Carles20vt wrote on 2021-10-12, 18:19:

I tried to see what line of the ISA bus hangs in the system. After some experimentation looking at the ISA bus pinouts, I found that the computers boot up if you cover all the data lines (D0 to D7).

I checked the CGA card diagrams fount at -0Cº but only saw a logic chip that handles these data lines to be Input or Output. I don't know how to test it or check the entire logic.

That logic chip you talk about is U66, a 74LS245, a bidirectional data driver. Either that chip is broken, or it gets bad control signals, causing it to disturb ISA operation. As you said, the POST card doesn't output anything, so we can be very sure that the card disturbs the ISA bus even if it is not addressed. When the card is not addressed, pin 19 of U66 should be high, keeping the chip off the ISA bus. Even without a scope, you can put a multimeter on that pin. It needs to stay near 5V all the time, i.e. the value on the meter shouldn't drop at all, not even from 4.8V to 4.5V. If you hit the reset button, attempting a new POST, the pin still needs to stay at the same voltage. The CGA card is not addressed before the first POST code is issued. If pin 19 stays at 5V all the time, and the CGA card still disturbs the bus, U66 is broken. If pin 19 is low, you need to trace the logic generating that signal.

Great detail mkarcher ! Curious if the Mono/Color switch was put in proper position for CGA.....not that being in wrong position would halt a boot AFAIK.....

mkarcher wrote on 2021-10-12, 18:38:

Carles20vt wrote on 2021-10-12, 18:19:

I tried to see what line of the ISA bus hangs in the system. After some experimentation looking at the ISA bus pinouts, I found that the computers boot up if you cover all the data lines (D0 to D7).

I checked the CGA card diagrams fount at -0Cº but only saw a logic chip that handles these data lines to be Input or Output. I don't know how to test it or check the entire logic.

That logic chip you talk about is U66, a 74LS245, a bidirectional data driver. Either that chip is broken, or it gets bad control signals, causing it to disturb ISA operation. As you said, the POST card doesn't output anything, so we can be very sure that the card disturbs the ISA bus even if it is not addressed. When the card is not addressed, pin 19 of U66 should be high, keeping the chip off the ISA bus. Even without a scope, you can put a multimeter on that pin. It needs to stay near 5V all the time, i.e. the value on the meter shouldn't drop at all, not even from 4.8V to 4.5V. If you hit the reset button, attempting a new POST, the pin still needs to stay at the same voltage. The CGA card is not addressed before the first POST code is issued. If pin 19 stays at 5V all the time, and the CGA card still disturbs the bus, U66 is broken. If pin 19 is low, you need to trace the logic generating that signal.

I had just tested it now and everytime is Hi (around 5v). So... I can assume that the IC is broken. I will order a new one and see what happens.

I tested a cheap CGA clone and I can see some small voltage decrements in some point.

Thanks a lot for the information! I will learn a little more about logics with this small finding 😁

Only a minor update.

I have just removed the U66 and prepared the board to install the replacement.

I removed the tape and tested it. The computer boots, and nothing happens. So, it is promising! 😁

The IC arrived at home sooner as expected. So I just installed it when I finished the work. (I can't wait a minute more). So I soldered it, put it on a test board (good known 386 board) with the POST card attached, and it started! Look at this!!! We have an image! And the POST card shows numbers, and the speaker counts the ram. It is promising.

So the IBM CGA was installed in his place, and then I was needed to set up using the Diagnostics Disk to set the "new" card. After that, I tested using the same tool to fine-tune the potentiometer on the mainboard to show the colours properly.

And after it, I tested (and played a little 😜 ) with these titles to test the card. Wow! CGA card indeed shows the games better-using composite! But the text is blurry. So, only games. OK, I can deal with it 😉

My dream is to have a CGA/EGA IBM monitor, but it is prohibitive to pay one, working or not.

So thanks a lot for helping with this topic. I learned a little from it. 😁

​​​​​​​Now I want to solve the last mystery that I have. The dead/crazy memory expansion board!

Good work !