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- Newsgroups: comp.sys.dec.micro
- Path: sparky!uunet!usc!rpi!news.columbia.edu!watsun.cc.columbia.edu!lasner
- From: lasner@watsun.cc.columbia.edu (Charles Lasner)
- Subject: Re: Pro/Venix Installation
- Message-ID: <1992Aug31.041611.11985@news.columbia.edu>
- Sender: usenet@news.columbia.edu (The Network News)
- Nntp-Posting-Host: watsun.cc.columbia.edu
- Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
- Organization: Columbia University
- References: <Bts2CG.MM0@watdragon.uwaterloo.ca> <1992Aug30.055216.23265@news.columbia.edu> <BttDwA.579@watdragon.uwaterloo.ca>
- Date: Mon, 31 Aug 1992 04:16:11 GMT
- Lines: 62
-
- In article <BttDwA.579@watdragon.uwaterloo.ca> kcwellsc@watdragon.uwaterloo.ca (Ken Wellsch) writes:
- >>I have heard some of this before, all too fuzzy though.
- >>
- >>I have seen actual DEC RD51 drives that come with metallic cutable jumpers on
- >>the DS lines. DS0, DS1, DS2 are intact and DS3 is cut. This means that this
- >>particular drive will select on 0 or 1 or 2 but not 3. I have been told that
- >>the reason for cutting the DS3 line is that in (presumably one of the PRO
- >>models, because it ain't a DECmate or RB), the controller uses the incompatible
- >>notion of *reading* back DS3 instead of potentially asserting it. Thus, a
- >>5 Meg has the DS3 set and a 10 Meg has it clear, so the machine "knows" what
- >>size disk it is. (This is back when there only were RD50 and RD51 supported.)
- >>
- >>Is there anything to this?
- >
- >I don't know. I can only mention what my docs have. The Pro pocket service
- >guide says to cut the "fourth from the back of the drive" jumper. They've
- >used a seven jumper dip leaving the 8th (toward the front of the drive) empty.
- >I don't have the Seagate info to know what the jumpers do. My old uVAX-1
- >docs say to cut the back two and the fourth (as for the Pro). I've forgotten
- >what the setting for the RD50 is, possibly all jumpered.
-
- I got out the RD51 (ST-412) in question. There are 8 possible socket-based
- jumpers, but DEC's plug can't do the last one. The first 4 are DS0,1,2,3, and
- then it gets unclear. I believe one of them is to make the disk do an
- automatic recalibrate when turned on, which is unnecessary since the controller
- issues a recalibrate command, it just makes it take longer to boot, and on
- some systems might make a time-out error happen. (Wasn't there a Rainbow ROM
- patch on account of this kind if problem?)
-
- So, this all seems consistent: if you ground DS3, then something in DEC's
- controller reads it, even though this is wildly outside of the normal spec
- for the ST-506-compatible MFM drives. If it's grounded, it's a 5 meg, and
- if it's not, it's a 10 meg, unless some other software "knows better" which
- may merely be the master boot code that was read in by the ROM in its belief
- that the disk was either 5 or 10 megs. A smart boot program could tell the
- ROM what it needed to know to correct the situation, etc. In most systems,
- it should be self-evident because the system is implicitly self-configured.
- Perhaps the system attempted is too feeble to make the correct guess so it
- falls back on the ROM's misunderstanding of the size, etc.
-
- There is a plausible alternate explanation for all of this:
-
- All disks *other than* the RD50 support buffered stepping of tracks, but the
- RD50 requires track step commands at the physical rate of the disk or slower.
- Faster disks can receive the steps serially, but at a high rate such as every
- few microseconds. Some even take advantage of this to realize a performance
- improvement by using a so-called "rampup/rampdown" algorithm. The idea is
- that you can move the head during a seek too fast to stop on the desired
- track, but not too fast to realize where it is. If the controller has the
- opportunity to realize that the disk is seriously in the wrong are, the
- controller (the one built into the newer disks, not the host machine's
- controller!) can affect a high-speed stroke to more quickly get close to
- the desired track, then slow down at the end to finish the seek to the
- desired track. Thus, a "smart" disk can have much lower worst-case seek
- times, even though a next-track seek is no faster all other things equal.
-
- Perhaps the PRO controller knows how to issue these high-speed seek command
- steps, and needs to know when it's an RD50 that it better not attempt to do
- it on.
-
- cjl
-
-