Massbus - was: Re: VAX 11/750

Rob Doyle radioengr at
Thu Feb 25 17:43:53 CST 2021

On 2/25/2021 1:37 PM, Josh Dersch via cctalk wrote:
> On Thu, Feb 25, 2021 at 12:27 PM Al Kossow via cctalk <cctalk at>
> wrote:
>> On 2/25/21 10:43 AM, Chris Zach via cctalk wrote:
>>> Oh this is fun stuff. Is there a specification write-up anywhere on the
>> MASSBUS overall?
>> there is some documentation on bitsavers
>> the register descriptions were intentionally obfuscated to prevent cloning
>> that was figured out to make SIMH work
> Interesting... Is this obfuscation documented anywhere (beyond looking at
> SIMH code?)  I was working on an RH11/RPxx emulation for the Unibone that
> was partially working (and that I should really get back to...)
> - Josh

I did a lot of reverse engineering of the RP06 and RM05 schematics.

The implementation in my KS10 FPGA passes all the DSRPA and DSRMB 
diagnostics that matter.  I didn't simulate all of the sector header 
hardware or ECC hardware since the disk already does that.

The register descriptions are at:

Rob Doyle.

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