From: kgermann@zeos.com (Ken Germann)
Subject: Re: No VL-BUS for 486/50 Mhz?
Organization: Zeos International, Ltd
Date: Sun, 11 Oct 1992 01:01:33 GMT

In article <92283.202328I18BC@CUNYVM.BITNET> Spartacus At CunyVM <I18BC@CUNYVM.BITNET> writes:
>In article <1992Oct07.153900.7878@zeos.com>, kgermann@zeos.com (Ken Germann)
>says:
>>The specification states that the max speed for a VL-Bus Card is 40Mhz due
>>to electrical limitations. You can place a VL-Bus Device directly on
>[...stuff deleted...]
>>with a 64-bit BUS. VL-Cards are backwards and forward compatible. i.e. You
>>can put a VL-Card that is 32-bit in a future 64 bit Vl-Bus connector and
>>it will work. You can put a VL-Bus card that is 64-bit in a 32-bit VL-Bus
>
>Great info Ken,
>however I have a question. if Vesa is limited to 40mhz,
>wouldn't this create another bottleneck for faster processors
>like the p5?

The maximum bus speed to the VL-Bus Device is 40Mhz. i.e. the Maximum
speed to a Video card or Hard disk controller is 40Mhz. This is 5
times faster than the standard AT bus speed. IF there is a VL-Bus device
surface mounted on the motherboard that is connected to the VL-Bus, 
you can access this device at up to 66Mhz.

By supporting a 50Mhz processor on the VL-Bus your are going to 
introduce unwanted wait states that will affect the performance
your system. Future revisions of the VL-Bus spec are supposed to 
support 50 Mhz or higher. The problem with most implementations of
the 50Mhz processor is that the clock cycle is split to 25 Mhz before 
accessing memory. This is why it is important to talk to the technical
support people to find out if the chip set being used is a 50Mhz chip 
set. You will notice that a DX2-50 and DX50 from the same company will
have the same or similiar benchmarks. This is because the implemention
is the same. The difference in performance will only be noticeable when
you drop a 50Mhz cache on the motherboard/processor module. The Intel Cache
Chip set will only support 33Mhz. This may change in the near future. Also,
The Intel cache chip set is one of the fastest on the market. The next release
of a doubler processor from Intel is supposed to be a DX2-80. 


Also, as a side note there are physical limitations in copper. I am told, 
I haven't verified this, but there is a point n at which traces on the
motherboard will begin to vibrate. I am told this is somewhere around
99.9 Mhz. i.e. circa 100 Mhz. Please no flames. I am not an engineer;
I just listen to them every so often.

 or would a 64bit vl-bus operate faster when they
>become available.  I ask this question because VL-BUS is
>suppose to eliminate bottlenecks, but it seems to create another
>for the 486-50mhz...(Yup I know...the differences between 40 and
>50 is minuscule...but shouldn't this new bus leave alot of
>breathing space for future processors?
>It seems like the VL-BUS will be short lived at 40mhz particularly
>when 66mhz p5 or 100mhz cpus will be out later.

If you have a processor and bus running at 64 Bit, you are doubling
the performance without increasing the bandwith.

It is more probable to see the number of bits on the bus increased to
64. There are RFI problems as describe above at 100Mhz. There 
is going to be a point when we have to say, "How much is enough?".

We are brining the definition of main frame to new heights...
our desktops.


<include disclaimer.h>
This is my opinion and is not meant to discredit my employer or bring them
any harm....
-- 
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