[Info-vax] RB730 Integrated Disk Controller (R80/RL02) usable with VAX-11/750?

[John Wallace]

On May 3, 5:48 pm, SeanOBanion <s... at obanion.us> wrote:
> On May 3, 9:38 am, SeanOBanion <s... at obanion.us> wrote:
>
>
>
> > On May 2, 9:59 pm, Johnny Billquist <b... at softjar.se> wrote:
>
> > > On 2011-05-02 17:05, SeanOBanion wrote:
>
> > > > On May 2, 2:52 am, vaxorcist<vaxorc... at googlemail.com>  wrote:
> > > >> Can the RB730 Integrated Disk Controller (primarily developed for R80/
> > > >> RL02 drives with the VAX-11/730 and /725) be used with the VAX-11/750?
> > > >> This might have been a "low cost" solution to connect disk drives to
> > > >> the /750.
>
> > > >> Any hints welcome!
>
> > > >> Regards,
>
> > > >> Ulli
>
> > > > No.
>
> > > > The IDC is part of the 730 CPU, stealling CPU cycles in a hidden way
> > > > to get it's work done.
>
> > > Cute. I have never examined the IDC in detail... :-)
>
> > > > The later version of the 730 did away with the IDC for a single UDA50
> > > > Unibus adapter that also reduced the amount of memory the system could
> > > > support:
> > > > system memeory of a 730 sat on the Unibus.
>
> > > What??? Are you sure? The Unibus only have 18 address lines, which means
> > > 256K addressable. And some of that is needed for I/O. I would be very
> > > surprised if memory really sat on the Unibus.
>
> > >         Johnny
>
> > > --
> > > Johnny Billquist                  || "I'm on a bus
> > >                                    ||  on a psychedelic trip
> > > email: b... at softjar.se             ||  Reading murder books
> > > pdp is alive!                     ||  tryin' to stay hip" - B. Idol- Hide quoted text -
>
> > > - Show quoted text -
>
> > Yes, I'm sure.
>
> > My university bought a new 730 while I was there, and added a TU80
> > tape drive to the original R80/RL02 (along with the butilin TU58...),
> > and the system had the IDC and a DMZ32 serial  / parallel interface (8
> > or 16 lines, and a Centronics interface) already installed.
>
> > The TZ11 tape interface  (though it must have been called something
> > else but it was really a TZ11...) had to go in a Unibus expansion box
> > in the TU80 enclosure.  Along the way, the Unibus expansion cable from
> > the CPU to the TU80 encllosure, a white flat "ribbon" cable of bare
> > copper wires between a pair of platic sheets held in place a strategic
> > locations with foam, got a broken wire.  The FE was soldering  jumpers
> > on the memory card (cards?) and talking about how it (the memory card)
> > could be generating errors on the Unibus.  After getting bus errors at
> > an address higher the memory addresses, he concluded it was the cable
> > and brought a new one in the next day.- Hide quoted text -
>
> > - Show quoted text -
>
> Also see doc's in:http://www.bitsavers.org/pdf/dec/vax/730/

My understanding, and my reading of figures 1-1 and 1-2 in the VAX730
Memory System Technical Diagram on bitsavers, is that although the CPU
and the memory are both on the UNIBUS,  the UNIBUS is not the only (or
preferred) CPU to memory path - there is an alternate non-UNIBUS path
between CPU and memory, without the UNIBUS limitations of addressing
and speed.

If the UNIBUS was the sole path, then as others have pointed out
already, there'd be a problem getting more than 18bits of system
memory on a 730-family box. And 730-family boxes could have up to 5MB,
iirc.