">ADDRESS</span>
<span class="nt">001040</span> <span class="nt">105737</span> <span class="nt">177404</span> <span class="nt">L1</span><span class="o">:</span> <span class="nt">TSTB</span> <span class="o">@</span><span class="p">#</span><span class="nn">RKCS</span> <span class="o">;</span><span class="nt">CHECK</span> <span class="nt">RKCS</span> <span class="nt">RDY</span> <span class="nt">BIT</span>
<span class="nt">001044</span> <span class="nt">100375</span> <span class="nt">BPL</span> <span class="nt">L1</span> <span class="o">;</span><span class="nt">LOOP</span> <span class="nt">IF</span> <span class="nt">BUSY</span>
<span class="nt">001046</span> <span class="nt">032737</span> <span class="nt">000100</span> <span class="nt">177400</span> <span class="nt">L2</span><span class="o">:</span> <span class="nt">BIT</span> <span class="p">#</span><span class="nn">100</span><span class="o">,@</span><span class="p">#</span><span class="nn">RKDS</span> <span class="o">;</span><span class="nt">CHECK</span> <span class="nt">RKDS</span> <span class="nt">ARDY</span> <span class="nt">BIT</span>
<span class="nt">001054</span> <span class="nt">001774</span> <span class="nt">BEQ</span> <span class="nt">L2</span> <span class="o">;</span><span class="nt">LOOP</span> <span class="nt">IF</span> <span class="nt">BUSY</span>
<span class="nt">001056</span> <span class="nt">010037</span> <span class="nt">177412</span> <span class="nt">MOV</span> <span class="nt">R0</span><span class="o">,@</span><span class="p">#</span><span class="nn">RKDA</span> <span class="o">;</span><span class="nt">WRITE</span> <span class="nt">SEEK</span> <span class="nt">TARGET</span> <span class="nt">TO</span> <span class="nt">RKDA</span>
<span class="nt">001062</span> <span class="nt">012737</span> <span class="nt">000011</span> <span class="nt">177404</span> <span class="nt">MOV</span> <span class="p">#</span><span class="nn">11</span><span class="o">,@</span><span class="p">#</span><span class="nn">RKCS</span> <span class="o">;</span><span class="nt">WRITE</span> <span class="nt">SEEK</span> <span class="nt">CMD</span> <span class="o">+</span> <span class="nt">GO</span> <span class="nt">TO</span> <span class="nt">RKCS</span>
<span class="nt">001070</span> <span class="nt">000207</span> <span class="nt">RTS</span> <span class="nt">PC</span> <span class="o">;</span><span class="nt">RETURN</span> <span class="nt">TO</span> <span class="nt">CALLER</span>
<span class="nt">001000</span> <span class="p">.</span><span class="nc">END</span> <span class="nt">START</span>
</pre></div>
</td></tr></table>
<p>At first this code was generating no seek activity on the drive. Decided to try swapping out the BC11-A
drive cable, and that produced some limited success -- drive seeks, but some bits of the cylinder address
are still apparently not making it across the cable.</p>
<p>The BC11-A cables are problematic. They seem flaky and fragile, and many of my spares seem bad. Any given
cable may beep out fine on the bench, and yet fail consistently in use... It looks like what's up next is a
voyage through my box of spares, swapping in cables looking for one that works reliably. Failing that, I'll
need work on some sort of modern replacement, since original BC11-A in good shape are getting hard to find.
It will be sad if at the end of this journey I can't boot the machine for mere lack of a good cable between
the drives and controller!</p>
<p><img src='/images/pdp11/rk05-cartridge-foam_thumbnail_tall.jpg' title='Foam inside the hub of an RK05 pack -- could be bad news if this is decaying like other DEC foam...' onclick='pswipe("pdp11",54);'/>
<img src='/images/pdp11/rk05-running_thumbnail_tall.jpg' title='RK05 running offline from controller; pack spinning and heads loaded' onclick='pswipe("pdp11",55);'/>
<img src='/images/pdp11/BC11-woes_thumbnail_tall.jpg' title='A box of BC11-A cables to be sorted through...' onclick='pswipe("pdp11",56);'/></p></div>
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