From: me00222@epx.cis.umn.edu ()
Subject: Summary:Q: What are the speed of I/O bus on 386 and 486 machines?
Organization: University of Minnesota
Distribution: usa
Date: Fri, 14 Aug 1992 15:32:50 GMT


This is the summary of what I have  received.  I appreciate who helped me.
I hope it may help others too.


-------------  My posting --------------------------------------------------

Subject: Q: What are the speed of I/O bus on 386 and 486 machines?

Hi! netters,

        This is the second try for the similar problem but more specific.

I have two memory cards installed on my 286-8 machine.  Each cards have
150ns and 120ns chips respectively.

My question is "Will it work on the faster machines i.e. 386/33 or 486/33?"

Different people gave me totally different answers.

1. Since I/O bus speed is always 8 Mhz despite of the cpu speed, the memory
card will work but slow.
        My question for this theory is then : If I change the chips for the
faster one,say 53ns, will it make the difference on running Windows as an
example?

2. I/O bus speed is different from cpu to cpu.  386 machines use 16Mhz I/O
bus and EISA machines use 32Mhz I/O bus speed ( I asked him if he was
confused between bus width(bits) and speed(Mhz).  He confirmed that it's speed).
        My question for this theory is then : How come memory board advertise-
ment doen't specify the appropriate CPU?  How about 486?

I have to make it clear that I fully understand that I have to use appropriate
chips for the specific speed of CPU WHEN the chips are installed on MOTHER-BOARD.  My question is when the chips are installed on the memory card.
I also understand that some proprietory memory cards (usually non-standard
32 bit card) runs as fast as chips on the motherboard.

Thank you for you help in advance.

------------What I received -----------------------------------------------

From: REE700A@MAINE.BITNET

   EISA is 8.25 MHz x 4 Bytes = 33 MBytes/sec.  Your friend is brain dead.
 
   On the other hand, bus speeds (ISA) do vary from system to system.  A low
speed of 8 MHz and a high speed of Sys_clock are allowed on various systems
that I have seen.  The most common settings for ISA speeds hover around the
8-12 MHz range.  Placing 53 nS RAM on those boards won't help, the board
itself will most likely limit you to the speed it was originally built for.

---------------------------------------------------------------------------

From: johnl@iecc.cambridge.ma.us (John R. Levine)

>1. Since I/O bus speed is always 8 Mhz despite of the cpu speed, the memory
>card will work but slow.

That's the right answer, though "slow" is a bit weak.  "Glacial" perhaps.

>	My question for this theory is then : If I change the chips for the
>faster one,say 53ns, will it make the difference on running Windows as an
>example?

It'll make no difference at all.  The speed rating on the chips is the
miniumu cycle time at which they promise the chip will work.  The actual
cycle time at which the chip runs depends on the system it's installed in.

>2. I/O bus speed is different from cpu to cpu.  386 machines use 16Mhz I/O
>bus and EISA machines use 32Mhz I/O bus speed

Typical bus speed these days for a 386 and better is 8 MHz since nobody
(except maybe you) puts memory on the I/O bus any more.  EISA buses can
indeed run at 32 MHz but only when using an EISA memory card.

>	My question for this theory is then : How come memory board advertise-
>ment doen't specify the appropriate CPU?  How about 486?

Generic memory cards are for 16 bit 286 machines.  As noted above they'll
work in faster machines but very slowly.  Most 386 and 486 motherboards
now have sockets for enough motherboard RAM that there's little call for
memory expansion cards for them.  For instance, my 486 EISA board can take
32 MB and it'll be a long time before I need more than that.

Regards,
John Levine, johnl@iecc.cambridge.ma.us, {spdcc|ima|world}!iecc!johnl

--------------------------------------------------------------------------
From: bich@hpscit.sc.hp.com (Bich Tran)

I/O speed is ALWAYS 8Mhz regardless of the CPU. That's how they can maintain
the compatability across the x86 Intel platform. Since each I/O cycle takes
from 3-4 BCLK (8MHz), the proper access time of DRAMS is much less than 127ns.
The possible difference between computers is in the I/O wait state, with
the fastest running with no wait state, hence 3 BCLK for each I/O. So if
the memory board inyour 286-8 is running with 1 wait state ( the system
inserts 1 extra clock in 1 I/O bus cycle), and your new system has no wait
state I/O bus, the memry can lock up. Changing to faster DRAMs may or may not
help since the timing on the memory board may be slow to catch up with the bus.
So please check the memory board manual for thing like wait state.

bich