5.25"/3.5" FDD pinouts (from CPM-L digest)

Date:         Mon, 7 Apr 1997 13:55:17 -0400
Reply-To:     Jeff Jonas 
Sender:       CPM-L Mailing List 
From:         Jeff Jonas 
Organization: Jeff's House of Electronic Parts
Subject:      Re: Osbornes and 3.5" Drives?

In article <199704041023.a23084@k2.maus.de> Axel_Berger@k2.maus.de
        (Axel Berger) writes:
>You can. 3.5" and 5.25" are fully hardware compatible and your Osborne
>will never notice the difference - unless the 5.25 are HD drives. As
>3.5" drives are able to step faster and draw less current, this
>direction of swapping is totally uncritical. The other way round
>sometimes proves more tricky.

Yes I agree.
To elaborate:
both 3.5" and 5.25" drives have the same 34 pin interface.
3.5" disks spin at 300 RPM thus the 250k/500k data rates.
5.25" disks spin at 300 RPM for all but the 1.2 Meg capacity
(which is 360 RPM, thus the ratios
15 sectors per track / 18 sectors per track
= 300 RPM / 360 RPM
= 1.2 meg / 1.44 Meg)
[well, SOME drives had jumpers to override that too]
(all 8" floppy disks spin at 360 RPM too)

Most old systems didn't use pin 2, 34.
That's GOOD NEWS since modern 3.5" floppy drives place signals there
that the old controllers can't handle.
The ready/disk changed lines changed from the
"XT" generation drives to the "AT" generation drives.
Older floppy drives had jumpers for
- drive select 0-3
- where to place the status signals
The "AT" floppy drives assume the "AT" signals and usually allow
only setting the middle 2 drive selects, thus the cab'e twist nonsense.

for completeness, here are the pinouts, thanks to other contributers!

Mini/Micro Floppy Interface

Pin#    Description             Alternate Functions
----    -----------             -------------------
1       GND                     Eject, Disk Change Reset
3-33    Odd pins are GND
2       High Density
4       Head Load               In Use, Eject
6       Drive Select 3
8       Index Pulse +
10      Drive Select 0          Motor On A      \  IBM twisted
12      Drive Select 1          Drive Select B   \ cable - both
14      Drive Select 2          Drive Select A   / drives are
16      Motor On                Motor On B      /  strapped DS1
18      Direction
20      Step
22      Write Data
24      Write Enable
26      Track Zero +
28      Write Protect +
30      Read Data +
32      Select Head
34      Disk Changed +          Ready +

+ signal from drive to controller

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Last modified: 26th. August, 2001