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RE: [N8VEM-S100:5481] 80386 CPU Board - Component type/value clarification

I think I did 4 prototypes on this board Dave, Played around a bit with the caps early on.   Did not seem to make a difference (at these low frequencies).   The Intel manual has a whole section on board layout. See one page attached here.   I ended up with 0.1’s and two 1uF Tants on C71 & C54.  Probably more important is the actual power supply.  Doing the 80486 I’m learning a lot! On that board I’m bring special wide traces directly to the chip. Also if you can get your hands on one, get a 74H05.  There are so many of the 3Amp regulators that are out of specs,  it’s ridiculous.  While I have a board running at 4.8V’s with one, I really don’t like it.


I am also looking into these newer LM2576’s. See attached. Know anything about them.


BTW, I’m also doing a direct SMD 32MB  RAM daughter board (like our 16MB board). Again a tight  board fit (see an non optimized version).   I will order 4 for myself but since its completely untested I’m not going to announce it yet. But just FYI.


Let me know how the 80386 works.  The only thing I’m not comfortable with on that board is the “kludge” to stretch out the pWR* signal.  I hope to do it better (clock independent) on this 80486.






From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of David Fry
Sent: Friday, October 31, 2014 2:37 PM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:5481] 80386 CPU Board - Component type/value clarification


Hi John,


I am slowly starting to populate my 80386 PCB with capacitors and have a query regarding C51 and C74 (positioned either side of the 80386 CPU).

The schematic would suggest that these are 0.1uF multilayer ceramic capacitors like the rest, but the pictures on most versions of your 80386 boards would seem to indicate you have used a tantalum bead capacitor in these two positions.


Please can you clarify what type & value of capacitor should be used here.


regards and thanks


David Fry

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Attachment: Caps Around 80386.pdf
Description: Adobe PDF document

Attachment: 32MB RAM2.JPG
Description: JPEG image

Attachment: 32MG RAM1.JPG
Description: JPEG image

Attachment: LM2576 5V regulator.pdf
Description: Adobe PDF document