An historical nit about FDDs

Tom Gardner t.gardner at
Fri Jul 13 13:04:33 CDT 2018

The 23FD used on most S/370 mainframes had a two solenoid and swash plate actuator, pulse the in  solenoid to go in and the out solenoid to go out.
IBM SJ in those days was very cost conscious and preferred mechanical parts over electronic, hence this mechanism instead of the stepper motor implement by most (all?) other FDD manufacturers.

Chuck and Paul:
It's all relative - key to disk systems were taking off and the IBM 3470 blessed the market.  The 1973 Shugart Associates business plan acknowledged Memorex as the then market leader in FDDs - MRX had Mohawk Data Systems and had shipped product to 45 potential customers.  SA estimated the market to be 327k units in 1973 growing to 633k by 1977, a big number in the 1970s and big enough to attract venture capital.  The big customer turned out to be Wang which SA won.

The FDD and FD were invented by a number of folks at IBM most of whom did join Al at Memorex see:

The first product was the SA900 not the SA800; it had a step/direction interface, and my guess now it was to save an IC or two in the circuits to drive the stepper motor.


 -----Original Message-----
From: Jon Elson [mailto:elson at] 
Sent: Thursday, July 12, 2018 6:41 PM
To: Paul Berger; General at; Discussion at and Off-Topic Posts
Subject: Re: An historical nit about FDDs

On 07/12/2018 01:40 PM, Paul Berger via cctalk wrote:
> IBM created the 8" diskette as an inexpensive and reliable means of 
> loading microcode and shipped the first read only drives in 1971.
I am quite certain the original FDD on the 370/168 used a pair of solenoid coils to ratchet the head in and out.  I think the mechanism was a leadscrew and toothed wheel.  I heard a 370/168 loading a microcode overlay and it sounded like a machine gun, even in a pretty loud machine room.

I think the same scheme was used in the 370/145.


-----Original Message-----
From: Chuck Guzis [mailto:cclist at] 
Sent: Thursday, July 12, 2018 12:05 PM
To: Paul Berger via cctalk
Subject: Re: An historical nit about FDDs

On 07/12/2018 11:40 AM, Paul Berger via cctalk wrote:

> 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.

One thing that escapes modern sensibilities is how expensive the first floppy disk systems were.  If you purchased one of the early microcomputers (IMSAI, Altair), a single-drive floppy disk system would
run more than the CPU unit.   Remember, there were initially no LSI
floppy controllers--on the MDS, Intel rolled their own as a 2-board Multibus set.  Some early systems used USART chips. IMSAI used another
8080 MPU for their controller.

Data separation was a fairly new problem too, as floppy ISV and general signal stability was not as good as most hard drives.  You're essentially using flexible, disposable media.

So initially, the market was not terribly large.


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