VAX Q-bus identical to PDP-11 Q-bus?

allison allisonportable at
Thu Dec 7 13:22:16 CST 2017

On 12/07/2017 12:44 PM, Paul Koning via cctalk wrote:
>> On Dec 7, 2017, at 11:50 AM, Jon Elson via cctalk <cctalk at> wrote:
>> ...
>> MSCP is a software protocol.  Any device that has a driver available for the PDP-11 operating system you want to use can use that device.  
> True with small variations.  A sufficiently large disk might not be supported on some OS because the on-disk structure is limited in what device size it can handle.  (This applies to RSTS for example.)  Some devices use obscure MSCP mechanisms that might not be in all drivers -- for example, the RA80 uses host-based bad block replacement, which is quite a complicated process; I know RSTS supports that but it might be omitted in some other operating systems.
MSCP is supported by most for the common PDP-11 OSs.  Only the older
UNIX flavors lack a driver.
However it has the component of local intelligence in the controller and
if the controller supports
MSCP such as RQDX(1/2/3) it works with RDxx MFM disks, floppies both
5.25(RX50 and RX33)
and 3.5 (RX2x).  There are also the CMD and other controllers that are
MSCP but do SCSI disks,
Tapes and CDrom.   So it a software protocol for communicating with
controllers that understand that.
The interface to the devices were on the "other" side of that.

As such PDP11 and VAX support is there unless the OS in non-DEC in
origin and even then if the
time frame made it a marketing requirement the third party OS vendor had it.

An example is a RD54 on a Qbus PDP-11 running RT11.  You need the LD
driver to partition the
disk as RT supports only 32mb or smaller devices and the RD54 is 150.  
It was supported but
not ever sold that way.

Things like block replacement are options of the OS and the device IO is
then just a
interface.  The protocol for to talk to the device is a lower level
layer in most cases.

> Also, in DEC terminology, "supported" doesn't mean "it works in the software" but rather "we stand behind it".  That means tested, sold, handled by product support and field service, etc.  For example, the RP07 works in RSTS on an 11/70, but it is not "supported".  I'd expect the same is true for any number of MSCP or TMSCP devices that were intended to be sold only on VAXen -- they may very well work, but if you had plugged one in on a machine where they aren't supported, DEC would give you no help with any problems.

Usually true.  If you were an important enough customer they did.  More
often that that it was not a
device level thing it was a system level "supported configuration" and
help might be had, depending
on the field office.

> 	paul

More information about the cctalk mailing list