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Olivettti M20 ST missing chip

one1test

Member
Joined
Sep 16, 2022
Messages
20
Location
Turin, Italy
Hi all,
I have an M20 ST (yes, the one with cpu at center of the board) with an empty socket near the floppy flat cable.
Someone can check if it's normal or needs to be filled with a chip ? In case which is the code of the chip ?
Thank you in advance
Davide
 

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...but more resistors. Quick check is to see if all of the pins on one side of the socket are connected to Vcc. If so, it's a pullup. If not, it might be useful to see where the socket leads connect. There aren't a lot of 18 pin DIP ICs in the catalog, which makes me suspect resistor network.
 
To better understand my question this is a photo of the ST board
 

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This, rather bad, photo shows that holder unpopulated, this was on a twin-floppy M20.View attachment 1246296
Thank you Chris. I will leave the holder empty.
My board is not responding.. It seems to be stuck at boot. Looking with oscilloscope i have some i/o signals on ram and cpu but no output on the monitor (no rhombus) no signal at all
i tried to pull out all ics with socket except for cpu- 3 rom (pa63 -pa77 - pa78 and 6545 video chip .
I tried also to replace the 6545 ic , the cpu and the 3 roms with a working ones but the board still not responding. Anyone knows which ic do the initial reset on these boards ?
Thanks in advance
Davide
 
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Sorry, but I might have misled you... I have just remembered that the particular motherboard I took that picture off was from a machine I had which I can not recall booting - I had 3 here at the time (a few years ago now) and I'm easily confused ! ... What give me doubts is the pictures of the `G` rev. board in this article, which seems to show a chp there.:-


on the, rather useful http://www.z80ne.com website.
 
Very nice find Chris !! It's a 9+9 pin DIP IC. Unfortunately the photo have not the necessary resolution to read the code in the chip. On the other hand, looking on the D rev. board i can't find any 9+9 IC....
 
Yes, i have two 128k cards and a 32k. I tried to switch between them in all combinations but no boot
 

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Sorry I did not introduce myself. I recently bought an M20 and an extra PCB so I was looking for some documentation, but I may help with some schematics.

The device next to the floppy connector is called WD2143M-03.

Screenshot 2022-10-24 at 11.54.56.png

Screenshot 2022-10-24 at 12.00.02.png
 
Great Scott !! You have found the missing chip for my board !! Thanks OLI-FIX !!
I will set R8 for 1,4 V thought TP1 but how can i set R11 ?
Do you have full schematic ?
Thanks a lot !! :)
Davide
 
I forgot welcome to the forum.
I have 4 Olivetti M20 to fix.
I have 2 motherboard fixed and i am still checking all the 5 1/4 drives. Someone need repairs...
Happy to exchange infos, docs and hints with you and the other members of the forum :)
Davide
 
Im not sure if it is a good idea to publish some incomplete work. It contains only about 75% of the wirings. Also the revision is a bit unsure. The CPU is in the center.

The DRAMs can be 16k or 64k-bit.
 

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I have two M20s in storage. They both came with the smaller memory on the mother board but I converted the mother board to use 64K chips, on one of them.
I also connected a hard drive to one. The original Olivetti drive is impossible to find. I used a ST251 as I recall ad wasted most of the space. I used a controller
from a TRS80 5Meg hard drive. It did need a little modification as I recall. It needed bus buffers between the M20 and the controller and there was some
modification, I forget what, where the heads cable is connected.
I used a used drive but found I needed to clear Track 0. As the Olivetti expected bad track data there ( I never did find the format of that data but what the PC
put there didn't work. I had to write software to erase the track. Also, you need the right boot floppy version that supports the hard drive. The disk image does
need to be modified. You need to find the two locations that set the track step rate. The ST251 can't step at the rate of the Olivetti drive. You need to change the
two locations to auto-stepping for the ST251. There is a separate location for the read and for the write code,as I recall. It wasn't hard to find as you just look
for the write I/O instructions.
That is about all I recall. I don't know if I wrote this info down someplace.
Both machines have flaky keyboards. I've taken them apart but can't see any reason they should be flaky.
Dwight
 
Hello,
i still waiting for the missing ic to be delivered.
I have 2 Olivetti HD-E/10 10mb external hard drives. I hope to have time to check them soon.
Davide
 
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