486 motherboard repair - No Ram detected

[opensys]

Hello,

I have a Digicom DIGIS-486NL motherboard that don't boot, he complains about no ram with 2 SIMM 1MB modules, it stops at postcode12, with tree long beeps.
So checked for signals in ram slots, and found all ok except A9 pin, the pin have some signal for 2 seconds at boot time and get high level, and stays there.
I follow the signal to a 74fct373 and them goes to 82C495 pin 79, the TAG6/MA9, that have the same signal behavior.
All the other lines A0-A10 have signal and clocks.
So my questions are:

- may the 82C495 IC was bad ?
- What can I do to diagnose more about this issue ?
- Some other ideias about it ?

References:
https://ftpmirror.your.org/pub/misc/bitsavers/components/opti/dataSheets/82C495XLC_82C206_PC_AT_Chip_Set_199410.pdf
https://theretroweb.com/motherboards/s/digicom-digis-486nl


Thanks for your time.

[squadchannel]

did you insert memory into BANK0 and BANK1 respectively?
battery replaced?

to start with.

[opensys]

Yes, I try that, changes the modules to other bank, same behavior.
I have two in bank0 and two in bank1, or 4 in bank0 or 4 in bank1 post code stops at 12/13
About the battery, was removed at many years, no acid damage.

[squadchannel]

Some boards won't start up unless the battery is installed.

[opensys]

No problem, I put a cr2032 with a adapter, the 3.3v power exists in F82C206 the RTC IC.
About the ram line A9 this is normal behavior, have clock activity about 1/2 second then goes way and stay high after power on ?

[Ian.M]

Those look like 30 pin SIMM  sockets.  30 pin SIMMs are 8 bit (9 with parity), so as the i486 is a 32 bit processor with a 32 bit bus, you *MUST* have four matched SIMMs in the *SAME* bank for it to have any hope of booting.   Two wont do it, nor will splitting four across two banks.

[squadchannel]

I was wondering why 256k becomes 1MB. that makes sense.

[opensys]

ok, I just have 4 modules, no replacements, but all in same bank, same result.

[opensys]

4 new SIMM 30 pins modules, same beaver, stop in the same post.

[akatrinii]

What CPU are you using?  Check the Jumper settings are set correctly.

Reseat BIOS chip, clean the legs with fine sandpaper, apply deoxid spray.

Clean ram contacts with IPA, apply deoxit spray on the Ram slots aswell.

Many times I've seen 486 boards not working just from bad contacts.

[Haenk]

4 new SIMM 30 pins modules, same beaver, stop in the same post.

"new" as in "new", or a set of tested working vintage modules?
Check the manual for manufacturer qualified modules...

[Psi]

Do you have a videocard installed,  beep codes aren't always correct at identifying the real issue. Also maybe its meaning no VRAM due to no videocard, not system ram

I vaguely recall a few things about sims.
Are they the right type, I think some are different.
- The word FB (Fast page) comes to mind. I think there were speed grades.
- I think there was single and double-sided versions. I cant recall if double sided ones meant you could install less chips or if it was just how the bits were split up, But i'm pretty sure they all had to be the same/right type