Date   : Thu, 22 Jul 1999 17:53:47 +0100
From   : Mike Tomlinson <mike@...>
Subject: Re: DFS Installation

In article <0182399E7179D211824400A0C9EA408A03C0A5@...>,
Jon Bloomfield <Jon.Bloomfield@...> writes

>I have just installed a DFS to my BBC Model B, and am having some problems
>with it.

Have you fitted the Acorn 8271 DFS or the Acorn 1770 upgrade, or a third
party DFS?  Some third party upgrades used, um, "creative" fitting
methods.

I'll assume you have the Acorn 8271 DFS.  Some thoughts:

Ensure you have an issue 4 or issue 7 motherboard.  Issues 1, 2 and 3
have serious defects requiring correction before they will work with a
disc upgrade.  Issue number is screenprinted on the board.

>The interface seems to be working more or less, because I can see the
>catalogue entries of my disk collection, but the drive is refusing to access
>any tracks other than track 0.

Check:

Link S9 (just behind the 8271 or 1770) is open (if this is wrong, you
will get a garbage catalogue)
Link S2 I *think* should be closed (on LHS edge of board, next to Econet
circuitry.)  If closing it produces a garbage catalogue on a known good
disc, open it again.

Check:

Link S18 is north
Link S19 is east
Link S20 is north
Link S21 is east-west (twice)
Link S22 is north
Link S32 is west
Link S33 is west

Check:

you have not bent out/under any pins on IC80 (7438) as this buffers the
step signal, or on the 8271.  If you have an oscilloscope check pin 36
on the 8271 and pin 6 on ic80 (7438) for pulses when the drive should be
stepping.

you have not bent or pushed in any pins on the floppy disc connector
underneath the machine (this is common in schools)

ic87 is fitted (74LS123)

Does the drive make any unusual noises (whining, chattering, grinding)
when it should be stepping?  If so, indicates too fast a step rate (see
later)

>Does anyone know enough about disk-drives/DFS to offer some suggestions ?

If you've tried the drive on a Master and it's OK, the problem is in the
Beeb.  As you've said, the interface seems to be mostly working with the
exception of stepping.  The Step signal leaves pin 36 of the 8271 and is
buffered by ic80, whose pin 6 is connected to pin 20 of the floppy
connector.  Check R37 is present (1k)

74LS38/HC38 etc. are not suitable replacements for ics 79 and 80, which
must be vanilla 7438s.

>P.S. The keyboard links are not set (I didn't get a DIP module with the DFS
>chips) is this the problem ?

could be.  With the 8271 DFS, no links selects the slowest step rate and
all drives should work at this speed.  With the 1770/1772 DFS, no links
selects a fast step rate (3mSec IIRC) which many 5.25" drives can't cope
with (and you will get error 18).  The 1772 has different step rates to
the 1770.  If you look at the 1770 DFS Installation guide (which should
be on Mark de Weger's doc site) this came with an addendum sheet showing
the DIP switch settings for the 1770 and the 1772.  Hopefully Mark will
have also scanned the addendum - if not, I'll have to have a hunt
through my BBC notes.

Note that the DFS ROM will still report 'Acorn 1770 DFS' even if a 1772
is fitted.  The only way to check for 1772 is to look at the chip.

Solder in a DIP switch and start by closing switch 4.  This should
select the slowest step rate for the 1770, 12mSec IIRC.  The 1770 can do
12, 6, and 3 mSec - it's the 3mSec setting that most 5.25" drives have a
problem with.

You can simulate the DIP switch settings with *FX255, though I've found
the willingness of various DFS ROMs to pick up the settings varies.  The
Acorn DFSes seem to pick up the settings with *DISK, whereas others need
a warm break.  Might be safer to fit a DIP switch.

If any of the above is wrong, please feel free to correct me.  Most of
it has come from my dodgy memory.

-- 
Mike Tomlinson