Why did you connect the mouse while the system was on? Serial mouse is not hot-plug capable.

Lets go back to: "so I disconnected the hard drive (idk why I did that), and it worked. Then I went and connected it again, and the PC stopped responding completely."

Does this mean the computer will POST with the drive removed? I would say the drive is bad. (or cable in backwards, but since you didn't move that, rule that out). Pretty random failure after simply hot-plugging a serial mouse, if true.

dominusprog wrote on 2025-07-13, 14:54:

Why did you connect the mouse while the system was on? Serial mouse is not hot-plug.

I always thought a serial mouse could be plugged in with the PC turned on. I only ever heard about PS2 devices that could damage something; meanwhile, I never damaged anything by plugging in a serial mouse.

Major Jackyl wrote on 2025-07-13, 15:07:

Does this mean the computer will POST with the drive removed? I would say the drive is bad. (or cable in backwards, but since you didn't move that, rule that out). Pretty random failure after simply hot-plugging a serial mouse, if true.

No, it was only a one-time thing. The drive is a CF-IDE adapter with a practically new CF card. The cable can't be plugged in backwards in this motherboard, nor can it be plugged in backwards to the adapter. Also forgot to mention, I got it set up in a way where I got the CF adapter on the primary IDE and a DVD+HDD on the secondary IDE. Disconnecting the adapter made the PC boot.

I always thought a serial mouse could be plugged in with the PC turned on. I only ever heard about PS2 devices that could damage something; meanwhile, I never damaged anything by plugging in a serial mouse.

Well.. The serial port and serial mouse aren’t exactly hot plug capable, but the RS-232 port on PC has protective diodes.
Normally, the used UART FiFo is even able to cope with shorts.
So *normally*, the serial port itself is quite safe and good to tinker with.
Completely different to parallel port, which goes "poof" if you just make weird looks. 😂
But can't speak about the serial mouse, since it's not standardized. It depends on the model.

Back in the late-90's a friend of mine had an XT form factor 286 PC and he did the same as OP and resulted in the same error OP experienced. So this could definitely happen. We were to young back then to properly troubleshoot. We were around 10. I wasn't present when he connected the mouse, but after that the PC couldn't recognize the hard drive. We never quite could fix it even with replacing the hard drive controller card. but booting from floppy and loading games from there still worked.

So things like this can happen.

Jo22 wrote on 2025-07-13, 15:41:

Well.. The serial port and serial mouse aren’t exactly hot plug capable, but the RS-232 port on PC has protective diodes.
...
But can't speak about the serial mouse, since it's not standardized. It depends on the model.

Even if I did fry the diodes, the PC should still boot up, right? I could replace the diodes myself without much issue if that happened, so that's not the issue.
The mouse is a Microsoft-branded one.

RandomStranger wrote on 2025-07-13, 16:13:

Back in the late-90's a friend of mine had an XT form factor 286 PC and he did the same as OP and resulted in the same error OP experienced. So this could definitely happen. We were to young back then to properly troubleshoot. We were around 10. I wasn't present when he connected the mouse, but after that the PC couldn't recognize the hard drive. We never quite could fix it even with replacing the hard drive controller card. but booting from floppy and loading games from there still worked.

The thing is, in my case the PC doesn't boot or give any errors through the PC speaker, even though it did beep at the beginning. After some tinkering, it stopped completely. I doubt I fried the IDE, and even if I did, I'm more concerned about why the motherboard seems dead.

RandomStranger wrote on 2025-07-13, 16:13:

Back in the late-90's a friend of mine had an XT form factor 286 PC and he did the same as OP and resulted in the same error OP experienced. So this could definitely happen. We were to young back then to properly troubleshoot. We were around 10. I wasn't present when he connected the mouse, but after that the PC couldn't recognize the hard drive. We never quite could fix it even with replacing the hard drive controller card. but booting from floppy and loading games from there still worked.

So things like this can happen.

