Re: Apple //c & Disk II

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <3ml992$a3v@vixen.cso.uiuc.edu>,
Jim Wong <jd-wong@uxa.cso.uiuc.edu> wrote:
>
> [damaged IIc after possibly plugging Disk ][ in backwards]
> 
> First, what is likely to have been damaged in the first attempt?  The
> internal drive?  The floppy controller?  The Disk II?  Everything?

Quite possibly everything.  Here is a list of what would have been
connected:

Disk ][ write protect output, write data input, read data output and
drive enable input tied together and connected to IIc ground signal.

Disk ][ +5V inputs connected to IIc -12V and write request signals
(almost certainly destroying the write request output of the IIc disk
controller).

Disk ][ write request and -12V inputs connected to IIc +5V outputs.

Disk ][ phase select inputs all connected to IIc +12V.

Disk ][ +12V inputs connected to IIc phase select outputs (all shorted
together).

Disk ][ ground inputs connected to IIc write protect input, write data
output, read data input and drive enable output (all shorted together).

None of this will be very healthy.  All IIc signals will be connected
in parallel to the internal drive (except the drive select line).

> Second, how can you tell which way the 20-pin connector of the adapter
> cable should be oriented relative to the Disk II?  The original Disk II
> connector cable was threaded through a pair of what appear to be magnets
> inside the drive housing.  Is it necessary to do the same with the
> adapter cable?

That is probably a ferrite core, to reduce noise radiation from the
cable.  It is not crucial.

The safest method is to identify pin 1 on the Disk ][ cable (red
stripe on a grey cable) and ensure that it is connected to pin 1 of
the adaptor.  If the adaptor was not made with an identifiable pin 1,
or it has been wired up with the cable backwards, then find out where
pin 1 actually is and clearly label it.  (The pins at the DB-19 end
should be numbered, and pin 1 at this end is connected to pin 1 at the
IDC-20 end.)

If the cable has been removed from the Disk ][ mechanism, you need to
identify which way round it is supposed to go by looking at the
circuit board.  I don't have an open drive handy, but I think the pins
are numbered.

> Finally, what is the correct mapping between the pins of the connector
> inside the external drive and the pins of the DB-19 connector at the
> end of the cable.

The main problem is the numbering systems used at each end are
different.  The IDC-20 pins are numbered to match the wire numbers on
the cable - pins are numbered across the connector (1 and 2 in the top
row, 3 and 4 in the next row, etc.)  The DB-19 pins are numbered along
the rows, and the numbers are labelled on the connector.

In a correctly wired adaptor, most of the wires go straight through
(IDC-20 pins 1 through 16 go to DB-19 pins 1 to 8 and 11 to 18).  The
remaining pins are:

IDC-20   DB-19
17       not connected
18       19
19       not connected
20       10

DB-19 pin 9 is an external interrupt input on the IIc.  On the IIgs or
IIe, it is the second drive select signal.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand