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From: ronb@adds.newyork.NCR.COM (Ron Beausoleil)
Newsgroups: comp.sys.ibm.pc
Subject: Fastback DMA failure (8237A-5) defective chip
Message-ID: <1247@adds.newyork.NCR.COM>
Date: 19 Nov 89 17:38:51 GMT
Distribution: usa
Organization: Applied Digital Data Systems Hauppauge, NY
Lines: 28


   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??