From: (Roger Ivie)
Newsgroups: comp.sys.dec
Subject: Re: VAXstation 3520 info?
Date: 14 Jan 96 18:13:29 MDT
Organization: Utah State University
Message-ID: <>

In article>, (Eric Smith) writes:
> I just bought a VAXstation 3520.  I'm not sure if I got a good deal or
> not, but I didn't pay too much.
> Can anyone tell me the general specs?  Which generation of VAX CPU
> does it use?  Are there newer plug-compatible VAX CPU cards I could
> put in to upgrade it?

The VAXstation 3520 is a dual-CPU CVAX-based workstation. The 3540 had
four CPUs. The system uses a proprietary bus called Mbus, which on paper
can run 26MB/s, but in reality doesn't ever come close. The Mbus is a
write-back cache bus; each agent in the system has a cache and can
intercept memory transactions for which it owns the data, supplying the
data instead of the memory. The difficulty with this is that it takes
time to knock the local processor of the internal pin-bus so you can fetch
the data from its cache, so accessing data that is in a cache is lots slower
than accessing data in main memory. The cache also doesn't have an invalid
state, so once something gets in a cache it can be difficult to get it
back out.

The system has a three-board graphics engine with its own dedicated
MicroVAX II. 

You can get a QBus adapter for it. There are two variants: the FTAM, which
supports only a TK70 controller, and the FQAM, which supports a wider range
of Qbus peripherals. The FTAM is built around the CQBIC chip which, 
combining its scatter/gather mechanism with the real-world timing of the
Mbus, can require up to 20 microseconds to complete a Qbus transaction; the
TK70 controller is supported because it has a particularly long timeout. The
FQAM does really seriously evil things to guarantee it can meet standard
Qbus timings, but is a) slow and b) makes certain absolutely nothing else is
happening on the system when the Qbus is active.
Roger Ivie               | "They say that heaven is like TV.          |  A perfect little world that doesn't really |  need me." -- Laurie Anderson