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Commodore PC30-III issue with RAM test

D

demonlg

Veteran Member
Joined
May 21, 2016
Messages
575
Location
Italy
Hello again,

i have a PC30-III recovered in a abandoned house! This machine is the first revision with battery externally of the RTC chip. This computer results totally dead, no power completely. After repair power supply the machine not boot, no video output, nothing.

I have checked, revised, cleaned all the battery area and i have found only two pcb wire interrupted, and three logic chip faulty, the chip involved in a reset circuit and keyboard controller circuit. Replaced all with new one and repaired the wires i have modded the board to use a DALLAS 1287 RTC chip, i have another working PC30-III with this and i have removed component not used with DALLAS.

Then i have tested the VGA ISA card in another computer and it results faulty and replaced with another working, rhis machine not post anything without video card and hangs during post. Then i have found a PAL chip faulty, replaced with my working PAL from my PC30-III working and now i have a POST and boot screen but......

I see video card model installed, boot screen with commodore BIOS version and it start to test the RAM memory but it stop to 256k and going to reset loop.. then again 256k and reset...

I have tested all chip RAM with my PC30-III and with DRAM tester all results ok, and all logic involved to memory controller and not involved, the delay lines too, and all socketed chip, CPU too, and all PAL chip on my PC30-III, all is working perfectly, but on this board not, it's stay to same problem of 256k ram and reset.

The RTC dallas chip have external battery disconnected and resetted, don't have BIOS setting saved. The keyboard controller is ok tested, but i don't have any access to BIOS settings with pressing CRTL+ALT+ESC keys...

Any help to me to other check?

Seems all wires are ok, tested connection with chips and schematics. Suspect to CPU socket, but i'm not sure..

Thanks

Emanuele
 
Adding, if i remove the DALLAS 1287 the issue is the same, this DALLAS chip have battery discharged end removed from another computer, totally different of this...but my other PC30 boots without DALLAS chip inserted and give me a cmos error.

And, when i power up computer the leds on keyboard don't blink, at first time, i don't know if is normal or not, i have to check in other my PC30-III.

I have removed and tested all RAM chip, all are good, mistery!
 
Do you have a card that can check what is written to port hex 80?
 
I strongly recommend buying one of those cards. The other option would be to burn a modified BIOS that replaces writes to port 80 with writes to whichever address the parallel port has - in that case you could simply for example connect LEDs with drivers to the 8 data bits (maybe even use LEDs with decent intensity with just series resistors connected to the data bit pins on the parallel port).

==========

In general it might be something related to RAM parity testing. I.E. maybe it checks 256k and then gets a parity error when testing the next 256k block, and some hardware fault causes the parity error to actually generate a NMI rather than just BIOS reading it in some port or so?

Is it possible to set jumpers for how much RAM it has? If so try setting it to lower and lower settings and see if it makes a difference.
 
Ok, i'll try to set jumpers to obtain 128k total memory or 256k total memory and check if it continue to bootstrap, i have to cut trace because jumper are not present but only holes and track to short 2 pins.

Thanks for now

Emanuele
 
Hello, i have resolved the problem on PC30, Memory RAM resulsts defective with another ram tester, i have checked with scope all datalines and one chip results with output little lowest respect other chips. i have replaced all chip with new one and the machine boots !

Then i have modified the board for use DALLAS RTC chip and now is perfectly working with an old IDE Harddisk or CF direclty connected on IDE bus on mainboard.

Thanks for support

Emanuele
 
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