NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/sys/ibm/pc/misc/12681

Internet Message Format | 1992-09-15 | 1.9 KB

Path: sparky!uunet!noc.near.net!mars.caps.maine.edu!maine.maine.edu!ree700a
Organization: University of Maine System
Date: Tuesday, 15 Sep 1992 12:06:31 EDT
From: <[e-mail]>
Message-ID: <[e-mail]>
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: EISA Board Compatibility
References: <[e-mail]>
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Lines: 24

  Just to add on to Fred McCall's correct answer (almost)
EISA is 8.25 MHz x 4 bytes, and ISA is variable (by motherboard).
In the BIOS extended setup, there are settings for bus clock speed.  NTL,
assume that the ISA and EISA clock speeds are the same.  Assume a hypothetical
16 bit EISA card and compare it to an ISA card (in an EISA system).  The EISA
card will still be capable of faster transfers because ISA does not support
burst mode DMA, in which one data item (byte, word or dword) is transfered
per clock cycle!
  In ISA, there are multiple bus clock cycles per transfer which lower the
8 MHz bus clock's effective bus speed to something between 2 million to
4 million 16 bit transfers per second (2-4 bus cycles per transfer) as opposed
to 8.25 million transfers per second for the EISA card.
  Note that the presence of ISA cards does not interfere with the ability
EISA cards to acheive burst rate transfer.
  The big misconception here is between clock speed (Mega Hertz) and data
transfer rate (Mega Bytes / second).  EISA acheives a 33 MB/s transfer rate
(peak) by performing 8.25MHz transfers of 4 byte data items.
  ISA cards in EISA systems acheive (around) 8 MB/s transfer by transfering
2 bytes every other clock cycle at 8.25 MHz.
  ISA cards in an ISA system acheive around 4 MB/s transfer by transferring
2 bytes every fourth clock cycle at 8 MHz.  (Although some systems push ISA
to as much as 16 MHz --> 8 MB/s.)

  Jeff Andle   DoD#3005