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Re: [N8VEM-S100:6238] Re: 80386 CPU Board - A possible solution

John -

Not much time to play today (too much snow!).

Switching SMBv2 K3 8<-->16 bits makes no difference.

I don't have a 74F138 or 74F10 here, but have ordered them.  Should be here midweek weather permitting.

I did have 74S138 and 74S10 - unfortunately the problem persisted with them.

I jumpered the parallel port board for ECh--EFh so I could use the output LED's to see what the ports should contain.

I can set and clear the EE,01/00 bit fine on the parallel port board, but it stays "stuck on" with the SMBv2 board.

- Gary

On 2/7/2015 11:47 PM, John Monahan wrote:

Perhaps the following might help Gary…

In my travels is working with the 80486-40 I noticed that with one (of my two) SMB-V2’s it would not switch back to the Z80 using the Output a 1 to bit 0 of port 0EEH.  


However if  I jumpered K3 2-3 so it used only 8 bit port addressing and that immediately corrected the problem.


When I set K3 to 1-2 (16 bit port addressing) the problem returned.   I suspected the problem is U31B is not getting the address lines up in time on its pin 5 to clock the correct data into U33.    If I use a 74F138 for U27 and a 74F10 for U18 the problem disappeared.  I.e. I could always switch back to the Z80.


Could you try these in your system.