Massbus - was: Re: VAX 11/750

Nigel Johnson nw.johnson at
Wed Feb 24 10:12:43 CST 2021

On 2021-02-24 11:06 a.m., Jon Elson via cctalk wrote:
> On 02/24/2021 08:06 AM, Chris Zach via cctalk wrote:
>> 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.
> OOOOhhh, the RP07!  I have stories about that drive!
> It was a Burroughs mainframe drive they reprogrammed for DEC.  It had
> a number of issues.
> The worst one was that due to some microcode bugs, it was possible to
> trash an occasional track descriptor record.  When a bad TDR was read,
> the drive totally locked up, you had to cycle power to reset it.  VMS
> recovered from it quite gracefully, however.  They finally fixed it,
> it required replacing about 10 boards.  Then, there was a procedure to
> rewrite all the TDRs.  The DEC FE assured me it would not touch user
> data.  Well, after running the procedure, it came up as a blank disk! 
> YIKES!  A quick check at the binary level showed data was still on the
> drive, but the RMS-11 home block had been overwritten.  Well, I had a
> program that could examine the home blocks, and I knew where to find
> the backup home blocks.  So, I hacked that program to copy the backup
> block to the main one, and then change the checksums to agree.
> The other issue is the voice coil could suck dirty air in during long
> seeks, so they had ti replace the HDA and put a filter over the air
> exhaust from the voice coil.
>> 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.
> MASSBUS was not anything like a UNIBUS, it was a lot closer to an IBM
> channel bus and tag cable.
>>> 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)?
> Well, just the guts inside an RM05, that was a 9762 drive relabeled by
> DEC.
Actually the RM05 was a 9766 - the 9762 was an RM03 when DEC re-packaged it

> Jon

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