flash (or ide) storage for unibus 11?

Fred Cisin cisin at xenosoft.com
Sat Nov 28 20:57:29 CST 2015

On Sat, 28 Nov 2015, Jerome H. Fine wrote:
> it did not cause a problem.  The media could be LLF using either
> index hole and then read and written using the other index hole.

changing the rotational position of index will not cause a problem with 
WD style controllers - they will read along the track until they find the 
sector that they are looking for.

HOWEVER, on NEC 765 type of controllers, the controller is blind 
momentarily after the index pulse.  If you have a sector starting "too 
soon" after the index pulse, the controller won't be able to read it, and 
if a sector straddles the index pulse, you may get a read error.

Disabling the index pulse (which is needed for LLF) during read and write 
will get around that problem.
If other errors occur, such as "requested sector header not found", they 
may be misinterpreted by the drive -  it may notice lack of index, and 
misreport "time-out"

On some 5.25" drives (some Teac models, for example), the drive uses the 
index pulse as part of its determination of "being ready", so on those 
drives, any blocking of the index pulse should be done between the drive 
and the controller, not by blocking the sensor.

So, . . .
WD style: no problem with index pulse timing relative to data
NEC765 style: index pulse is necessary during LLF, but may need to be 
blocked during read/write, although a few of the newest drives may not be 
happy without index.

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