Massbus - was: Re: VAX 11/750

Chris Zach cz at
Wed Feb 24 11:26:27 CST 2021

Yep. I actually had one. The lid flew off the top of it while we were 
driving it back from Eastern shore to Baltimore on Rt. 50 but the rest 
of it was intact. Very different MBA controller from an RM02/03.

Unfortunately when Doug closed up the storage place someone swiped MC 
from there before I could pick it up, and at the same time the RP07 
vanished. I'm guessing that the 07 was junked as MC has resurfaced in a 

> 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.
> Jon

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