Subject: Re: Why a pdp-11

From: [email-masked]
Date: Thu, 22 Dec 94 13:38:20 EST

Field Circus told me the same story about never turning the drive off, which
was a bummer because it draws 4A at 110V even with the controller off.  Besides
keeping the heads flying, they mentioned something about the motor bearings
getting worn and wobbly after a few years, and how the gyroscopic effect is
Our Friend.  One thing that stressed me out is that air filters were no longer
available even then (at least not where I could find them), and they were
supposed to be replaced every 6 months or so.
 
SS fixed head disk -- I've never heard of a DS one.  The RS08 is also SS and
the drive can be stripped to expose it, according to the FS guys removing or
replacing the platter was utter hell, they had to set up a tent (like a mini
clean room) and the only way to get the platter on straight was by wrestling
for hours with four screws in the hub, loosen one, tighten the opposite one,
measure again, repeat until angry.
 
Oxide -- actually, I vaguely remember something about the coating being
something really ridiculous, like platinum.  Didn't know it had magnetic
properties...
 
DF/RF incompatibility -- well they aren't *that* different.  They differ in
all the details of course, but basic things are similar, like they both use
the 6601/6603 (DMAR/DMAW) instructions to load the low 12 bits of the disk
memory address from the AC and start a read or write (TSS/8 borrowed these
opcodes for its RFILE/WFILE IOTs), and I believe they used the same data
break locations.  The RS08 doesn't have a photocell like the DS32 does, so
they simulated it from the timing track.  Geometry is none of the programmer's
business so you don't notice any differences there (actually the TSS/8
comments consistently refer to 4KW tracks, maybe they were on the DS
(doubt it) but tracks are definitely 2KW on a RS08, they didn't even know
and their code still worked).
 
My favorite quote from the RF/RS manual:  (I don't remember the numbers but
you get the idea)
 
Power consumption: 600W
Heat dissapation: 350W
 
So like what, it's spinning faster and Faster and FASTER!!!  Or else those
front panel lights are a lot brighter than they look...  Or maybe the bus
drivers can handle miles of cable at 0ns slew.  :-)
 
John Wilson

---

From: Charles Lasner <[email-masked]>
Date: Mon, 26 Dec 1994 15:58:06 -0500 (EST)

> supposed to be replaced every 6 months or so.

Yes, and without powering the drives down.  BTW, I know someone who did 
target practice with the used filters.

> measure again, repeat until angry.

No!  All of them are single-sided.  The DS is just the arbitrary notation 
- DS32 - as opposed to DF32.  It's DEC's way of specifying an expander 
disk.  The RF gets RS expanders, etc.

> something really ridiculous, like platinum.  Didn't know it had magnetic

As far as I know it's a magnetic coating.  But in any case, it really 
crashes after a few stops!

> and their code still worked).

Yes, the RF is the "big brother" of the DF.  They could have made them 
proper subsets of each other, but chose to instead put an elaborated 
interface into the RF, thus the incompatibility past the power-on state 
of the RF registers mapping the low-order transfer bits in the same way 
as the DF did, etc.

BTW, the photocell and its associated interrupt cannot be disabled in the 
DF32.  Some users modified the drive to do it RF style, but in the 
original version, you couldn't change it, etc.

Eventually, the DF32 was done over in a posibus version - DF32D - 
complete with DS32D expanders, etc.  This one can delete the photocell 
interrupt, etc.

> drivers can handle miles of cable at 0ns slew.  :-)

Was the 600 W the turn-on power, or the all-the-time power?  Also, are 
they considering the interface power or just the platter?

> John Wilson

cjl