SMD disk specifications

Eric Smith spacewar at
Sun Dec 15 00:58:44 CST 2019

On Sat, Dec 14, 2019 at 2:47 AM jim stephens via cctalk <
cctalk at> wrote:

> CMD was the name of the division that manufactured the disks,

The division was Magnetic Peripherals Inc. (MPI). I've never heard of it
having been called CMD either before or after it was called MPI. There was
an (AFAIK) entirely unrelated company named CMD Technology Inc. that made
disk controllers.

FWIW. Not heard of CDC calling it anything other than SMD.

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.

> It was their golden goose, so though it's not really a spec, reading the
> specs of any CDC
> drive of an equivalent capability as you are looking for is probably
> what any standard would contain.

There's most definitely an interface specification that is not specific to
any given drive. For original SMD/MMD/CMD, it is CDC document 64712400,
which went through many revisions, some of which can be found on bitsavers:

The technical specifications of any given drive model give a substantial
faction of the interface specification, but not necessarily all of it.
There are optional features that may not be implemented in a particular
drive model, and a model may have minor deviations from the spec. Also the
interface specification may allow broader ranges of characteristics than
are documented for any given drive. If you're designing an SMD controller
or an SMD drive, you should definitely design to the interface
specification, though looking at individual drive model specs is also

Unfortunately the later CDC specification for the SMD-E interface (a
superset of the SMD interface) does not seem to be available anywhere. I
think the ANSI specification covers SMD-E, but I'm not 100% certain.

More information about the cctech mailing list