Re: HD floppy on a IIgs?

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

Bill Garber <willy46pa@garberstreet.com> wrote:

> "David Empson" <dempson@actrix.gen.nz> wrote in message 
> 1jjgq8y.97z2a21kjfoqaN%dempson@actrix.gen.nz">news:1jjgq8y.97z2a21kjfoqaN%dempson@actrix.gen.nz... 
> > mojoehand <mojoehand@gmail.com> wrote: 
> > 
> >> On Jun 1, 10:53 am, Warren Ernst <wer...@gmail.com> wrote: 
> >> > On May 31, 11:20 pm, mojoehand <mojoeh...@gmail.com> wrote: 
> >> >
> >> > > I know that you need a special interface card to support a 1.44 MB
> >> > > floppy drive on a IIe. Assuming I had a compatible drive, does the
> >> > > IIgs Smartport support HD drives?
> >> >
> >> > > In other words, can I take a HD drive from a vintage Mac and use it on
> >> > > the IIgs? Can I format, read and write HD floppys with it? If so, is
> >> > > it possible to read/write MSDOS disks?
> >> >
> >> > Just to be thorough, if you take a 1.44MB High Density 3.5" floppy
> >> > drive from a suitably old Macintosh and plug it into the GS SmartPort,
> >> > the drive will act like a normal 800KB floppy drive. It's the magic of
> >> > the "SuperDrive Controller Card" that makes it read and write 1.44MB
> >> > floppies.
> >> >
> >> > You should also note that Apple used two different types of HD floppy
> >> > drives on old Macs, and it is the older ones that are shared with the
> >> > Apple II. Basically, if the drive came from a 68K Mac, it should be
> >> > good to use. If it came from a PowerPC-era Mac, then it is not only
> >> > less likely to work properly with ProDOS disks, but you can't just
> >> > pull the mechanism and expect it to work with an Apple II.
> >> 
> >> Thanks to all for the input. I suspected that it wouldn't get me
> >> anywhere, but I wanted to make sure.
> >> 
> >> Pie in the sky question - Is it possible to write a IIgs driver that
> >> would make the Smartport work with the 1.44 MB drive?
> > 
> > No. The IWM controller chip can't transfer data fast enough (nor can it
> > handle MFM-encoded data). In addition, a 1 MHz 6502 or 65C02 isn't fast
> > enough to keep up with the data stream.
> > 
> > The SuperDrive controller achieves the feat by using a SWIM chip instead
> > of an IWM, and has its own CPU (a 2 MHz 65C02) and RAM for buffering.
> 
> However, a combination of external hardware, and a software driver could.
> I am not capable, yet, of creating such a system, but, it has crossed my
> mind many times, and I am working towards being able to do so. Of course,
> now that I have mentioned it in a public forum, someone else may get on
> it and create one before I can do it. That's just how it goes.   8^) 

That's feasible. Effectively equivalent to something like the UniDisk
3.5, which has its own CPU and a dual-purpose IWM which is set to either
communicate with the host computer (using the SmartPort protocol), or
with the drive hardware.

It wouldn't exactly be fast, since the SmartPort interface would be a
bottleneck (including 8-bit to 7-bit conversion, 250 kbps throughput and
protocol overhead).

The SuperDrive card's only hardware bottleneck is the 1 MHz Apple II I/O
bus. It can read buffer an entire track into its on-card memory, and
transfer blocks pretty quickly into the host, but write performance
isn't very good in ProDOS-8 due to rotational latency when writing 1
block at a time through the ProDOS or SmartPort firmware interface. In
principle, GS/OS should be able to get better throughput with a native
driver. Interleave is important: 1:1 (the default for preformatted 1.44
MB disks) is horribly slow, 2:1 is more reasonable (but halves
read/write performance on a PC).

-- 
David Empson
dempson@actrix.gen.nz