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 

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.



-----Original Message-----
From: Chuck Guzis []
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

So far as near as I can tell the earliest FDDs (IBM 23FD Minnow and
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.


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