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Re: [N8VEM-S100:4755] Progress with the 8080 board

We should probably keep a "to do list" for V2 Josh.

1.   Add MWRT with jumper.
2.   For I/O port data out, address lines A8-A15 should be pulled low. (so only ports 0-255 addressed).
3.   Address lines A16 to A23 always pulled low (in "master" mode)

Space is tight on the board as it is.  #2 & #3 will require 74LS244's.  You can shave off some space for the SD card transistor etc. As best I can tell  E2 pin 10 does not do anything.

On Wednesday, July 30, 2014 5:24:51 PM UTC-7, Crustyomo wrote:

Thank you for testing this out and getting it working.  I'm not having the same pull up problem on sOUT.

This board was designed for use with the ALTAIR and IMSAI FP boards, but I agree and in V2, I will add mWRT generation and jumper select it for the flexibility.

You are right about the high address bus.  The 8080A places the I/O address on both the upper and lower 8 bits.  On my initial version, I was decoding the upper 8 bits for use with I/O and Memory banks, but later changed to add it's own decoder chip on the lower address to support Z80 CPU's that might want to access this card as a peripheral.

What should I do with those upper address lines?  Should I just ground them? or pull them high/low through a resistor network?
Something for V2.

Thanks for the schematic corrections, I have updated my copy and will release another revision soon.

I should also point out, if anyone is wanting to use the 16550 UART chip,  then RP2 must be changed to 47K.  Thanks to testing with Fabio, he found that the 16550 data bus output is getting pulled marginally high with a 4.7K.  The 47K will suffice to prevent the bus from floating yet not interfere with the 16550's.


From: mon...@vitasoft.org
To: cru...@hotmail.com
CC: n8ve...@googlegroups.com
Subject: [N8VEM-S100:4755] Progress with the 8080 board
Date: Wed, 30 Jul 2014 11:44:12 -0700

Hi Josh solved the I/O problem (see below).  Since a few others may be in the board assembly stage I thought I would write up my progress so far for the group!


I am running the board in my “test box” that has no front panel.   Just the V2-SMB and Propeller Console I/O.  I always start with a minimal system so I don’t “blow out” board drivers etc.


First one needs to remember that unlike the Z80,  the 8080 does not treat the upper 8 bits of an I/O port the same way. On our Z80 board (and most others) the upper 8 bits are set ort Zero.  Not so on this board. So on all the N8VEM/S100Computers boards (and perhaps others), ports need to be jumpered so that only the lower 8 bits are factored into port addressing.  All our boards have this option.   Not a nice solution since with 16 bit CPUs the ports appear on every 256 byte boundary.  Setting the jumper K3 to 2-3 on the V2-SMB allowed the routine below to work correctly.


The IEEE-696 upper address lines float.  This will not allow 696 RAM boards to work unless they are adjusted.  Our N8VEM/S100Computers boards generally have a jumper to ignore these lines but again will not work with 16 bit CPU’s.  Our V2-SMB has an option to force these address lines to ground when this board is the master but release them for slave/16 bit CPU’s.


Second the board does not generate the S100 MWRT signal. (On front panel systems, it’s generated there).  One can be generated on the SMB with the  JP11 jumper.


There seems to be a problem with the sOUT signal (s100 Pin 45) on J8.  It’s been pulled high all the time – at least on my board.    I bent out J8 pin 6 and connected it directly to the S100 bus (bending out the pin connection on the extender board). I will try and follow traces to figure what is going on here.


Minor schematic errors.

Pins 8 & 9 of E2 are switched.

Pins 1 & 19 of I8 and H8 should be  labeled D-IN, D-OUT, etc.


Still a work in progress, but wanted to save others reinventing the wheel.







From: John Monahan [mailto:mon...@vitasoft.org]
Sent: Tuesday, July 29, 2014 7:21 PM
To: 'Crusty OMO'
Subject: Problem with sOUT with the 8080 board


Hi Josh.

I running into a bit of a problem with the 8080 board. I use this code


BEGIN:0009 3E34         MVI   A,'4'             ;FOR QUICK HARDWARE DIAGNOSTIC TEST

     000B D301         OUT   01H

     000D DBEF         IN    IOBYTE

     000F D301         OUT   01H

     0011 C30900       JMP   BEGIN


I can get a “4” on the screen no problem.  But I found no matter what value I have the dip switches for port EF I get a blank on the screen.

With the SMB switches set to 33H one normally see “3’s”


Analysis tracked things down to the fact that sOUT was not being generated on the bus.  The LED on the SMB in fact did not flicker.

I bent out pin 6 of J8 and bent out the edge connector pin going to the bus and jumpered J8 pin 6 to the S100 line #45.  Now sOUT flickers (as it should).  I find that pin 45 on the board is being pulled high (and warming up the J8 IC).    Before I dig further, wanted to know if you see this.


For configuration..

SMB, Propeller Console I/O board (Ports 0 & 1) and the 8080 board nothing else in bus.

Note have to generate MWRT on SMB -  J11, need to tell people this as it’s not generated on your board if no FP.

I have JP8 LHS jumper open the other 7 closed

JP9 LHS jumper closed all the rest open. 

So 8K ROM at 0, the rest RAM

No R15 jumpers so all I/O ports via bus

JP4 1-2,  JP1 2-3, JP6 2-3, JP7 1-2


No A5 ,A6 , J5 or J3 (but J7 is present).




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