Massbus - was: Re: VAX 11/750

Paul Koning paulkoning at
Thu Feb 25 11:14:51 CST 2021

> On Feb 25, 2021, at 2:40 AM, Eric Smith via cctalk <cctalk at> wrote:
> On Wed, Feb 24, 2021 at 7:06 AM Chris Zach via cctalk <cctalk at>
> wrote:
>> 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.
> There's nothing in the hardware that prevents it from being done, but DEC
> OSes don't support it, and it's generally not a good idea. Tape activity
> would block disk activity, because it doesn't have anything like modern
> "tagged command queuing".

I know most DEC OS don't support this but I'm pretty sure that at least one of them does.  Not sure which.

A given Massbus can only do one transfer at a time, so yes, a tape transfer in progress would mean you can't start a disk transfer (though you can start the seek).  For that matter, if one disk is transfering another can't start either.  So if you want max throughput, you want to spread the disks over several massbus adapters, and keep the tape separate.

But if you need lots of disk space, you may have to put tape and disk on the same Massbus since any given system has a limit on Massbus adapters (4 for the PDP-11/70?) and unit numbers are 3 bits.  For PDP-11 systems, not likely to be an issue.  For mainframe (PDP-10), it might be.  I know mainframe systems at times had dozens of disk drives.  One I used (CDC not DEC) had 22 RP04 class drives.


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