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Re: S-100 80286 CPU board write-up

Hi John
Excellent news :-)
I was kinda holding off on starting my 80286 board in case the mods/fix ended up being extensive (I have the S-100 Ram/Rom board on order also)
Just wondering if will you be investigating why the final board will only manage 11Mhz when the prototype manages 12Mhz ?
It's not a big issue but would be nice to get to 12Mhz if possible.
regards and thanks
David Fry

On Saturday, October 26, 2013 7:06:09 PM UTC+1, monahanz wrote:

Good news, It now looks like our new 80286 Master/Slave S100 bus CPU board works fine with not only our 4MG Static RAM board bit also the 1M RAM/ROM board.

The problem I outlined below was due to the fact that I had jumpers wrong on both the CPU and ROM/RAM boards.  The 80286 works fine with both boards at 11MHz (with no wait states).  


One of the reasons for “upgrading” the ROM/RAM board was to simplify the “rats nest” of jumpers on the earlier boards for the diverse range of RAM & RAM chips (including flash RAM)  that board can use.  The new RAM/ROM boards should be here soon. If you have the older board, consider switching over (it uses all the same chips/parts) .   For new users it is a very useful memory board.


One remaining bug I see with the 80286 board  is that some older S100 Static RAM boards (particularly those that use PALs, such as the Godbout 128K static "RAM21" board or the BG Computers 256K Static RAM board), need a patch. You need to bend out pin 2 of U56 and jumper it to pin 9 of U68 - the bus controller.  This seems to provide the early MWRITE status signal these boards require.  Interestingly this is also the case for our 8088 CPU board (jumper K5,2-3),  but not the 8086 board.

I have written up a detailed account for the 80286 boards construction here:-




And a description of the RAM/ROM board here.










From: John Monahan [mailto:mon...@vitasoft.org]
Sent: Sunday, October 20, 2013 10:53 AM
To: 'n8ve...@googlegroups.com'
Cc: 'cct...@classiccmp.org'; Andrew Lynch (andr...@sbcglobal.net)
Subject: S-100 80286 CPU board write-up


Hi everybody. I have just written up construction notes on this 80286 CPU board.

Please see here:- for details:-



Unfortunately there are some strange issues with the board running with RAM boards other than our 4MG Static RAM board

(described here:-)



With that RAM board,  the 80286 is rock solid at 11MHz (no I/O or ROM wait states required).


Please see the bottom of the page for details.


I am traveling all of next week but will explore further when I get back.

Sorry about the bad news