If you don’t get back to the Z80 with 74Sxx’s the 74Fxx’s probably will not work. S’s are faster, just sink a lot of input 2-3 TTL’s I think.
With your parallel port setup, remove the K4 jumper completely on the SMB-V2. Be sure P36 has 1-2 & 3-4. With a logic probe show that you can lower the 1-2 jumper by outputting a 1 on bit 0 with the Z80. The 80386 should switch in. Them within your 80386 monitor output a 0 on bit 1. Check the P36, 1-2 jumper now returns high. The Z80 should come back on. Do you get this?
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Gary Kaufman
Sent: Sunday, February 8, 2015 7:28 PM
Subject: Re: [N8VEM-S100:6242] Re: 80386 CPU Board - A possible solution
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.
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.
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