Massbus - was: Re: VAX 11/750

Paul Koning paulkoning at
Wed Feb 24 14:43:01 CST 2021

> On Feb 24, 2021, at 9:06 AM, Chris Zach via cctalk <cctalk at> wrote:
>> 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.

Overpriced?  Compared to what?  SMD didn't appear until later; I know Massbus existed in 1974, perhaps earlier.  What else would do the job back then?

> 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.

Electrically it works.  I believe at least one system/OS supports this; TOPS-10 perhaps?  But most operating systems do not, and yes, that would be a driver issue.

Similarly, not all OS support RS03/04 and RM/RP on the same Massbus (RSTS doesn't for example).

Meanwhile, speaking of the RP07: DEC never supported that on any PDP-11, but it actually works just fine on the high performance Massbus of the 11/70.  In the RSTS group we had one on the big 11/70 system ("ARK" because at one point it had "two of everything").  So RSTS/E will happily drive such a device, if you can manage to power it up.

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

:-)  I think IBM's "bus and tag" 360 channel cables are even thicker than a Massbus cable.

> 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....

Yes, but you'd need some driver work, since Massbus disk drivers have to deal with the actual drive topology.  So you'd have to define a Massbus ID value for "RM81" and pick the appropriate sector/head/cylinder count.  Depending on the OS this may be fairly easy or not so easy.  I know how to do it in RSTS (at the source level -- doing it by patches might also be doable).


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