From: ghazi@wizofhoz.uucp (Ozzie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re:  Why there are 40 Mhz VXLS
Date: 15 Aug 1992 03:11:47 GMT
Organization: Not an Organization

In article <16e38mINN8kj@grapevine.EBay.Sun.COM> smckinty@sunicnc.France.Sun.COM (Steve McKinty - Sun ICNC) writes:
>In article <63971@cup.portal.com>, Tony-Preston@cup.portal.com (ANTHONY FRANCIS PRESTON) writes:
>	> The VXL-30 comes in 25, 40, and 50 Mhz versions.  They use 68030's
>	> that are rated at those speeds.  A 40 Mhz cpu chip is more expensive
>	> and maybe the MMR people want a lower cost unit.

	The VXL comes in two configs, 25 and 40 mhz, the 40 mhz unit has a
50 mhz 030 in it but MicroBotics puts a 40 mhz oscillator in it. If it has
a 50 mhz oscillator, it was your supplier who put in the oscillator and
called it a 50. Microbotics does not advocate changing the oscillator, but
do say the unit has been tested to 50 mhz.

>	>Overdriving a 40Mhz or 50 Mhz 68030 to 60 mhz may work,
>In general it won't. As you say, the higher spec parts are more expensive, and

	Over driving the WILL WORK, it's a simple oscillator change. From
the information I've managed to get from Motorola, when electronics are
tested they are generally under rated by 20-25%. This is done in a great
many industries, for a variety of reasons, most just common sense. You'd be
pissed if you bought a CPU for 400-500 and have it fail every time it got
close to it's upper end specs. So there is a substantial margin to work
with. In theory, if you have a 50 mhz 68030, you should be able to push it
to 60-65 mhz before heat starts to degrade it's operation or your board
starts to melt or smolder before your eyes.

>if Motorola could get a 50MHz part to work at 60, they'd sell it as a
>60MHz part & take the extra cash.

	Again it's the safty margin that determines the speed, if a part
starts to fail at say 33 mhz then it is rated at 25. That's why you pay a
premium for high speed parts, because they are tested in this fashion.

>reliably work at (you may get a 50MHz part that runs at 51, but you won't
>get an extra 20% performance. TANSTAAFL!)

	Not true, I use my 68882 at 40 mhz and experimentd a bit at 60 mhz
with the MC68030(50 mhz rated), but some adventurous VXL users have pushed
theirs to 50 mhz, using 25 mhz 68882's. They do this by using fans or
thermo-coolers on the chips to keep them cooled, it's heat that is the
killer for electronics. So if you can keep it cool it will run at speeds
not intented, of course you do this at your own risk.

	Against an A2000 with a GVP 40 mhz combo, my system runs at roughly
20% better. CPU_Speed shows the combo at 17+ of a regular Amiga, while the
VXL 50/40 combo shows a consistant 21+ of a regular Amiga.

	If I run SI, MIPS etc. it tells me my CPU is at 50 mhz and my FPU
is at 40 mhz, so you can and do get a 20% increase. If your really
adventurous you can even push it to higher, say 100% in some cases, the
limiting factors are heat and the rest of the system, can it take the
higher speeds without crashing.

--

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