Yup, another IBM 5160 failed motherboard

[modem7]

The logic probe sampling frequency is 2 MHz.

I think "sampling" is the wrong word to use for a logic probe.
But yes, the 2 MHz quoted for your logic probe is a limit. Beyond that, alternating digital signals may not be expected to show as alternating ('pulsing') on your logic probe.

But your earlier posts suggest that your logic probe is showing an alternating signal ('pulsing') for the 4.77 MHz clock.

That is not enough? I need like 10 MHz sampling frequency to capture data been transmitted at 4.77 MHz ?

Just because the 8088 CPU is receiving a 4.77 MHz clock does not mean that addresses and data are changing at 4.77 MHz.
The 8088 data sheet contains some timing diagrams.
You have a good 5160 motherboard, and so you can confirm that your logic probe shows activity ('pulsing') on the 5160's address and data buses.

[tonata]

Sorry. My fault. I was referring to a clone of the logic analyzer "Saleae Logic Analyzer". I read in the comments that at some point the logic analyzer stopped with message "device was not able to keep up with the sample rate".

[maxtherabbit]

I think "sampling" is the wrong word to use for a logic probe.

No, it's exactly the correct word

[modem7]

I think "sampling" is the wrong word to use for a logic probe.

No, it's exactly the correct word

There is a kit for a logic probe shown at [here]. That probe does no sampling.

[tonata]

Question.
There are 4 banks of memory. Where is the physical location on the motherboard for the memory between 640kb and 1024kb?
I am checking here and so some is real RAM until 768 KB and above it it is all mapped and thus it is not physical?

[jafir]

There isn’t any, unless you’ve got an expanded memory card. That space is just for add-in hardware like ISA cards. If you’ll notice, the items in that image you linked to are mostly expansion cards, like graphics cards.

[ibmapc]

Question.
There are 4 banks of memory. Where is the physical location on the motherboard for the memory between 640kb and 1024kb?
I am checking here and so some is real RAM until 768 KB and above it it is all mapped and thus it is not physical?

There isn’t any, unless you’ve got an expanded memory card. That space is just for add-in hardware like ISA cards. If you’ll notice, the items in that image you linked to are mostly expansion cards, like graphics cards.

That's true. Unless you perform the 1megXT mod.

[tonata]

I thought that these ROMs are copied into the RAM. Instead they are simply mapped.

I have a question about U2:
IMG_20201130_182758_small.jpg
Mine looks rusty. Is it a good candidate for replacement?

[modem7]

Mine looks rusty. Is it a good candidate for replacement?

Perhaps a personal thing. I would not be replacing that pictured chip based only on its present appearance, but others here might.

[tonata]

When the POST configures a pin in 8255 as an output then what voltage is expected?