Massbus - was: Re: VAX 11/750

Paul Koning paulkoning at
Wed Feb 24 19:16:11 CST 2021

> On Feb 24, 2021, at 7:06 PM, Noel Chiappa via cctalk <cctalk at> wrote:
>> From: Chris Zach
>> technically the MASSBUS cable is just an extension of the Unibus
> No.
> For one thing, the MASSBUS has no lines for carrying memory addresses. So
> there is no way to even build a box that 'translates' MASSBUS to UNIBUS; the
> semantics ('the things you can say', basically) of the two busses are so very
> different.

It's faintly true if you look at the Massbus adapter, rather than the cable.  But only faintly.  There's a good reason why the big disks on many DEC machines were Massbus devices until MSCP arrived.  It's quite clear on Unibus PDP-11s, which needed Massbus both for speed and for a cleaner answer to more-than-18 bit addressing.


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