From: russ@hale.cts.com (Russ Golish)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: 3-Chip SIMM Modules
Date: Mon, 14 Sep 92 04:57:21 PDT
Organization: Hale Telecommunications San Diego CA

There most definitely IS a difference between 9-chip SIMMs and the newer
3-chip variety.  The 3-chip SIMMs will not work (correctly or at all)
in many motherboards and memory boards.  This incompatibility is most
likely to occur with somewhat older systems and memory boards, where
"older" is inherently a function of the hardware manufacturer.  For AST,
"older" is about two years and is typically isolated to low-end systems.
 
The problem has been explained to me many times DD it involves DRAM
refreshing.  At first I was skeptical of this problem definition, but
experience has changed my mind.  The difference is that the new 3-chip
SIMMs utilize a different, faster, and much more sophisticated refresh
method.  As "older" machines and memory boards did not incorporate this
faster refresh timing into their designs, they are more likely to exhibit
problems.  Myself and several SIMM brokers have noticed that big brand
vendors such as IBM, Compaq, AST, etc., exhibit problems more frequently
than typical "OEM/Clone" boards.  One possible explanation is that these
vendors design to much tighter specs DD all system components are known.
Another is that there are More of those systems out there, and/or More
of those systems are having RAM upgraded, etc., among many others.
 
Reports of software-related problems were fairly heavy 2-6 months ago.
Many users reported that MS Windows experienced parity errors when under
heavy load, and 3-chip SIMMs seemed to be a fairly common element.  I
heard detailed reports that some OS/2 users could not use the 3-chip
SIMMs AT ALL.  The system would boot DOS-5 fine, however OS/2 would not.
These were truly ordinary systems, and apparently a quick swap to 9-chip
SIMMs solved the problem completely.  I know, I know:  Solar Flares!
 
Another problem with 3-chips is strictly hardware and well understood.
The problem is caused by a mismatch between the (2) 4-bit wide DRAMs and
the Parity DRAM.  The mismatch is the speed, NOT that the ICs are from
different vendors.  The parity chip MUST BE the Same Speed or Slower than
the other ICs (within a given tolerance.)  The stamped or rated speed is
not necessarily the true IC speed.  One notable case is DRAM made by NMB,
which routinely performs 20ns faster than rated speed.  3-chip's which
contained (2) 4-bit wide DRAMs (non-NMB) and a NMB parity DRAM were prone
to fatal parity anomalies.  Many Brokers blacklisted this configuration.
Definitely not NMB's fault DD strictly the fault of the SIMM assembler.
 
Like all new "HW standards" in our multi-vendor world, it takes a while
for everyone to get their timing signals in sync.  This "standard" was
better than most DD nearly everyone is in sync now.  In general, there
are very few compatibility problems using 3-chip modules in later Rev.
motherboards.  Good news indeed DD 3-chips are significantly cheaper...
 
- Russ -
 

HALE TELECOMMUNICATIONS - Public Access Node, San Diego CA 619/660-6734 HST
russ@hale.cts.com (Russ Golish)   14-Sep-92 at 5:01a
Newsgroups: comp.sys.ibm.pc.hardware