From: chet@netcom.com (Eric Chet)
Subject: Re:  Why there are 40 Mhz VXLS
Date: Sat, 15 Aug 92 17:37:09 GMT
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 

In article <ghazi.04xr@wizofhoz.uucp> ghazi@wizofhoz.uucp (Ozzie) writes:
>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.

Also the 50Mhz version from microbotics has a different PAL set designed
for 50Mhz.  You could have problems with a 40Mhz PAL set at 50Mhz, the logic
was not designed for 50Mhz.

>
>>	>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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>
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