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Re: [N8VEM-S100:6766] Re: S100 8088 CPU Board Crashing/Unstable

David —

I just started it up again and here’s what I see. I’m using a ceramic AMD 8088, an OKI 82C84A-5 and a UMI 82C88. I don’t have any pull-ups installed on the pins mentioned below. If I run “MEM” I get an “Unassigned vector CCH” error and it returns to the monitor. I’ve also noticed that it’s not as stable as I remember, and after a few minutes it goes into lala land. I may try the 4.7k pull-ups tonight on the 82C88 and see if that improves things.


Rich Cini
Collector of Classic Computers
Build Master and lead engineer, Altair32 Emulator

From: David Fry <dgf...@googlemail.com>
Reply-To: S100-Post <n8vem...@googlegroups.com>
Date: Saturday, April 18, 2015 at 1:54 PM
To: S100-Post <n8vem...@googlegroups.com>
Subject: Re: [N8VEM-S100:6766] Re: S100 8088 CPU Board Crashing/Unstable

Hi Rich,

you say 'the system is stable without the pull-ups'

Do you mean it is stable when the 8088 has control of the bus ?
did you solve your original problem with the 8088 then ?

regards & thanks

David Fry

On Saturday, April 18, 2015 at 6:34:40 PM UTC+1, AltairManRich wrote:
Thanks David. As of now, the system is stable without the pull-ups, but I haven't used it in a while. I'll run it tonight and see if I have any issues with MEM. I'll also look at which chips I'm using. My system is fully loaded. 

Sent from my iPhone

On Apr 18, 2015, at 1:17 PM, David Fry <dgf...@googlemail.com> wrote:

Hi folks,

A quick update on progress made.

The new bus controller IC (D8288) arrived this morning, a ceramic Intel part and not the ceramic AMD part I ordered but I'll let that one go.

Fitting the Intel bus controller brought an immediate improvement to the situation but not a 100% cure, the CPU card still intermittently locks up but FAR less than it did. I tried the NEC V20 CPU I have and that now also seems to work and I can get into MSDOS and copy files from one folder to another, something I previously couldn't do.

I still have the Intel CPU on order and will give that a try when it arrives but in the meantime I dug out my IBM Technical Reference Manual for the PC-XT to see how IBM implemented the D8288. What immediately jumped out at me was that on the Memory & I/O read write lines (pins 12, 8, 13, & 7) they have implemented 4.7k pull up resistors, checking the D8288 data sheets from 3 different manufacturers revealed no mention of the need for these resistors.
I therefore conjecture that IBM have used them to sharpen the leading edges of these control signals to counteract the capacitance of all the combined gate inputs the D8288 is driving across the motherboard (correct me if I'm wrong).

Referring to Johns 8088 schematic I cannot see these resistors implemented but on the Sergey Xi8088 he seems to have used 10k pull ups on these 4 signals.
So over the next few days I will add the 4.7k resistors and conduct longer term testing (outside of a CPU swap) to see if the pull up resistors add any value to the problem.

Rich, if you are reading this, give the pull ups a go before swapping out the bus controller.

Oh... and can someone else try running the MSDOS command MEM, it still hangs my 8088 CPU but not the 80286, would appreciate feedback on this one.

best regards

David Fry

On Friday, April 17, 2015 at 10:36:00 PM UTC+1, David Fry wrote:
Hi everyone,

I've recently plugged my 8088 CPU board into my S100 system and have been having problems with the 8088 CPU board crashing/locking up.
My system comprises all S100Computers.com boards

Prop I/O Console V01
16MB V3 Static Ram Board
V2 MSDOS support board
V2 Z80Master CPU Board
V1 8088 CPU Board (Slave)
V1 ZFDC Floppy Controller
V2 S100 IDE/CF Board

The 8088 board was inspected for damage before construction, All resistors/capacitors/sil arrays are new & checked with multimeter/capacitance meter before soldering to the board,
All TTL logic chips (TI branded) are new, all jumpers were initially set as per Johns webpage and double checked and the 3 x 47pf capacitors are in the correct locations.

The symptoms are similar to what AltairManRich had been experiencing in a few previous posts but no conclusion has been posted.

Basically, I can boot my system into the Z80 monitor and transfer control to the 8088 CPU which signs on with it's banner, I can press 'K' and bring up
the monitor menu, most monitor commands seem to work fine but menu option 'J' memory test (specifying 1000,2000 as parameters) always results in the 8088 CPU card going into LaLa Land
as Rich puts it, just filling the screen with spaces and scrolling, only a hard reset recovers the system. Occasionally the 8088 CPU will go into a HALT state rather than scrolling spaces.

I can 75% of the time get the 8088 CPU card to boot MSDOS 4.01 sometimes it hangs but most of the time I can get a directory listing, but copying a file to a folder will cause the
system to freeze, also running the MSDOS command MEM always results in a system lockup.

If I drop the 8088 CPU card out of the system and place my 80286 card into the system everything mentioned above works fine without an issue so the fault would seem to be on the 8088 CPU board.

Studying the jumpers and schematic I can improve matters if I jumper K5 2-3, K6 2-3, K7 2-3, K11 2-3, K13 2-3 which obviously is tweaking the bus signal timing to come more from
U68 the bus controller IC, the monitor memory test 'J' then seems to work fine but the system is still not totally stable under DOS and I can get it to crash. Lowering the CPU speed to 4.77Mhz makes no difference the fault remains.

My CPU is a Fujitsu MBL8088-1 and the bus controller IC is a UMC82C88, both of these IC's were supplied as 'NOS' but I can't remember the vendor now. 
I also have a NEC V20 CPU I have just had delivered from UTsource, if I fit this IC then the system stability is much worse

So I have now ordered a 'NEW' Intel 8088-2 and a 'NEW' AMD8288 (non-cmos) bus controller to see if that changes the situation.

Are there any other recent 8088 CPU board builders having problems with this board ?
Rich, did you fix your board ?


David Fry

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