Re: G3 B&W vs 7600

["Jeff Walther" <trag%io.com@localhost>]

> Hi,
>
> On Fri, 16 May 2008, Michael Lorenz wrote:
>> Depends what you need. The 7600 uses a 50MHz CPU bus, the G3s are
>> faster ( 66MHz or 100MHz ) but that might be sufficiently compensated
>> by a big and fast L2 cache.
>
> From what I understand when you put an excelerator card in the older mac,
> it will increase the bus speed. Although, it may never get over 50 mhz.

On x500 and x600 models, the bus speed of the logic board is controlled by
the CPU card.  Of course, there is a maximum workable bus speed dependent
on the quality of the motherboard, so the CPU card is limited in how fast
it can drive the logic board.

In fact, all the clock signals for the major memory/CPU bus components
come off of the CPU card in these models.  There are six CLK pins (9 - 14)
in the CPU slot pinout which carry clock signals from the CPU card to the
components on the logic board.  The pin out for the CPU socket is here:
<http://www.io.com/~trag/x500_CPU_Pinout>.

If you examine a CPU card for these models, you'll notice there is always
a clock buffer (distribution) chip on board to divide the oscillator
signal into six or more phase matched buffered clock signals.

The CLKID pins (three of them) can be tied to ground or left open in
various combinations (presumably they're lightly tied to Vcc on the logic
board).   They are a signal from the CPU card to the Hammerhead controller
of the speed range (e.g. 40 - 45 MHz, 55 - 60 MHz) into which the CPU card
falls.  Hammerhead uses this hard-wired signal to set certain timing
registers at boot time.

My understanding is that these timing control registers are also settable
through software, or perhaps NVRAM, but I don't know anything about their
addresses, or exactly what parameters they are setting.

Some adjustable CPU cards are adjustable up to 65 MHz or so but in
practice will not operate beyond the low 50s.  Usually, these cards have
no provision to adjust the CLKID pins as the bus speed setting is
increased.

The most successful card I've seen at making those adjustments was one of
the adjustable 604e based cards.  I can't remember the model now, and it
is right on the tip of my brain.   Anyway, it was rated for 225 MHz, and
wouldn't run much over 240 or 250 MHz.  But the bus speed would get up to
62 MHz in every logic board I tried it in.

Most accelerator cards are just designed to run the bus at 50 MHz.

Jeff Walther