Equinox Megaport serial boards (LONG)

[Brian Litzinger]

>From article <8367 at killer.DALLAS.TX.US>, by jonm at killer.DALLAS.TX.US (Jon Meinecke):
> This sounds like a really neat board.
> 
> Perhaps someone more knowledgealbe in hardware details can explain why
> 
> Mr Lukas writes:
>> ...DMA or I/O cycles are much longer than 16 bit memory cycles on the AT
>> bus...
> 

In ATs DMA cycles operate at one fourth the clock rate.  

> 
> Also, what does it mean to say that the card "can support 16MHz bus speeds"?
> I thought that AT-compatible bus speed is 8MHz...

[Flame resistant suit ON...]
I do not claim to be an expert on Megaport boards and I do not know
if their product suffers from any of the following problems. I am simply
providing the following information to answer Mr. Meinecke's general
inquiry into these kinds of guestions.
[Flame resistant suit OFF...]

The IBM AT bus, if you can all it that, most people consider runs at
8Mhz.  (It used to be 6Mhz on the original IBM AT).  The 286 processor
requires a clock of 2 times the processor speed.  I.E. and 8 Mhz
processor uses a 16 Mhz Master Clock.  In the IBM AT the Master Clock
drives the 286 Processor, and the Master Clock divided by 2 drives
the bus. Therefore, the bus runs at 8Mhz.

Many AT compatible and 386 AT machines use this same strategy.

So imagine you want your system to go faster. You could simply
replace the 16Mhz Master Clock, with a 20Mhz Clock.  Your system
might work if the electronics and memory can still operate at
this higher rate.  You might even design a system specifically to
do this.  However, your AT bus is now operating at
20Mhz/2, or 10Mhz, instead of 8.  

Many peripherals will be able to continue to operate correctly
at this higher speed, but many will not.

To their credit, some manufactureres use seperate clocks for the
processor and the bus, thus avoiding this problem.

On the otherhand, some people might not call an AT bus operating at
greater than 8Mhz a problem, they might call it higher performance.

In my experience, including working for Award Software, and Wyse
Technology, I would avoid using greater than 8Mhz bus speed systems.
It only leads to problems.

Another performance point often mentioned is 16 bit transfers.

By using the Memselect16 and LA address lines a peripheral can do
16 bit transfers.  Unfortunately, using the LA address lines only
allows you to decode memory in 128K chunks.  To get finer precision
you must use the SA address lines which occur approximately 50ns
later than the LA address lines.

This is not a problem for VGA/EGA cards because they can decode from
0xa0000 to 0xbffff, a 128K region.  Some cards 
use various tricks to decode the SA address fast enough to do
16 bit transfers.  However, all the schemes I have seen 
will not work in all computers.

So would also recommend avoiding peripheral cards that try to decode smaller
than 128K regions for 16 bit transfers.

<>  Brian Litzinger @ APT Technology Inc., San Jose, CA
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