12" Floppy Disks

Johnny Billquist bqt at update.uu.se
Tue Sep 15 13:30:35 CDT 2015

On 2015-09-15 20:25, Paul Koning wrote:
>> On Sep 15, 2015, at 2:19 PM, Johnny Billquist <bqt at Update.UU.SE> wrote:
>> On 2015-09-15 19:03, Paul Koning wrote:
>>>> On Sep 15, 2015, at 12:28 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:
>>>> ...
>>>> AFAIK the RK04 never existed. The RK02 and RK03 were re-badged Diablo Model 30
>>>> drives, the 02 being low density and the 03 high density. The RK05, very well known
>>>> was a DEC drive of the same capacity, physical track format, etc as the RK03 (that is
>>>> 'high density, about 2.5 MBytes on a disk).
>>> There was also an RK08, as I recall -- same platter as the RK11/RK05 but 16 sectors per track instead of 12.
>>> If I remember correctly, that same 16-sector platter was used in the IBM 360 model 44 which had an RK05 lookalike built into the CPU cabinet (on the side).  I never used it; not sure if OS/360 supported it, that might have been PS/44 only.  If yes, one wonders if it was fixed size sectors, a rather un-OS-like thing to do...
>> Actually, the disk pack don't change the designation here.
>> The RK05 was used both on PDP-8 and PDP-11 systems. On a PDP-8 you use a disk with 16 sectors, while on the PDP-11 you use a disk with 12 sectors. And, of course, on the PDP-8 you have 256 12-bit words per sector, while on the PDP-11 you have 256 16-bit words.
>> So it ends up being the same number of data bits per track.
>> So the disk packs themselves had different designations. But the drive is identical.
>> As far as I know, there never was a RK08. The RK06 and RK07 were the last two with that designation.
> Yes, my mistake.  I was thinking of the RK05 class drive as used on a PDP-11, which apparently is "an RK05 drive and RK8 controller".

Yeah. Except I bet you meant "RK05 with an RK11 controller" above. Or 
else "as used on a PDP-8". :-)

I like RK05. It's very reliable. And plenty for a PDP-8... Heck, OS/8 
have to treat it as two disks, since it is so huge. :-)


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