[Simh] RD10 is probably not the Burroughs IA2

[Timothe Litt]

On 28-Feb-16 14:40, Paul Koning wrote:
>> On Feb 28, 2016, at 11:29 AM, Timothe Litt <litt at ieee.org> wrote:
>>
>> On 27-Feb-16 14:19, Bob Supnik wrote:
>>> Thanks, Tim. Burroughs made a lot of fixed head disks at the time. I
>>> can't identify the model, but the IA2 (see page 7-4 of the B6700
>>> Hardware Handbook, on bitsavers) seems like a possibility. It has 7552
>>> sectors per surface vs 8000, but Burroughs sectors were longer than
>>> DEC sectors (180 x 8b = 1440b vs 32 x 36b = 1152b), so perhaps DEC
>>> format had more sectors per track.
>>>
>> Maybe.  But I don't think it's the IA2.  Probably the previous
>> generation Burroughs disk.
>>
>> The RC10 manual (DEC-10-I5AA-D) states that the disk makes one
>> revolution in approximately 34ms. 
>> (17 ms access time).  That corresponds to 1765 RPM, an unlikely number
>> since motors tend
>> to be a multiple of the line frequency (50/60 Hz).  1800 RPM is 33.33
>> ms/rev.    That's "approximately
>> 34 ms", and my guess at the correct speed.
> A hair under 1800 rpm is quite plausible.  Synchronous motors turn at line frequency or an integer fraction of it, because that't the rate at which the flux pattern turns around the motor poles.
>
> But induction motors, which are the most common power machinery motors until you get to very large sizes, turn a hair slower, and slow down slightly more under load.  This is called "slip"; it comes from the fact that the induced field in the rotor is not fixed to the rotor.  For example, a randomly picked 3 phase motor (Leeson 1.5 hp) has a spec of 1725 rpm.
>
>
All true.  I should have qualified my comment to magnetic disk drives of
this era, which do tend to be related line frequency.  There isn't much
load once the disk is up to speed - just friction losses.  The disks of
this era are quite massive; inertia takes over. 

I don't know about the RD10.  The RP drives, which came a bit later,
nominally require 3 phase power, but use 1 phase, 1HP motors.

Here are some semi-random examples:

IBM 350 - 1200 RPM
IBM 1405 - 1200 RPM
IBM 1301 - 1800 RPM
IBM 1311 - 1500 RPM
BUR IA2 - 1500 RPM
BUR IC3 - 1200 RPM
BUR IC4 - 750 RPM
BUR IIB2 - 860 RPM
BUR IIB4 - 500 RPM
BUR IIB6 - 1500 RPM
RP01 - 2400 RPM
RP02 - 2400 RPM
RP03 - 2400 RPM
RP06 - 3600 RPM
RP07 - 3633 RPM
RS04 - 3600 RPM
RM02 - 2400 RPM
RM03 - 3600 RPM
RM80 - 3600 RPM

Of these 19, 15 are even multiples of 50 and 60.

I have one reference, which is of uncertain quality, that claims the
RD10 is 1800 RPM.  I still think this is the most likely, and that the
"34 ms" is simply imprecision in the manual.

In any case, the more compelling argument is the second one: the
sectors/track density numbers don't make sense.



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