Fastback DMA failure (8237A-5) defective chip
Ron Beausoleil
The defective DMA chip has been known about at my work for a long time!
The defective chips are mfg'ed from AMD. The problem that exsists in the
defective chips deals with multiple DMA channels active at one time. The
CHIP has three channels. On all XT's one channel is used for refresh.
This channel is always active! The floppy and hard disk use the other
channels. FASTBACK uses concurrent floppy and hard disk DMA. Of course
a problem with DMA will show up very soon.
The reason for the failure is very complex. The easiest way to explain
it. Is to say any time a channel is active AND a write to the dma command
register is made, the write command end up in the DMA data transfer. Also
the command never made it to the register. So when you expect to get the
DMA transfer for the new command, it never will happen. LOCK-UP CITY.
This problem is only found on XT's using AMD dma parts. AT's using this
part do not have this problem. Different circuit design.
BTW, we found this problem on a CPM product long before IBM personal
puter became infamous. Seems the policy of chip mfg'er is to dump defective
parts into different designs until all parts are gone, then fix the next mask.
Sorry to say that! But remember the defective 8250-B's We found out about
those parts 1 and 1/2 years before TAIWAN got the defects. How do I know??
We marked defective devices and sent them back. We had to find some good ones
so we tested and tested. Rejects were returned. LO and BEHOLD when the stink
hit the fan, I found our mark on some parts, at local fea sales.
So if you are BUYING PCB's from flea markets and you review the parts on the
boards. Look closely at the DATE codes. If the board is NEW, then why would
it have a part (2) years old. Think about it??
Glendell R. Midkiff
message of 'bad DMA chip'. I then tried installing Fastback+ ver 2.01
and there was no indication of any problem. DMA speed configured to 'fast
mode'. If there really is a hardware problem, why does one version of Fastback
work and the other won't. Smells fishy to me. Anyone have any ideas?????
--
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|Glen Midkiff osu-cis!dsacg1!gmidkiff |
|From the Internet: gmid...@dsac.dla.mil |
|Phone: (614)-238-9643 @DLA, Systems Automation Center, Columbus, Oh. |
David Lesher
Simple,
Fastback+ bears little or no relation to Fastback-un+.
They were written by different authors, at different times.
The author of +, aware of the DMA problem, went to great
lengths to avoid it if possible, and if not, detect it during
installation.
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Bob Calbridge
` I tried installing Fastback Ver. 5.14 on my Northgate 386/16 an got the
` and there was no indication of any problem. DMA speed configured to 'fast
` mode'. If there really is a hardware problem, why does one version of Fastback
` work and the other won't. Smells fishy to me. Anyone have any ideas?????
Reminds me of another Fastback Plus problem we ran into. We were running
Fastback on a number of machines to make sure that the one we decided on was
going to work with all of our software. Of course, you have to install
Fastback Plus and test the DMA etc. Well, since one of the machines we were
testing had a high density floppy drive, when it came time to put a disk in
the drive we stuck in a 1.2Meg disk. We kept getting a DMA failure. It seems
that even though Fastback Plus knew ahead of time that there was a 1.2M drive
it was asking for a 360K disk to be inserted in the drive. Once we realized
this everything worked fine. Wierd stuff.
Bob
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Phil Ngai
| This problem is only found on XT's using AMD dma parts. AT's using this
|part do not have this problem. Different circuit design.
The real story (although I am not an official spokesman) is that AMD
did the first generation of the DMA controller and licensed the design
to Intel who modified it a little and documented the enhancement. Boca
Raton designed to the Intel data sheet and didn't check to see if it
was downward compatible with AMD's design. It wasn't but until
Fastback came along there was no software which exposed the problem.
Since then AMD has incorporated the Intel enhancement in their chips
(it is a very simple change) and most motherboard makers use circuits
which are compatible with both generations of DMA controllers.
--
Phil Ngai, ph...@diablo.amd.com {uunet,decwrl,ucbvax}!amdcad!phil
AT&T Unix System V.4: Berkeley Unix for 386 PCs!
John Horeth
I have version 1.01 of Fastback Plus and tried to bring up on MS-DOS 4.01. The
DMA test repeatedly failed. I called Fifth Generation (authors of Fastback)
and they said that version 1.01 would not run on 4.01 (it does on 3.3), but
that their lastest version would. I proved their claim by getting version
2.01 to work on MS-DOS 4.01. So I now have a copy of Fastback Plus V2.01 on
order.
So the moral of the story is not only does your version of Fastback Plus count
but also your level of MS-DOS.
---
jo...@DRD.com | John M. Horeth, II
{uunet,rutgers}!drd!john | Horeth Programming Services, Inc. - (918)622-9232
| c/o DRD Corporation (918)743-3013
Jason Merrill
mysteriously hangs in the middle of the Medium test. I don't get a message of
any sort...the hard drive light stays on, but nothing happens. I've tried
using formatted and non-formatted floppies, running the computer at all
available speeds...any thoughts?
Disclaimer: I'm not even sure my opinions are mine...they certainly aren't
anyone else's.
Steve Elias
>The real story (although I am not an official spokesman) is that AMD
>did the first generation of the DMA controller and licensed the design
>to Intel who modified it a little and documented the enhancement. Boca
>Raton designed to the Intel data sheet and didn't check to see if it
>was downward compatible with AMD's design. It wasn't but until
>Fastback came along there was no software which exposed the problem.
believe me, some of us were working on software back then
that did expose the problem in a big way. not an easy bug
to find. we couldn't afford an in circuit emulator for 8088,
even if there was around that worked...
>Since then AMD has incorporated the Intel enhancement in their chips
>(it is a very simple change) and most motherboard makers use circuits
>which are compatible with both generations of DMA controllers.
what is the motherboard patch which precludes this bug?
>AT&T Unix System V.4: Berkeley Unix for 386 PCs!
yeah!
--
-- Steve Elias ; e...@spdcc.com ; 6179325598 ; 5086717556 ; { *disclaimer(); }
Phil Ngai
| believe me, some of us were working on software back then
| that did expose the problem in a big way. not an easy bug
| to find. we couldn't afford an in circuit emulator for 8088,
| even if there was around that worked...
Sorry to hear that. I meant there wasn't any piece of software which
was popular enough to force all the clone makers to wake up and handle
the problem.
| what is the motherboard patch which precludes this bug?
As I understand it (I haven't actually done this) the problem is that
the Read and Write lines on the old 8237 were not gated with the CS
line. There is a recovery time on either or both of the read and write
lines and a read or write which are too close together can violate
this recovery time EVEN when not addressed to the 8237. The solution
is to use two AND gates to condition the read and write lines going
into the 8237 so that they only go active if CS is also active.
As a user, I think I'd just swap the 8237 for a newer one. They
can't be very expensive.
--
Phil Ngai, ph...@diablo.amd.com {uunet,decwrl,ucbvax}!amdcad!phil