Massbus - was: Re: VAX 11/750

Chris Zach cz at
Wed Feb 24 08:06:07 CST 2021

> I happen to have a massbus adapter and the cabling with terminator for my 11/750, but I never came across any massbus peripherals. Considering the price back then and the size for these, they seem nowadays quite rare, which is a real pity.

Sort of. MASSBUS was a very overpriced and to be honest weird system 
that DEC developed in an attempt to be like IBM and charge very high 
markups for generic peripherals. While it did support a variety of 
interface formats (I don't think the RS03 and 04 were SMD, and I know 
the RP07 was *not* SMD) the industry quickly centered on SMD and that 
was that.

Yes, the TM02 and TM03 formatters allowed MASSBUS to connect to Pertec 
drives, but I don't think you could run a tape drive and a disk drive on 
the same MASSBUS channel anyway. Wonder why, technically the MASSBUS 
cable is just an extension of the Unibus, so it shouldn't matter much 
what combination of cables you used. Maybe they just wrote different 
drivers or something.

> Anybody knows if there was conversion kit to equip CDC 9762 and 9766 type SMD drives into MASSBUS drives much like the TU81 could be turned into a TA81 (SDI)?

I have heard that the difference between what comes out of a MBA (which 
is huge, several quad height boards in an interface box) and true SMD is 
the inversion of some differential signals. Whatever it is the changes 
to a 9762 were very minimal, probably just one of the cards in the card 
cage. It's quite possible there is no difference, I'll leave that to 
someone else.

My guess is they removed the TM03 formatter from the TU81 and just put 
in a SDI formatter which was nothing more than yet another closed source 
version of a Pertec formatter. Maybe DEC lost the contractor who built 
MASSBUS cables as they were better used to tie ships to a dock.

(Yes, I did use a MASSBUS cable to tow a car once. Worked fine)

I still have an RM80 drive in my house here, I fire it up every 5 years 
or so. Mostly for when I get my Decsystem/20 back together.

It has the RM80 personality module which is the difference between the 
R80 and RM80. To be honest I'd bet that one could put an RA81 
personality module into it and make an RA80 drive....

Far more interesting would be to see if one could put the RM80 
personality module into an RA81 drive and turn that into a 456mb MASSBUS 
drive. THAT would be interesting....


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