RC25 (was Re: Modifying microcode)

[Ethan Dicks]

On Tue, Jun 5, 2018 at 2:53 PM, Robert Armstrong <bob at jfcl.com> wrote:
>   I too have heard that RC25s and PDP-11s were used in nuclear subs for some kind of sonar thingie.  I've no idea how that worked, except that maybe DEC gave all the good drives to the Navy and the rest of us got the crappy ones.

Hi, Bob,

Perhaps.

>   They worked as long as you didn't spin them down or try to change the removable pack.

Ah... that may be part of it.  I never did.  I loaded one removable
pack and never swapped it out.

> The removable part would crash at the drop of a hat

:-(

> and, of course given the clever shared spindle design, if the removable part wouldn't spin up then neither would the fixed part.

Right.

I have a different 11/725 now (long story but I screwed up and the old
one went away at a friend's company when they were purchased and shut
down)... I've had that replacement 11/725 for 10 years but it didn't
come with a removable pack so I mothballed it out at my farm.  I have
only gotten a couple of removable packs in the past year and have not
been able to spend time fiddling with it, but it's going to be a full
cleaning and overhaul (dust, fans, PSU test, TU58 roller...)

>   I have (I think) three drives, or maybe just two.  Two are internal drives for the 725 and one is in the table top enclosure.  None work.

No fun.

> If anybody has any tips for fixing them, or even just a kludge to spin up the Winchester part without needing the removable part to work too, I'm all ears.

I didn't want to bung up my only drive, but if I had 2, what about
this?  Put a foam or rubber wedge between the head arms of the
removable pack (to keep them from thwacking together) and defeat any
sort of interlock or removable cardridge sensor (I think there's a mag
sensor at one corner?)  The idea is to make the drive think there's a
pack loaded.  It might take doing something with a real pack housing
and removing the platter in case the lower mass screws up the spin
rate.

Of course, if the internal drive electronics lose their mind because
there's no signal coming from the removable pack heads, then it will
likely go into Fault and not work.  This is all just speculation on my
part but I can envision ways that it doesn't keep the drive
electronics happy.

>   AFAIK the LESI ("Low End Storage Interconnect") protocol is not documented anywhere, unlike SDI or MASSBUS which are.  If it is, I've never found it.

Nor have I.

> I have several UNIBUS KLESI boards and I've often thought the same thing, but I'm not really interested in trying to reverse engineer the protocol w/o documentation.

Same.  That's what really slows me down - not wanting to start without
a single idea of how it works.

And then I think of how much work it would be to convert one of my
COMBOARDs from a 3rd-party comms controller into a 3rd-party disk
controller (I have full schematics and specifications and source for
all the COMBOARDs so that's not starting from zero) but between
designing and writing new firmware and writing drivers for every
possible OS (RT-11 and VMS at the absolute front of the list), before
I even get started, I just find something less insane to do.

-ethan