Wait a moment, was it really a serial mouse or serial port?
Because, there had been bus mice with a DE-9 connector in that time .
My 286 PC had it on-board, too and the label was "MOUSE". It wasn't short-protected, also.
This port didn't work with my Genius mouse I got, so I never used that port, but ordinary "SERIAL" port (also DE-9).

Part of RS-232 is that shorting pins has to be tolerated. And Windows 95 even advised hot-plugging a serial mouse if no mouse was detected (but not PS/2 of course).

You could try reflashing the BIOS.

Another option, as stupid as it may sound, is to just disconnect everything and take all the components apart, put everything away and try again in 1 week.

jakethompson1 wrote on 2025-07-13, 18:28:

Part of RS-232 is that shorting pins has to be tolerated. And Windows 95 even advised hot-plugging a serial mouse if no mouse was detected (but not PS/2 of course).

Yeah, that's why I do it.

devius wrote on 2025-07-13, 20:08:

You could try reflashing the BIOS.

Another option, as stupid as it may sound, is to just disconnect everything and take all the components apart, put everything away and try again in 1 week.

I would love to be able to reflash the BIOS, but I don't own any device that is capable of flashing chips. When the PC was still up and running, I wanted to reflash the BIOS on my S3 Virge, but because there is not a way to do that via software, I just swapped the chip from some other S3 card I have. While doing research I found that there is a way to reflash the chip using some network card. I even got one that should be compatible, but until I get the PC running again, there is no point since the S3 Virge was meant to be used in this PC.

It does not sound stupid at all. Every time when I run out of options when troubleshooting, I just leave the hardware and come back after a few days. It works wonderfully almost every time.

dominusprog wrote on 2025-07-13, 14:54:

Why did you connect the mouse while the system was on? Serial mouse is not hot-plug.

Partially true - the serial port (RS-232) is definitely hot-plugable (speaking as someone with a LOT of serial port
experience - I designed the NewBridge 10xx series of serial data switches and multiplexers).

So, hardware-wise, no damage should occur - but software can't be guaranteed. A super-simple OS
(like DOS) generally needs the mouse to be connected when it's driver loads. More complex multi-threading
OSs (like Winblows, Linux) etc. may detect the mouse "on the fly - but this is often not done
because lots of other "stuff" could be connected to a serial port and you don't necessarily want to go
"twiddling bits" every time anything is connected.

DOS "knows" there's a serial mouse because you are loading a mouse driver..
Other OS's often look for one at startup - some keep checking. others no...

None of which explains why the system "died" ... if it doesn't even POST, could be a
hardware problem caused by static discharge (unlikely as RS-232 ports should be pretty well
protected) - otherwise most likely software has gotten itself confused...

DaveDDS wrote on 2025-07-13, 23:06:

Partially true - the serial port (RS-232) is definitely hot-plugable (speaking as someone with a LOT of serial port experience - […]
Show full quote

dominusprog wrote on 2025-07-13, 14:54:

Why did you connect the mouse while the system was on? Serial mouse is not hot-plug.

Partially true - the serial port (RS-232) is definitely hot-plugable (speaking as someone with a LOT of serial port
experience - I designed the NewBridge 10xx series of serial data switches and multiplexers).

So, hardware-wise, no damage should occur - but software can't be guaranteed. A super-simple OS
(like DOS) generally needs the mouse to be connected when it's driver loads. More complex multi-threading
OSs (like Winblows, Linux) etc. may detect the mouse "on the fly - but this is often not done
because lots of other "stuff" could be connected to a serial port and you don't necessarily want to go
"twiddling bits" every time anything is connected.

DOS "knows" there's a serial mouse because you are loading a mouse driver..
Other OS's often look for one at startup - some keep checking. others no...

None of which explains why the system "died" ... if it doesn't even POST, could be a
hardware problem caused by static discharge (unlikely as RS-232 ports should be pretty well
protected) - otherwise most likely software has gotten itself confused...

Good point, but why would anyone take a risk when it can easily be avoided?