SMD disk specifications

jim stephens jwsmail at
Sun Dec 15 04:05:33 CST 2019

On 12/14/2019 10:58 PM, Eric Smith wrote:
> According to CDC and MPI documents on various drives, and the 
> SMD/MMD/CMD specification document:
>     SMD = Storage Module Drive
>     MMD = Mini-Module Drive
>     CMD = Cartridge Module Drive
> These were separate groups of products, though they obviously had 
> similarities, and the interface was essentially identical.
Those definitions are correct, but there was no no difference in the 
data interface that we ever worried about with our product.

I don't know that I recall SMD being a product as much as a reference to 
the interface they had though.

I've got an MMD160 I hope to restore.  When we were selling systems, we 
had both 80mb CMD and 80mb MMD's and they had identical specs as far as 
heads and tracks.  Main difference was the CMD top disk could crash as 
it wasn't totally sealed.  And our application had no provision to use a 
removable platter.

But we ran with the CMD without any problem just to be sure.

There was an EMD line that CDC came out with which had two drives in the 
space occupied by an MMD.  If I recall I think the transfer rate was 
about twice the rate of the MMD, with larger numbers of bits / track.

For our design, we used a CRC chip to generate and check the recording.  
When the EMD came out, we couldn't get our design modified to use the 
preferred ECC chip, and the logic couldn't keep up with the data rate.  
So the controller we had stuck with the original interface.


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