floppies and the SCB6120

[Ethan Dicks]

On 6/12/06, David Griffith <dgriffi at cs.csubak.edu> wrote:
> For comepleteness and just for fun, shouldn't the SBC6120 be able to
> support floppy disks?  How hard would it be to come up with an interface
> board?

Given that a _real_ PDP-8 has either an RX8E w/RX01 or RX02, a
third-party OMNIBUS board and a propietary interface to a smart drive
(a la DSD), or in the case of later DECmates, a local Western Digital
FDC chip and a lot of firmware to emulate the functionality (similar
IOT scheme) of the earlier techniques, I would say that it depends on
which floppies you are trying to support.

If you want to support an RX50 on a WD chip, I would think it would be
as "easy" as building a daughter card for the SBC-6120 stackable
connector that has a WD chip and whatever support components are
required, then write a lot of ROM code.

If you wanted to plug a _real_ RX01 or RX02 into it, that might be
easier - just an IOT decoder on that daughter card, plus the shift
registers, etc., to serialize/deserialize the bit stream on the RX
cable (i.e., 80% of an RX8E).  Given how large the "RX8L" that vrs
prototyped (and that I have one of) is, I think you'd be hard pressed
to make an RX-6120(?) the same size as the SBC-6120 without
programmable parts and/or surface mount.

Since the SBC-6120 comes with IDE, if you wanted to write ATAPI
support into the ROMs, I suppose you could add support for LS-120
floppy drives (IBM-type HD floppies plus 120MB "uberfloppies").  As
currently implemented, though, I'm reasonably certain the SBC-6120
only talks to "true" IDE devices (including PCMCIA flash and
CompactFlast).

-ethan