Re: Looking for a Micro SCI disk controller

[mjmahon@aol.com (Michael J. Mahon)]

Wayne Stewart wrote:

>Bill Garber wrote:
>
>> What would one of these controllers look like?
>> I've never seen one and also have no clue what
>> they are for. If you could send me a picture of
>> it, I would be very appreciative.
>
>I have a couple of pics here. Second pics the same, just a bit larger
>http://www3.telus.net/waynes/micro_sci.jpg
>http://www3.telus.net/waynes/micro_sci_large.jpg

Nice pictures.

Judging from the proximity of the two pots to the 74LS123,
I expect that they are used to adjust the period of the two
monostable flipflops in the '123.

I cant make out the values of the pots or the capacitors,
but a likely function would be data separation of the read
data stream from the disk drives.  One monostable pulse
could set the beginning of the interval to sense a pulse
(or its absence) and the second could set the end of the
interval.

This is a function which is carried out by the ROM-based
state machine in the normal Disk ][ Controller.  MicroSci
would have needed a license to use that invention, and so
may have opted for a more conventional approach.

The time intervals may have been adjustable only in early
controllers, as experience confirmed that fixed settings
provided adequate reliability (and lower manufacturing
costs).

These pots are, of course, unrelated to disk speed
adjustment (which requires signals present only within
the drive).

-michael

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