With 1MB RAM on-board, I expect you have 640KB conventional memory + 384KB extended memory. The onboard extended memory is at address range 1024KB (1MB) to 1408KB (1.375MB), so that's what you should set as start address on your RAM card.

Try SW1/1, SW1/2 and SW1/4 in the ON position, and SW1/3, SW1/5, SW1/6 and SW1/7 in the OFF position to get the required starting address.

Thanks for quick reply!

The ram count now shows 1152 KB so it count some RAM above 1024 KB at least but the issue persists. Do you think it could be related to RAM configuration?
SW2 has:
1/2, 2/2, 3/2 ON
4/2, 5/2, 6/2, 7/2, 8/2 OFF

JanOlausson wrote on 2022-04-18, 15:45:

Thanks for quick reply! […]
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Thanks for quick reply!

The ram count now shows 1152 KB so it count some RAM above 1024 KB at least but the issue persists. Do you think it could be related to RAM configuration?
SW2 has:
1/2, 2/2, 3/2 ON
4/2, 5/2, 6/2, 7/2, 8/2 OFF

OK, so this means the 1024KB planar RAM is now accessible reliably, and 128KB provided by the RAM card also works. I can't find documentation for that RAM card (actually, a multifunction card) except for what you can find on the TH99 mirrors. The jumper setting I gave you for "start at 1.38MB" is guessed from the start addresses documented in that mirror. I can't tell you whether this is an officially supported configuration, or misbehaviour of the RAM card is to be expected. Do you get the full 4096KB with the DIP switch settings I suggested and the Cirrus card removed?

Great! Without VGA card I now get the full 4096 KB. So far so good.
But as soon as I add the VGA card I get strange error on start up.
It stops on RAM count as:
01152 KB OK
180000 0009 201

Not sure what the numbers below RAM are but I suspect it's some kind of memory address or error code.
I have disabled the cards all other functionality. I used this documentation: https://stason.org/TULARC/pc/io-cards/C-D/UNI … rd-MF-3000.html

JanOlausson wrote on 2022-04-18, 18:51:

Great! Without VGA card I now get the full 4096 KB. So far so good. But as soon as I add the VGA card I get strange error on sta […]
Show full quote

Great! Without VGA card I now get the full 4096 KB. So far so good.
But as soon as I add the VGA card I get strange error on start up.
It stops on RAM count as:
01152 KB OK
180000 0009 201

Not sure what the numbers below RAM are but I suspect it's some kind of memory address or error code.
I have disabled the cards all other functionality. I used this documentation: https://stason.org/TULARC/pc/io-cards/C-D/UNI … rd-MF-3000.html

The first part is a memory address. It indicates offset 512K into the extended memory. So the working memory is 640K convetional + 512 extended = 1152KB. The second part is the mask of failing bits (0009 means: data bits 0 and 3 read back different than what was written), and "201" is the IBM error code number for a memory error. Obviously something goes wrong on the ISA bus regarding access to extended memory, as soon as the Cirrus card is installed. It seems the issue depends on the address. Please check the cirrus card for short circuits on the LA17-LA23 lines, either shorts between each other, or shorts to ground or +5V. These contacts are C2 to C8 on the ISA connector, see https://old.pinouts.ru/Slots/ISA.shtml .

Wow, thanks a lot for all that info. I'll grab my multimeter and start checking for short circuits.

I tested all connections on card and found no short circuits (Except between +5 and ground respectively of course).
Not sure where to go from here. Maybe there is an ISA bus diagnostic tool.