Massbus - was: Re: VAX 11/750

Chris Zach cz at
Wed Feb 24 11:31:32 CST 2021

> 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.

That is one way to avoid a very long restore :-)

> 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, they had issues. Another one apparently is the heads will glue 
themselves to the drive after a few years, I was researching ways to 
unstick the heads without damaging them (hint: Never use Isopropyl to 
clean a RM03 disk head. It will unglue it and hilarity will result) but 
that whole thing was mooted when it vanished.

> MASSBUS was not anything like a UNIBUS, it was a lot closer to an IBM 
> channel bus and tag cable.

Ok, however the address and data bus feels a lot like a Unibus. And all 
of the registers are in the drive controller/adapters, it almost feels 
like you are tickling the adapter directly off the Unibus. Given Dec's 
joy with Unibus repeaters, bridges and windows it does feel like a 
window into the Unibus itself.


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