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Announcing an S100 Bus 80486 CPU Board


Experienced Member
Oct 19, 2008
San Ramon, CA
After over two years of prototypes I’m delighted to report that we now have for the first time ever a Master/Slave 80486 S100 Bus CPU board. This was a long road starting first with a 74LSxx based 80386, then a GAL based 80386, then a GPLD/GAL based 80386 and after numerous tries a CPLD/GAL based 80486. I learned a tremendous lot along the way not only about the CPU hardware but also learning how to program these CPU’s in Protected Mode. The 80386/80486 still remains the core of the Intel CPU line. Once you understand that CPU set the rest is easy.

Over the years we have had an 8088, 8086. 80286 & 80386 S100 boards. This 80486 will be more or less the end of that line. It incorporates all the elements of the previous boards into one relatively simple board. Because all the S100 bus signals are synthesized from the basic 80486 M/IO, C/D, R/W, ADS etc. CPU signals in the CPLD, in theory it should be possible to get this board to run with any S100 bus board. A special feature of the 80486 is its ability to dynamically change the bus width for 32, 16 or 8 bit data for each bus cycle. This complicates the board tremendously but allows the board to (again in theory) work with any 8 or 16 bit S100 board. With enough wait states even the very old pre-IEEE boards.

That said, the main use of the board should be with multi-megabyte RAM boards. The board boots up MSDOS V4.01 fine and will even work directly with our 8 bit VGA board.

I have decided to put out two different 80486 boards.

The first I call the “Baby 80486 CPU board”. It can address all available RAM on the S100 bus (up to 16MB). It has its own onboard EEPROM. It is intended as a board to get people started with the 80486 on the S100 bus and should replace any of the earlier Intel CPU boards.

The second board which I call the “Full Size 80486 CPU Board” is more complex. It can address (in theory) the full 80486 4GB address space accessing RAM above 16MB in 32 bit mode. This board is really intended for the more experienced users.

Both boards are described here in detail:-
http://s100computers.com/My System Pages/80486 Board/80486 CPU Board.htm

All work on the Baby 80486 is now done. I have tested the final version; it appears to run fine. If people would like to obtain one or more copies of this bare board please join or go the S100Comnputers forum


and add you name to the "order list" for the tread with the same name as this one.

The cost of these boards will be $20 + shipping. Payment via PayPal, only after you receive your board. It will take ~4 weeks to come in from China weeks (PCBCART). BTW, these boards are extremely dense and have extra thick copper layers for better signals over our “normal” S100 boards. It took many tries with KiCAD to rout these two layered boards.

The Full Size 80486 is not ready yet. Will probably be one month later.
Typically I get swamped with requests for first time boards, so please reply only to this posting – that way I can track who ordered what.

By way of the next steps in this tread…. I’m currently doing a 16 bit Cirrus based XVGA board, trying to get a multi-GB DRAM board going and really long term, looking into putting the Intel Quark CPU on the bus. Other ongoing projects here include Raspberry and Edison CPU boards.

Stay tuned!
John Monahan