Sector size of CDC Sabre 850 with SMD-4 controller

[Hans van Staveren]

Some time ago I put an article in Sun-spots describing what we did to get
our Sabre 850's to run with a XY451 controller. I recommended a sector
size setting of 594 bytes and 69 sect/track including slip-sector.

Now we have changed controllers to the SMD-4 and being a software type I
never thought about having to restrap the disks, so we didn't. Now we are
getting lots of problems with some of the Sabre's and suddenly this number
that I read in the XY7053 controller manual (XY7053==SMD-4) hit me in the
back of the head.  It says that the XY7053 needs a minimum of 88 bytes of
overhead per sector, so 512+88 is, lets see, uh about 600 bytes per sector
minimum.

Having asked some local hardware gurus about how this 600-594=6 bytes
short sector could behave, they all started saying grave things, about
overwritten sector headers, head settling time, and other lowlevel stuff.
That did it.  We are going to restrap, reformat, and refill our disks.
Next step is 603 bytes/sect. A lot of work. Yuck!

If there is anyone still running with the settings I recommended, and
about to change controllers, beware! If there are external opinions about
the wiseness of our redo everything decision reply please.

You learn something new every day.

	Hans van Staveren
	Vrije Universiteit
	Amsterdam, Holland