Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!utastro!bigtex!james
From: james@bigtex.cactus.org (James Van Artsdalen)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: How long to hold the IOCHRDY (IO Channel Ready) signal ?
Keywords: IOCHRDY, AT-BUS, ISA
Message-ID: <48210@bigtex.cactus.org>
Date: 3 Oct 90 03:03:37 GMT
References: <188@alderan.uucp>
Reply-To: james@bigtex.cactus.org (James Van Artsdalen)
Organization: Institute of Applied Cosmology, Austin TX
Lines: 34

In <188@alderan.uucp>, bsteh@alderan.uucp (Herbert Bsteh) wrote:

> According to various hardware reference manuals (Compaq, Intel)
> we found different informations about how long we may hold the
> IOCHRDY signal. Our Intel manual says something like :
> "IOCHRDY must not be negated for longer than 15 us" - which does
> make sense to me because of the REFRESH signal. Our Compaq manual
> says "This signal should not be hold longer than 2.5 us ...".

15us is reasonable.  2.5us is way too short: there are lots of cards
that drive IOCHRDY longer than 2.5us.  Network cards often go over
15us.  VGAs in 800x600 256 color mode often drive IOCHRDY for more
than 20us.

The winner has to be one card that I have seen drive IOCHRDY for 30ms
(milliseconds) after reset.

The 15us refresh rate is a hold-over from the original IBM designs.
Modern memory systems usually don't need this much refresh.

> For our application it is necessary, that the bus agent holds the
> IOCHRDY for about 10-12 us. Is there any good reason for restricting
> the "hold-time" to not longer than 2.5 us.

Nope.  Maybe Compaq meant 25us.

> and what problems could arise if we go up to the limit of 15 us ?

None.  You can stretch it out beyond that too, though I wouldn't go
much beyond 20us.  So many cards on the market already have such
delays that all ATs support it.

Incidentally, don't forget the floppy.  All of this assumes that
you're not using the floppy.  If that needs to work at the same time,
then < 15us is the limit.  Under DOS this usually isn't a problem, but
with unix or OS/2...
-- 
James R. Van Artsdalen          james@bigtex.cactus.org   "Live Free or Die"
Dell Computer Co    9505 Arboretum Blvd Austin TX 78759         512-338-8789