Massbus - was: Re: VAX 11/750

Jon Elson elson at
Wed Feb 24 10:06:50 CST 2021

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.


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