An historical nit about FDDs

Paul Berger phb.hfx at
Thu Jul 12 13:40:48 CDT 2018

IBM created the 8" diskette as an inexpensive and reliable means of 
loading microcode and shipped the first read only drives in 1971. 
Memorex did ship a hard sectored R/W drive some time in 1972 likely 
because Alan Shugart had jumped ship from IBM to Memorex.  Some sources 
credit Shugart with inventing the diskette drive but that is not true he 
had given the job to others on his team.   The Memorex 650 seems to have 
an interface very similar to the IBM drives oddly enough, using step in 
and step out signals.  The next year IBM shipped its first R/W drive the 
single sided 33FD "Igar" a single sided soft sectored drive and larger 
capacity.  It was first used in the 3741/42 key to diskette machines but 
was used in a wide variety of IBM machines, as far as I know IBM never 
sold the bare drives.

Because IBM never sold the drives themselves and the market impact of 
the first Memorex drive may not have been really big, there was no real 
standard so when Shugart Associates released the SA800 its proved to be 
very popular and its interface became the defacto standard.


On 2018-07-12 3:02 PM, Tom Gardner via cctalk wrote:
> Hi Chuck
> I agree it is easy to convert but I am surprised that a start-up would have the guts to change the "standard," whether it was Memorex, Potter or Century.  I think before the 33FD Memorex was the market leader but I could be wrong.  I've asked some SA founders the question.  Does anyone know any Potter or Century FDD people from the early 70s?
> The early HDD interfaces I am aware of used a control cable with an 8-bit bus and a set of tag lines to define the bus - much more expensive to implement than the Step In/Step Out.
> Regards,
> Tom
> -----Original Message-----
> From: Chuck Guzis [mailto:cclist at]
> Sent: Wednesday, July 11, 2018 11:38 AM
> To: Tom Gardner via cctalk
> Subject: Re: An historical nit about FDDs
> On 07/11/2018 11:12 AM, Tom Gardner via cctalk wrote:
>> Anyone know where the Step/Direction version of the FDD interface
>> originated.
>> So far as near as I can tell the earliest FDDs (IBM 23FD Minnow and
>> Memorex
>> 650/651) used Step In/Step Out. The IBM 33FD Igar used direct control
>> of the motor.
>> The earliest Step/Direction FDD I can find is the Shugart 800 which
>> first shipped in September 1973.
> Shugart is probably it, unless there's a hard drive interface that precedes it.  Mostly a minimal bit of logical difference between the Step in/Step out  and Step/Direction.  One can be converted to the other rather easily.
> --Chuck

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