Random change WTT: KA694

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Sun Jun 24 13:43:48 2001

At 02:14 PM 6/24/01 -0400, you wrote:
>Hello Chuck,
>
>I didn't know such a cpu existed. Is it actually qbus? Some of the
>cpus I have seen on Ebay look like a Qbus card with a plastic connector
>rather than a card edge. I was under the impression that after the
>KA660? that DEC went to a design that wasn't really Qbus, for the cpu.
>I am just going by what I see on Ebay, Though. What's the real story on
>this?


Hi Chad,

In the 4000/300 DEC decided to make a "new kind" of Q-bus box, one where
the CPU was upgradable again.

In the early MicroVAXen the CPU was a single card, memory was a single
card(s), disk interfaces, and then ethernet. Since the Q-bus is limited to
about 3Mbytes/sec that set a limit on the I/O performance of the machine.
With the KA640 DEC put ethernet, disk I/O (DSSI), and memory on the same
card. This gave them a direct disk channel (4Mbytes/second), ethernet not
on the bus (saves interrupts and bus latency) and some local memory. I did
not work as well as they would have hoped apparently. But the ultimate
expression of this idea was the KA660 which used Q-bus "fingers" had only
DSSI and Ethernet on the CPU card and pushed the memory bus that ran over a
50 pin ribbon cable to its limit.

For the 4000/300 they knew 64bit wide memory was going to help them get
performance gains and they knew that leaving the processor on the Qbus
meant they had to connect things with ribbon cables that could have easily
(and less expensively) been connected on the backplane. So a new bus was
invented (I don't know if it is named or not) and the "CPU Box" was born.
This borrowed from the mainframe VAXen of the time that had the
CPU/memory/bus interconnects in one box, and then either BI, XMI, or even
UNIBUS boxes hanging off of that. In the 4000/300, 400, 500, 600, 700,
700a, 705, and 705a. The backplane consists of about 5 "slots" on the right
edge that hold the CPU / memory / DSSI interconnects and to the left of the
CPU is the Q-bus. The KA670,-675,-680,-690,-691,-692,-693,and -694 CPUs use
this bus. (note they could not go to KA7xx because that would interfere
with the module numbers of the 700 series processors :-) Everything from
the KA675 were variations on the NVAX implementation of the VAX
architecture (as is the 4000/90, 4000/105, etc)

So you're correct in that the back plane connector on the KA694 looks
almost like a eurocard connector but it actually plugs into a backplane
that has Q-bus slots to the left of the CPU.

So the KA660 is the fastest "true Q-bus" processor and the KA694 is the
fastest "integrated Q-bus" processor. The 4000/105 has a desktop enclosure
and actually brings the Qbus out to a pair of connectors on the back of the
box, These connect with cables to a B215F or B213F expansion box where the
first card in that box is the Q-bus extender. I don't consider the 105 to
be an integrated Q-bus processor like I do the 4000/705a.

--Chuck
Received on Sun Jun 24 2001 - 13:43:48 BST