Re: Using Mac 20 meg HD on A2 drive port???

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

Edhel Iaur, Esq. <edhel@LOSETHECAPITALWORDSbigfoot.com> wrote:

> On Fri, 21 Nov 1997 02:28:47 +1300, dempson@actrix.gen.nz (David
> Empson) wrote:
> 
> >Not by much.  The bit time of the SmartPort protocol is 4 microseconds,
> >which is the same as a 5.25" drive.  The only benefit gained is that you
> >have a full 7-bit data path instead of a 6-bit encoded format, so you
> >save some CPU decoding overhead, and get an extra 17% throughput.
> 
> Would those //c hard drives sold by Sequential be the same way?

Yes.  A genuine SCSI interface is capable of much faster transfer rates
than the SmartPort.  For starters, SCSI is parallel.

With a DMA-capable SCSI card in a IIgs, transfer rates up to 1 megabyte
per second are possible, though typical rates are more like 150 to 300
kilobytes per second.  In a IIe, the maximum transfer rate is limited
because the card cannot steal every clock cycle to perform DMA, and the
operating system only asks for 512 bytes at a time, so I'd expect
somewhat lower rates.

SmartPort has a maximum transfer rate of 7/8 of 250 kilobits per second,
i.e. about 27 kilobytes per second.  This is for a UniDisk 3.5 or other
intelligent SmartPort device.  A Disk ][ has a maximum transfer rate of
about 23 kilobytes per second.  An Apple 3.5 drive is about twice that.
These are the ideal rates - practical transfer rates will be limited by
rotational latency, seek time, and some overhead for decoding the
6-and-2 encoded data.

Even the original non-DMA Apple SCSI card can achieve transfer rates of
80 kilobytes per second in a IIgs.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand